--- /dev/null
+Getting started with kmemcheck
+==============================
+
+Vegard Nossum <vegardno@ifi.uio.no>
+
+
+Introduction
+------------
+
+kmemcheck is a debugging feature for the Linux Kernel. More specifically, it
+is a dynamic checker that detects and warns about some uses of uninitialized
+memory.
+
+Userspace programmers might be familiar with Valgrind's memcheck. The main
+difference between memcheck and kmemcheck is that memcheck works for userspace
+programs only, and kmemcheck works for the kernel only. The implementations
+are of course vastly different. Because of this, kmemcheck is not as accurate
+as memcheck, but it turns out to be good enough in practice to discover real
+programmer errors that the compiler is not able to find through static
+analysis.
+
+Enabling kmemcheck on a kernel will probably slow it down to the extent that
+the machine will not be usable for normal workloads such as e.g. an
+interactive desktop. kmemcheck will also cause the kernel to use about twice
+as much memory as normal. For this reason, kmemcheck is strictly a debugging
+feature.
+
+
+Downloading
+-----------
+
+As of version 2.6.31-rc1, kmemcheck is included in the mainline kernel.
+
+
+Configuring and compiling
+-------------------------
+
+kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of
+configuration variables must have specific settings in order for the kmemcheck
+menu to even appear in "menuconfig". These are:
+
+- ``CONFIG_CC_OPTIMIZE_FOR_SIZE=n``
+ This option is located under "General setup" / "Optimize for size".
+
+ Without this, gcc will use certain optimizations that usually lead to
+ false positive warnings from kmemcheck. An example of this is a 16-bit
+ field in a struct, where gcc may load 32 bits, then discard the upper
+ 16 bits. kmemcheck sees only the 32-bit load, and may trigger a
+ warning for the upper 16 bits (if they're uninitialized).
+
+- ``CONFIG_SLAB=y`` or ``CONFIG_SLUB=y``
+ This option is located under "General setup" / "Choose SLAB
+ allocator".
+
+- ``CONFIG_FUNCTION_TRACER=n``
+ This option is located under "Kernel hacking" / "Tracers" / "Kernel
+ Function Tracer"
+
+ When function tracing is compiled in, gcc emits a call to another
+ function at the beginning of every function. This means that when the
+ page fault handler is called, the ftrace framework will be called
+ before kmemcheck has had a chance to handle the fault. If ftrace then
+ modifies memory that was tracked by kmemcheck, the result is an
+ endless recursive page fault.
+
+- ``CONFIG_DEBUG_PAGEALLOC=n``
+ This option is located under "Kernel hacking" / "Memory Debugging"
+ / "Debug page memory allocations".
+
+In addition, I highly recommend turning on ``CONFIG_DEBUG_INFO=y``. This is also
+located under "Kernel hacking". With this, you will be able to get line number
+information from the kmemcheck warnings, which is extremely valuable in
+debugging a problem. This option is not mandatory, however, because it slows
+down the compilation process and produces a much bigger kernel image.
+
+Now the kmemcheck menu should be visible (under "Kernel hacking" / "Memory
+Debugging" / "kmemcheck: trap use of uninitialized memory"). Here follows
+a description of the kmemcheck configuration variables:
+
+- ``CONFIG_KMEMCHECK``
+ This must be enabled in order to use kmemcheck at all...
+
+- ``CONFIG_KMEMCHECK_``[``DISABLED`` | ``ENABLED`` | ``ONESHOT``]``_BY_DEFAULT``
+ This option controls the status of kmemcheck at boot-time. "Enabled"
+ will enable kmemcheck right from the start, "disabled" will boot the
+ kernel as normal (but with the kmemcheck code compiled in, so it can
+ be enabled at run-time after the kernel has booted), and "one-shot" is
+ a special mode which will turn kmemcheck off automatically after
+ detecting the first use of uninitialized memory.
+
+ If you are using kmemcheck to actively debug a problem, then you
+ probably want to choose "enabled" here.
+
+ The one-shot mode is mostly useful in automated test setups because it
+ can prevent floods of warnings and increase the chances of the machine
+ surviving in case something is really wrong. In other cases, the one-
+ shot mode could actually be counter-productive because it would turn
+ itself off at the very first error -- in the case of a false positive
+ too -- and this would come in the way of debugging the specific
+ problem you were interested in.
+
+ If you would like to use your kernel as normal, but with a chance to
+ enable kmemcheck in case of some problem, it might be a good idea to
+ choose "disabled" here. When kmemcheck is disabled, most of the run-
+ time overhead is not incurred, and the kernel will be almost as fast
+ as normal.
+
+- ``CONFIG_KMEMCHECK_QUEUE_SIZE``
+ Select the maximum number of error reports to store in an internal
+ (fixed-size) buffer. Since errors can occur virtually anywhere and in
+ any context, we need a temporary storage area which is guaranteed not
+ to generate any other page faults when accessed. The queue will be
+ emptied as soon as a tasklet may be scheduled. If the queue is full,
+ new error reports will be lost.
+
+ The default value of 64 is probably fine. If some code produces more
+ than 64 errors within an irqs-off section, then the code is likely to
+ produce many, many more, too, and these additional reports seldom give
+ any more information (the first report is usually the most valuable
+ anyway).
+
+ This number might have to be adjusted if you are not using serial
+ console or similar to capture the kernel log. If you are using the
+ "dmesg" command to save the log, then getting a lot of kmemcheck
+ warnings might overflow the kernel log itself, and the earlier reports
+ will get lost in that way instead. Try setting this to 10 or so on
+ such a setup.
+
+- ``CONFIG_KMEMCHECK_SHADOW_COPY_SHIFT``
+ Select the number of shadow bytes to save along with each entry of the
+ error-report queue. These bytes indicate what parts of an allocation
+ are initialized, uninitialized, etc. and will be displayed when an
+ error is detected to help the debugging of a particular problem.
+
+ The number entered here is actually the logarithm of the number of
+ bytes that will be saved. So if you pick for example 5 here, kmemcheck
+ will save 2^5 = 32 bytes.
+
+ The default value should be fine for debugging most problems. It also
+ fits nicely within 80 columns.
+
+- ``CONFIG_KMEMCHECK_PARTIAL_OK``
+ This option (when enabled) works around certain GCC optimizations that
+ produce 32-bit reads from 16-bit variables where the upper 16 bits are
+ thrown away afterwards.
+
+ The default value (enabled) is recommended. This may of course hide
+ some real errors, but disabling it would probably produce a lot of
+ false positives.
+
+- ``CONFIG_KMEMCHECK_BITOPS_OK``
+ This option silences warnings that would be generated for bit-field
+ accesses where not all the bits are initialized at the same time. This
+ may also hide some real bugs.
+
+ This option is probably obsolete, or it should be replaced with
+ the kmemcheck-/bitfield-annotations for the code in question. The
+ default value is therefore fine.
+
+Now compile the kernel as usual.
+
+
+How to use
+----------
+
+Booting
+~~~~~~~
+
+First some information about the command-line options. There is only one
+option specific to kmemcheck, and this is called "kmemcheck". It can be used
+to override the default mode as chosen by the ``CONFIG_KMEMCHECK_*_BY_DEFAULT``
+option. Its possible settings are:
+
+- ``kmemcheck=0`` (disabled)
+- ``kmemcheck=1`` (enabled)
+- ``kmemcheck=2`` (one-shot mode)
+
+If SLUB debugging has been enabled in the kernel, it may take precedence over
+kmemcheck in such a way that the slab caches which are under SLUB debugging
+will not be tracked by kmemcheck. In order to ensure that this doesn't happen
+(even though it shouldn't by default), use SLUB's boot option ``slub_debug``,
+like this: ``slub_debug=-``
+
+In fact, this option may also be used for fine-grained control over SLUB vs.
+kmemcheck. For example, if the command line includes
+``kmemcheck=1 slub_debug=,dentry``, then SLUB debugging will be used only
+for the "dentry" slab cache, and with kmemcheck tracking all the other
+caches. This is advanced usage, however, and is not generally recommended.
+
+
+Run-time enable/disable
+~~~~~~~~~~~~~~~~~~~~~~~
+
+When the kernel has booted, it is possible to enable or disable kmemcheck at
+run-time. WARNING: This feature is still experimental and may cause false
+positive warnings to appear. Therefore, try not to use this. If you find that
+it doesn't work properly (e.g. you see an unreasonable amount of warnings), I
+will be happy to take bug reports.
+
+Use the file ``/proc/sys/kernel/kmemcheck`` for this purpose, e.g.::
+
+ $ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck
+
+The numbers are the same as for the ``kmemcheck=`` command-line option.
+
+
+Debugging
+~~~~~~~~~
+
+A typical report will look something like this::
+
+ WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
+ 80000000000000000000000000000000000000000088ffff0000000000000000
+ i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
+ ^
+
+ Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A
+ RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
+ RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002
+ RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
+ RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84
+ RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000
+ R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e
+ R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8
+ FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000
+ CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
+ CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0
+ DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
+ DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
+ [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
+ [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
+ [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
+ [<ffffffff8100c7b5>] int_signal+0x12/0x17
+ [<ffffffffffffffff>] 0xffffffffffffffff
+
+The single most valuable information in this report is the RIP (or EIP on 32-
+bit) value. This will help us pinpoint exactly which instruction that caused
+the warning.
+
+If your kernel was compiled with ``CONFIG_DEBUG_INFO=y``, then all we have to do
+is give this address to the addr2line program, like this::
+
+ $ addr2line -e vmlinux -i ffffffff8104ede8
+ arch/x86/include/asm/string_64.h:12
+ include/asm-generic/siginfo.h:287
+ kernel/signal.c:380
+ kernel/signal.c:410
+
+The "``-e vmlinux``" tells addr2line which file to look in. **IMPORTANT:**
+This must be the vmlinux of the kernel that produced the warning in the
+first place! If not, the line number information will almost certainly be
+wrong.
+
+The "``-i``" tells addr2line to also print the line numbers of inlined
+functions. In this case, the flag was very important, because otherwise,
+it would only have printed the first line, which is just a call to
+``memcpy()``, which could be called from a thousand places in the kernel, and
+is therefore not very useful. These inlined functions would not show up in
+the stack trace above, simply because the kernel doesn't load the extra
+debugging information. This technique can of course be used with ordinary
+kernel oopses as well.
+
+In this case, it's the caller of ``memcpy()`` that is interesting, and it can be
+found in ``include/asm-generic/siginfo.h``, line 287::
+
+ 281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from)
+ 282 {
+ 283 if (from->si_code < 0)
+ 284 memcpy(to, from, sizeof(*to));
+ 285 else
+ 286 /* _sigchld is currently the largest know union member */
+ 287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld));
+ 288 }
+
+Since this was a read (kmemcheck usually warns about reads only, though it can
+warn about writes to unallocated or freed memory as well), it was probably the
+"from" argument which contained some uninitialized bytes. Following the chain
+of calls, we move upwards to see where "from" was allocated or initialized,
+``kernel/signal.c``, line 380::
+
+ 359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info)
+ 360 {
+ ...
+ 367 list_for_each_entry(q, &list->list, list) {
+ 368 if (q->info.si_signo == sig) {
+ 369 if (first)
+ 370 goto still_pending;
+ 371 first = q;
+ ...
+ 377 if (first) {
+ 378 still_pending:
+ 379 list_del_init(&first->list);
+ 380 copy_siginfo(info, &first->info);
+ 381 __sigqueue_free(first);
+ ...
+ 392 }
+ 393 }
+
+Here, it is ``&first->info`` that is being passed on to ``copy_siginfo()``. The
+variable ``first`` was found on a list -- passed in as the second argument to
+``collect_signal()``. We continue our journey through the stack, to figure out
+where the item on "list" was allocated or initialized. We move to line 410::
+
+ 395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
+ 396 siginfo_t *info)
+ 397 {
+ ...
+ 410 collect_signal(sig, pending, info);
+ ...
+ 414 }
+
+Now we need to follow the ``pending`` pointer, since that is being passed on to
+``collect_signal()`` as ``list``. At this point, we've run out of lines from the
+"addr2line" output. Not to worry, we just paste the next addresses from the
+kmemcheck stack dump, i.e.::
+
+ [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
+ [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
+ [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
+ [<ffffffff8100c7b5>] int_signal+0x12/0x17
+
+ $ addr2line -e vmlinux -i ffffffff8104f04e ffffffff81050bd8 \
+ ffffffff8100b87d ffffffff8100c7b5
+ kernel/signal.c:446
+ kernel/signal.c:1806
+ arch/x86/kernel/signal.c:805
+ arch/x86/kernel/signal.c:871
+ arch/x86/kernel/entry_64.S:694
+
+Remember that since these addresses were found on the stack and not as the
+RIP value, they actually point to the _next_ instruction (they are return
+addresses). This becomes obvious when we look at the code for line 446::
+
+ 422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
+ 423 {
+ ...
+ 431 signr = __dequeue_signal(&tsk->signal->shared_pending,
+ 432 mask, info);
+ 433 /*
+ 434 * itimer signal ?
+ 435 *
+ 436 * itimers are process shared and we restart periodic
+ 437 * itimers in the signal delivery path to prevent DoS
+ 438 * attacks in the high resolution timer case. This is
+ 439 * compliant with the old way of self restarting
+ 440 * itimers, as the SIGALRM is a legacy signal and only
+ 441 * queued once. Changing the restart behaviour to
+ 442 * restart the timer in the signal dequeue path is
+ 443 * reducing the timer noise on heavy loaded !highres
+ 444 * systems too.
+ 445 */
+ 446 if (unlikely(signr == SIGALRM)) {
+ ...
+ 489 }
+
+So instead of looking at 446, we should be looking at 431, which is the line
+that executes just before 446. Here we see that what we are looking for is
+``&tsk->signal->shared_pending``.
+
+Our next task is now to figure out which function that puts items on this
+``shared_pending`` list. A crude, but efficient tool, is ``git grep``::
+
+ $ git grep -n 'shared_pending' kernel/
+ ...
+ kernel/signal.c:828: pending = group ? &t->signal->shared_pending : &t->pending;
+ kernel/signal.c:1339: pending = group ? &t->signal->shared_pending : &t->pending;
+ ...
+
+There were more results, but none of them were related to list operations,
+and these were the only assignments. We inspect the line numbers more closely
+and find that this is indeed where items are being added to the list::
+
+ 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
+ 817 int group)
+ 818 {
+ ...
+ 828 pending = group ? &t->signal->shared_pending : &t->pending;
+ ...
+ 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
+ 852 (is_si_special(info) ||
+ 853 info->si_code >= 0)));
+ 854 if (q) {
+ 855 list_add_tail(&q->list, &pending->list);
+ ...
+ 890 }
+
+and::
+
+ 1309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
+ 1310 {
+ ....
+ 1339 pending = group ? &t->signal->shared_pending : &t->pending;
+ 1340 list_add_tail(&q->list, &pending->list);
+ ....
+ 1347 }
+
+In the first case, the list element we are looking for, ``q``, is being
+returned from the function ``__sigqueue_alloc()``, which looks like an
+allocation function. Let's take a look at it::
+
+ 187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
+ 188 int override_rlimit)
+ 189 {
+ 190 struct sigqueue *q = NULL;
+ 191 struct user_struct *user;
+ 192
+ 193 /*
+ 194 * We won't get problems with the target's UID changing under us
+ 195 * because changing it requires RCU be used, and if t != current, the
+ 196 * caller must be holding the RCU readlock (by way of a spinlock) and
+ 197 * we use RCU protection here
+ 198 */
+ 199 user = get_uid(__task_cred(t)->user);
+ 200 atomic_inc(&user->sigpending);
+ 201 if (override_rlimit ||
+ 202 atomic_read(&user->sigpending) <=
+ 203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
+ 204 q = kmem_cache_alloc(sigqueue_cachep, flags);
+ 205 if (unlikely(q == NULL)) {
+ 206 atomic_dec(&user->sigpending);
+ 207 free_uid(user);
+ 208 } else {
+ 209 INIT_LIST_HEAD(&q->list);
+ 210 q->flags = 0;
+ 211 q->user = user;
+ 212 }
+ 213
+ 214 return q;
+ 215 }
+
+We see that this function initializes ``q->list``, ``q->flags``, and
+``q->user``. It seems that now is the time to look at the definition of
+``struct sigqueue``, e.g.::
+
+ 14 struct sigqueue {
+ 15 struct list_head list;
+ 16 int flags;
+ 17 siginfo_t info;
+ 18 struct user_struct *user;
+ 19 };
+
+And, you might remember, it was a ``memcpy()`` on ``&first->info`` that
+caused the warning, so this makes perfect sense. It also seems reasonable
+to assume that it is the caller of ``__sigqueue_alloc()`` that has the
+responsibility of filling out (initializing) this member.
+
+But just which fields of the struct were uninitialized? Let's look at
+kmemcheck's report again::
+
+ WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
+ 80000000000000000000000000000000000000000088ffff0000000000000000
+ i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
+ ^
+
+These first two lines are the memory dump of the memory object itself, and
+the shadow bytemap, respectively. The memory object itself is in this case
+``&first->info``. Just beware that the start of this dump is NOT the start
+of the object itself! The position of the caret (^) corresponds with the
+address of the read (ffff88003e4a2024).
+
+The shadow bytemap dump legend is as follows:
+
+- i: initialized
+- u: uninitialized
+- a: unallocated (memory has been allocated by the slab layer, but has not
+ yet been handed off to anybody)
+- f: freed (memory has been allocated by the slab layer, but has been freed
+ by the previous owner)
+
+In order to figure out where (relative to the start of the object) the
+uninitialized memory was located, we have to look at the disassembly. For
+that, we'll need the RIP address again::
+
+ RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
+
+ $ objdump -d --no-show-raw-insn vmlinux | grep -C 8 ffffffff8104ede8:
+ ffffffff8104edc8: mov %r8,0x8(%r8)
+ ffffffff8104edcc: test %r10d,%r10d
+ ffffffff8104edcf: js ffffffff8104ee88 <__dequeue_signal+0x168>
+ ffffffff8104edd5: mov %rax,%rdx
+ ffffffff8104edd8: mov $0xc,%ecx
+ ffffffff8104eddd: mov %r13,%rdi
+ ffffffff8104ede0: mov $0x30,%eax
+ ffffffff8104ede5: mov %rdx,%rsi
+ ffffffff8104ede8: rep movsl %ds:(%rsi),%es:(%rdi)
+ ffffffff8104edea: test $0x2,%al
+ ffffffff8104edec: je ffffffff8104edf0 <__dequeue_signal+0xd0>
+ ffffffff8104edee: movsw %ds:(%rsi),%es:(%rdi)
+ ffffffff8104edf0: test $0x1,%al
+ ffffffff8104edf2: je ffffffff8104edf5 <__dequeue_signal+0xd5>
+ ffffffff8104edf4: movsb %ds:(%rsi),%es:(%rdi)
+ ffffffff8104edf5: mov %r8,%rdi
+ ffffffff8104edf8: callq ffffffff8104de60 <__sigqueue_free>
+
+As expected, it's the "``rep movsl``" instruction from the ``memcpy()``
+that causes the warning. We know about ``REP MOVSL`` that it uses the register
+``RCX`` to count the number of remaining iterations. By taking a look at the
+register dump again (from the kmemcheck report), we can figure out how many
+bytes were left to copy::
+
+ RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
+
+By looking at the disassembly, we also see that ``%ecx`` is being loaded
+with the value ``$0xc`` just before (ffffffff8104edd8), so we are very
+lucky. Keep in mind that this is the number of iterations, not bytes. And
+since this is a "long" operation, we need to multiply by 4 to get the
+number of bytes. So this means that the uninitialized value was encountered
+at 4 * (0xc - 0x9) = 12 bytes from the start of the object.
+
+We can now try to figure out which field of the "``struct siginfo``" that
+was not initialized. This is the beginning of the struct::
+
+ 40 typedef struct siginfo {
+ 41 int si_signo;
+ 42 int si_errno;
+ 43 int si_code;
+ 44
+ 45 union {
+ ..
+ 92 } _sifields;
+ 93 } siginfo_t;
+
+On 64-bit, the int is 4 bytes long, so it must the union member that has
+not been initialized. We can verify this using gdb::
+
+ $ gdb vmlinux
+ ...
+ (gdb) p &((struct siginfo *) 0)->_sifields
+ $1 = (union {...} *) 0x10
+
+Actually, it seems that the union member is located at offset 0x10 -- which
+means that gcc has inserted 4 bytes of padding between the members ``si_code``
+and ``_sifields``. We can now get a fuller picture of the memory dump::
+
+ _----------------------------=> si_code
+ / _--------------------=> (padding)
+ | / _------------=> _sifields(._kill._pid)
+ | | / _----=> _sifields(._kill._uid)
+ | | | /
+ -------|-------|-------|-------|
+ 80000000000000000000000000000000000000000088ffff0000000000000000
+ i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
+
+This allows us to realize another important fact: ``si_code`` contains the
+value 0x80. Remember that x86 is little endian, so the first 4 bytes
+"80000000" are really the number 0x00000080. With a bit of research, we
+find that this is actually the constant ``SI_KERNEL`` defined in
+``include/asm-generic/siginfo.h``::
+
+ 144 #define SI_KERNEL 0x80 /* sent by the kernel from somewhere */
+
+This macro is used in exactly one place in the x86 kernel: In ``send_signal()``
+in ``kernel/signal.c``::
+
+ 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
+ 817 int group)
+ 818 {
+ ...
+ 828 pending = group ? &t->signal->shared_pending : &t->pending;
+ ...
+ 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
+ 852 (is_si_special(info) ||
+ 853 info->si_code >= 0)));
+ 854 if (q) {
+ 855 list_add_tail(&q->list, &pending->list);
+ 856 switch ((unsigned long) info) {
+ ...
+ 865 case (unsigned long) SEND_SIG_PRIV:
+ 866 q->info.si_signo = sig;
+ 867 q->info.si_errno = 0;
+ 868 q->info.si_code = SI_KERNEL;
+ 869 q->info.si_pid = 0;
+ 870 q->info.si_uid = 0;
+ 871 break;
+ ...
+ 890 }
+
+Not only does this match with the ``.si_code`` member, it also matches the place
+we found earlier when looking for where siginfo_t objects are enqueued on the
+``shared_pending`` list.
+
+So to sum up: It seems that it is the padding introduced by the compiler
+between two struct fields that is uninitialized, and this gets reported when
+we do a ``memcpy()`` on the struct. This means that we have identified a false
+positive warning.
+
+Normally, kmemcheck will not report uninitialized accesses in ``memcpy()`` calls
+when both the source and destination addresses are tracked. (Instead, we copy
+the shadow bytemap as well). In this case, the destination address clearly
+was not tracked. We can dig a little deeper into the stack trace from above::
+
+ arch/x86/kernel/signal.c:805
+ arch/x86/kernel/signal.c:871
+ arch/x86/kernel/entry_64.S:694
+
+And we clearly see that the destination siginfo object is located on the
+stack::
+
+ 782 static void do_signal(struct pt_regs *regs)
+ 783 {
+ 784 struct k_sigaction ka;
+ 785 siginfo_t info;
+ ...
+ 804 signr = get_signal_to_deliver(&info, &ka, regs, NULL);
+ ...
+ 854 }
+
+And this ``&info`` is what eventually gets passed to ``copy_siginfo()`` as the
+destination argument.
+
+Now, even though we didn't find an actual error here, the example is still a
+good one, because it shows how one would go about to find out what the report
+was all about.
+
+
+Annotating false positives
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are a few different ways to make annotations in the source code that
+will keep kmemcheck from checking and reporting certain allocations. Here
+they are:
+
+- ``__GFP_NOTRACK_FALSE_POSITIVE``
+ This flag can be passed to ``kmalloc()`` or ``kmem_cache_alloc()``
+ (therefore also to other functions that end up calling one of
+ these) to indicate that the allocation should not be tracked
+ because it would lead to a false positive report. This is a "big
+ hammer" way of silencing kmemcheck; after all, even if the false
+ positive pertains to particular field in a struct, for example, we
+ will now lose the ability to find (real) errors in other parts of
+ the same struct.
+
+ Example::
+
+ /* No warnings will ever trigger on accessing any part of x */
+ x = kmalloc(sizeof *x, GFP_KERNEL | __GFP_NOTRACK_FALSE_POSITIVE);
+
+- ``kmemcheck_bitfield_begin(name)``/``kmemcheck_bitfield_end(name)`` and
+ ``kmemcheck_annotate_bitfield(ptr, name)``
+ The first two of these three macros can be used inside struct
+ definitions to signal, respectively, the beginning and end of a
+ bitfield. Additionally, this will assign the bitfield a name, which
+ is given as an argument to the macros.
+
+ Having used these markers, one can later use
+ kmemcheck_annotate_bitfield() at the point of allocation, to indicate
+ which parts of the allocation is part of a bitfield.
+
+ Example::
+
+ struct foo {
+ int x;
+
+ kmemcheck_bitfield_begin(flags);
+ int flag_a:1;
+ int flag_b:1;
+ kmemcheck_bitfield_end(flags);
+
+ int y;
+ };
+
+ struct foo *x = kmalloc(sizeof *x);
+
+ /* No warnings will trigger on accessing the bitfield of x */
+ kmemcheck_annotate_bitfield(x, flags);
+
+ Note that ``kmemcheck_annotate_bitfield()`` can be used even before the
+ return value of ``kmalloc()`` is checked -- in other words, passing NULL
+ as the first argument is legal (and will do nothing).
+
+
+Reporting errors
+----------------
+
+As we have seen, kmemcheck will produce false positive reports. Therefore, it
+is not very wise to blindly post kmemcheck warnings to mailing lists and
+maintainers. Instead, I encourage maintainers and developers to find errors
+in their own code. If you get a warning, you can try to work around it, try
+to figure out if it's a real error or not, or simply ignore it. Most
+developers know their own code and will quickly and efficiently determine the
+root cause of a kmemcheck report. This is therefore also the most efficient
+way to work with kmemcheck.
+
+That said, we (the kmemcheck maintainers) will always be on the lookout for
+false positives that we can annotate and silence. So whatever you find,
+please drop us a note privately! Kernel configs and steps to reproduce (if
+available) are of course a great help too.
+
+Happy hacking!
+
+
+Technical description
+---------------------
+
+kmemcheck works by marking memory pages non-present. This means that whenever
+somebody attempts to access the page, a page fault is generated. The page
+fault handler notices that the page was in fact only hidden, and so it calls
+on the kmemcheck code to make further investigations.
+
+When the investigations are completed, kmemcheck "shows" the page by marking
+it present (as it would be under normal circumstances). This way, the
+interrupted code can continue as usual.
+
+But after the instruction has been executed, we should hide the page again, so
+that we can catch the next access too! Now kmemcheck makes use of a debugging
+feature of the processor, namely single-stepping. When the processor has
+finished the one instruction that generated the memory access, a debug
+exception is raised. From here, we simply hide the page again and continue
+execution, this time with the single-stepping feature turned off.
+
+kmemcheck requires some assistance from the memory allocator in order to work.
+The memory allocator needs to
+
+ 1. Tell kmemcheck about newly allocated pages and pages that are about to
+ be freed. This allows kmemcheck to set up and tear down the shadow memory
+ for the pages in question. The shadow memory stores the status of each
+ byte in the allocation proper, e.g. whether it is initialized or
+ uninitialized.
+
+ 2. Tell kmemcheck which parts of memory should be marked uninitialized.
+ There are actually a few more states, such as "not yet allocated" and
+ "recently freed".
+
+If a slab cache is set up using the SLAB_NOTRACK flag, it will never return
+memory that can take page faults because of kmemcheck.
+
+If a slab cache is NOT set up using the SLAB_NOTRACK flag, callers can still
+request memory with the __GFP_NOTRACK or __GFP_NOTRACK_FALSE_POSITIVE flags.
+This does not prevent the page faults from occurring, however, but marks the
+object in question as being initialized so that no warnings will ever be
+produced for this object.
+
+Currently, the SLAB and SLUB allocators are supported by kmemcheck.
kasan
ubsan
kmemleak
+ kmemcheck
+++ /dev/null
-GETTING STARTED WITH KMEMCHECK
-==============================
-
-Vegard Nossum <vegardno@ifi.uio.no>
-
-
-Contents
-========
-0. Introduction
-1. Downloading
-2. Configuring and compiling
-3. How to use
-3.1. Booting
-3.2. Run-time enable/disable
-3.3. Debugging
-3.4. Annotating false positives
-4. Reporting errors
-5. Technical description
-
-
-0. Introduction
-===============
-
-kmemcheck is a debugging feature for the Linux Kernel. More specifically, it
-is a dynamic checker that detects and warns about some uses of uninitialized
-memory.
-
-Userspace programmers might be familiar with Valgrind's memcheck. The main
-difference between memcheck and kmemcheck is that memcheck works for userspace
-programs only, and kmemcheck works for the kernel only. The implementations
-are of course vastly different. Because of this, kmemcheck is not as accurate
-as memcheck, but it turns out to be good enough in practice to discover real
-programmer errors that the compiler is not able to find through static
-analysis.
-
-Enabling kmemcheck on a kernel will probably slow it down to the extent that
-the machine will not be usable for normal workloads such as e.g. an
-interactive desktop. kmemcheck will also cause the kernel to use about twice
-as much memory as normal. For this reason, kmemcheck is strictly a debugging
-feature.
-
-
-1. Downloading
-==============
-
-As of version 2.6.31-rc1, kmemcheck is included in the mainline kernel.
-
-
-2. Configuring and compiling
-============================
-
-kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of
-configuration variables must have specific settings in order for the kmemcheck
-menu to even appear in "menuconfig". These are:
-
- o CONFIG_CC_OPTIMIZE_FOR_SIZE=n
-
- This option is located under "General setup" / "Optimize for size".
-
- Without this, gcc will use certain optimizations that usually lead to
- false positive warnings from kmemcheck. An example of this is a 16-bit
- field in a struct, where gcc may load 32 bits, then discard the upper
- 16 bits. kmemcheck sees only the 32-bit load, and may trigger a
- warning for the upper 16 bits (if they're uninitialized).
-
- o CONFIG_SLAB=y or CONFIG_SLUB=y
-
- This option is located under "General setup" / "Choose SLAB
- allocator".
-
- o CONFIG_FUNCTION_TRACER=n
-
- This option is located under "Kernel hacking" / "Tracers" / "Kernel
- Function Tracer"
-
- When function tracing is compiled in, gcc emits a call to another
- function at the beginning of every function. This means that when the
- page fault handler is called, the ftrace framework will be called
- before kmemcheck has had a chance to handle the fault. If ftrace then
- modifies memory that was tracked by kmemcheck, the result is an
- endless recursive page fault.
-
- o CONFIG_DEBUG_PAGEALLOC=n
-
- This option is located under "Kernel hacking" / "Memory Debugging"
- / "Debug page memory allocations".
-
-In addition, I highly recommend turning on CONFIG_DEBUG_INFO=y. This is also
-located under "Kernel hacking". With this, you will be able to get line number
-information from the kmemcheck warnings, which is extremely valuable in
-debugging a problem. This option is not mandatory, however, because it slows
-down the compilation process and produces a much bigger kernel image.
-
-Now the kmemcheck menu should be visible (under "Kernel hacking" / "Memory
-Debugging" / "kmemcheck: trap use of uninitialized memory"). Here follows
-a description of the kmemcheck configuration variables:
-
- o CONFIG_KMEMCHECK
-
- This must be enabled in order to use kmemcheck at all...
-
- o CONFIG_KMEMCHECK_[DISABLED | ENABLED | ONESHOT]_BY_DEFAULT
-
- This option controls the status of kmemcheck at boot-time. "Enabled"
- will enable kmemcheck right from the start, "disabled" will boot the
- kernel as normal (but with the kmemcheck code compiled in, so it can
- be enabled at run-time after the kernel has booted), and "one-shot" is
- a special mode which will turn kmemcheck off automatically after
- detecting the first use of uninitialized memory.
-
- If you are using kmemcheck to actively debug a problem, then you
- probably want to choose "enabled" here.
-
- The one-shot mode is mostly useful in automated test setups because it
- can prevent floods of warnings and increase the chances of the machine
- surviving in case something is really wrong. In other cases, the one-
- shot mode could actually be counter-productive because it would turn
- itself off at the very first error -- in the case of a false positive
- too -- and this would come in the way of debugging the specific
- problem you were interested in.
-
- If you would like to use your kernel as normal, but with a chance to
- enable kmemcheck in case of some problem, it might be a good idea to
- choose "disabled" here. When kmemcheck is disabled, most of the run-
- time overhead is not incurred, and the kernel will be almost as fast
- as normal.
-
- o CONFIG_KMEMCHECK_QUEUE_SIZE
-
- Select the maximum number of error reports to store in an internal
- (fixed-size) buffer. Since errors can occur virtually anywhere and in
- any context, we need a temporary storage area which is guaranteed not
- to generate any other page faults when accessed. The queue will be
- emptied as soon as a tasklet may be scheduled. If the queue is full,
- new error reports will be lost.
-
- The default value of 64 is probably fine. If some code produces more
- than 64 errors within an irqs-off section, then the code is likely to
- produce many, many more, too, and these additional reports seldom give
- any more information (the first report is usually the most valuable
- anyway).
-
- This number might have to be adjusted if you are not using serial
- console or similar to capture the kernel log. If you are using the
- "dmesg" command to save the log, then getting a lot of kmemcheck
- warnings might overflow the kernel log itself, and the earlier reports
- will get lost in that way instead. Try setting this to 10 or so on
- such a setup.
-
- o CONFIG_KMEMCHECK_SHADOW_COPY_SHIFT
-
- Select the number of shadow bytes to save along with each entry of the
- error-report queue. These bytes indicate what parts of an allocation
- are initialized, uninitialized, etc. and will be displayed when an
- error is detected to help the debugging of a particular problem.
-
- The number entered here is actually the logarithm of the number of
- bytes that will be saved. So if you pick for example 5 here, kmemcheck
- will save 2^5 = 32 bytes.
-
- The default value should be fine for debugging most problems. It also
- fits nicely within 80 columns.
-
- o CONFIG_KMEMCHECK_PARTIAL_OK
-
- This option (when enabled) works around certain GCC optimizations that
- produce 32-bit reads from 16-bit variables where the upper 16 bits are
- thrown away afterwards.
-
- The default value (enabled) is recommended. This may of course hide
- some real errors, but disabling it would probably produce a lot of
- false positives.
-
- o CONFIG_KMEMCHECK_BITOPS_OK
-
- This option silences warnings that would be generated for bit-field
- accesses where not all the bits are initialized at the same time. This
- may also hide some real bugs.
-
- This option is probably obsolete, or it should be replaced with
- the kmemcheck-/bitfield-annotations for the code in question. The
- default value is therefore fine.
-
-Now compile the kernel as usual.
-
-
-3. How to use
-=============
-
-3.1. Booting
-============
-
-First some information about the command-line options. There is only one
-option specific to kmemcheck, and this is called "kmemcheck". It can be used
-to override the default mode as chosen by the CONFIG_KMEMCHECK_*_BY_DEFAULT
-option. Its possible settings are:
-
- o kmemcheck=0 (disabled)
- o kmemcheck=1 (enabled)
- o kmemcheck=2 (one-shot mode)
-
-If SLUB debugging has been enabled in the kernel, it may take precedence over
-kmemcheck in such a way that the slab caches which are under SLUB debugging
-will not be tracked by kmemcheck. In order to ensure that this doesn't happen
-(even though it shouldn't by default), use SLUB's boot option "slub_debug",
-like this: slub_debug=-
-
-In fact, this option may also be used for fine-grained control over SLUB vs.
-kmemcheck. For example, if the command line includes "kmemcheck=1
-slub_debug=,dentry", then SLUB debugging will be used only for the "dentry"
-slab cache, and with kmemcheck tracking all the other caches. This is advanced
-usage, however, and is not generally recommended.
-
-
-3.2. Run-time enable/disable
-============================
-
-When the kernel has booted, it is possible to enable or disable kmemcheck at
-run-time. WARNING: This feature is still experimental and may cause false
-positive warnings to appear. Therefore, try not to use this. If you find that
-it doesn't work properly (e.g. you see an unreasonable amount of warnings), I
-will be happy to take bug reports.
-
-Use the file /proc/sys/kernel/kmemcheck for this purpose, e.g.:
-
- $ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck
-
-The numbers are the same as for the kmemcheck= command-line option.
-
-
-3.3. Debugging
-==============
-
-A typical report will look something like this:
-
-WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
-80000000000000000000000000000000000000000088ffff0000000000000000
- i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
- ^
-
-Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A
-RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
-RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002
-RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
-RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84
-RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000
-R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e
-R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8
-FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000
-CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
-CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0
-DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
-DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
- [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
- [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
- [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
- [<ffffffff8100c7b5>] int_signal+0x12/0x17
- [<ffffffffffffffff>] 0xffffffffffffffff
-
-The single most valuable information in this report is the RIP (or EIP on 32-
-bit) value. This will help us pinpoint exactly which instruction that caused
-the warning.
-
-If your kernel was compiled with CONFIG_DEBUG_INFO=y, then all we have to do
-is give this address to the addr2line program, like this:
-
- $ addr2line -e vmlinux -i ffffffff8104ede8
- arch/x86/include/asm/string_64.h:12
- include/asm-generic/siginfo.h:287
- kernel/signal.c:380
- kernel/signal.c:410
-
-The "-e vmlinux" tells addr2line which file to look in. IMPORTANT: This must
-be the vmlinux of the kernel that produced the warning in the first place! If
-not, the line number information will almost certainly be wrong.
-
-The "-i" tells addr2line to also print the line numbers of inlined functions.
-In this case, the flag was very important, because otherwise, it would only
-have printed the first line, which is just a call to memcpy(), which could be
-called from a thousand places in the kernel, and is therefore not very useful.
-These inlined functions would not show up in the stack trace above, simply
-because the kernel doesn't load the extra debugging information. This
-technique can of course be used with ordinary kernel oopses as well.
-
-In this case, it's the caller of memcpy() that is interesting, and it can be
-found in include/asm-generic/siginfo.h, line 287:
-
-281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from)
-282 {
-283 if (from->si_code < 0)
-284 memcpy(to, from, sizeof(*to));
-285 else
-286 /* _sigchld is currently the largest know union member */
-287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld));
-288 }
-
-Since this was a read (kmemcheck usually warns about reads only, though it can
-warn about writes to unallocated or freed memory as well), it was probably the
-"from" argument which contained some uninitialized bytes. Following the chain
-of calls, we move upwards to see where "from" was allocated or initialized,
-kernel/signal.c, line 380:
-
-359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info)
-360 {
-...
-367 list_for_each_entry(q, &list->list, list) {
-368 if (q->info.si_signo == sig) {
-369 if (first)
-370 goto still_pending;
-371 first = q;
-...
-377 if (first) {
-378 still_pending:
-379 list_del_init(&first->list);
-380 copy_siginfo(info, &first->info);
-381 __sigqueue_free(first);
-...
-392 }
-393 }
-
-Here, it is &first->info that is being passed on to copy_siginfo(). The
-variable "first" was found on a list -- passed in as the second argument to
-collect_signal(). We continue our journey through the stack, to figure out
-where the item on "list" was allocated or initialized. We move to line 410:
-
-395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
-396 siginfo_t *info)
-397 {
-...
-410 collect_signal(sig, pending, info);
-...
-414 }
-
-Now we need to follow the "pending" pointer, since that is being passed on to
-collect_signal() as "list". At this point, we've run out of lines from the
-"addr2line" output. Not to worry, we just paste the next addresses from the
-kmemcheck stack dump, i.e.:
-
- [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
- [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
- [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
- [<ffffffff8100c7b5>] int_signal+0x12/0x17
-
- $ addr2line -e vmlinux -i ffffffff8104f04e ffffffff81050bd8 \
- ffffffff8100b87d ffffffff8100c7b5
- kernel/signal.c:446
- kernel/signal.c:1806
- arch/x86/kernel/signal.c:805
- arch/x86/kernel/signal.c:871
- arch/x86/kernel/entry_64.S:694
-
-Remember that since these addresses were found on the stack and not as the
-RIP value, they actually point to the _next_ instruction (they are return
-addresses). This becomes obvious when we look at the code for line 446:
-
-422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
-423 {
-...
-431 signr = __dequeue_signal(&tsk->signal->shared_pending,
-432 mask, info);
-433 /*
-434 * itimer signal ?
-435 *
-436 * itimers are process shared and we restart periodic
-437 * itimers in the signal delivery path to prevent DoS
-438 * attacks in the high resolution timer case. This is
-439 * compliant with the old way of self restarting
-440 * itimers, as the SIGALRM is a legacy signal and only
-441 * queued once. Changing the restart behaviour to
-442 * restart the timer in the signal dequeue path is
-443 * reducing the timer noise on heavy loaded !highres
-444 * systems too.
-445 */
-446 if (unlikely(signr == SIGALRM)) {
-...
-489 }
-
-So instead of looking at 446, we should be looking at 431, which is the line
-that executes just before 446. Here we see that what we are looking for is
-&tsk->signal->shared_pending.
-
-Our next task is now to figure out which function that puts items on this
-"shared_pending" list. A crude, but efficient tool, is git grep:
-
- $ git grep -n 'shared_pending' kernel/
- ...
- kernel/signal.c:828: pending = group ? &t->signal->shared_pending : &t->pending;
- kernel/signal.c:1339: pending = group ? &t->signal->shared_pending : &t->pending;
- ...
-
-There were more results, but none of them were related to list operations,
-and these were the only assignments. We inspect the line numbers more closely
-and find that this is indeed where items are being added to the list:
-
-816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
-817 int group)
-818 {
-...
-828 pending = group ? &t->signal->shared_pending : &t->pending;
-...
-851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
-852 (is_si_special(info) ||
-853 info->si_code >= 0)));
-854 if (q) {
-855 list_add_tail(&q->list, &pending->list);
-...
-890 }
-
-and:
-
-1309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
-1310 {
-....
-1339 pending = group ? &t->signal->shared_pending : &t->pending;
-1340 list_add_tail(&q->list, &pending->list);
-....
-1347 }
-
-In the first case, the list element we are looking for, "q", is being returned
-from the function __sigqueue_alloc(), which looks like an allocation function.
-Let's take a look at it:
-
-187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
-188 int override_rlimit)
-189 {
-190 struct sigqueue *q = NULL;
-191 struct user_struct *user;
-192
-193 /*
-194 * We won't get problems with the target's UID changing under us
-195 * because changing it requires RCU be used, and if t != current, the
-196 * caller must be holding the RCU readlock (by way of a spinlock) and
-197 * we use RCU protection here
-198 */
-199 user = get_uid(__task_cred(t)->user);
-200 atomic_inc(&user->sigpending);
-201 if (override_rlimit ||
-202 atomic_read(&user->sigpending) <=
-203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
-204 q = kmem_cache_alloc(sigqueue_cachep, flags);
-205 if (unlikely(q == NULL)) {
-206 atomic_dec(&user->sigpending);
-207 free_uid(user);
-208 } else {
-209 INIT_LIST_HEAD(&q->list);
-210 q->flags = 0;
-211 q->user = user;
-212 }
-213
-214 return q;
-215 }
-
-We see that this function initializes q->list, q->flags, and q->user. It seems
-that now is the time to look at the definition of "struct sigqueue", e.g.:
-
-14 struct sigqueue {
-15 struct list_head list;
-16 int flags;
-17 siginfo_t info;
-18 struct user_struct *user;
-19 };
-
-And, you might remember, it was a memcpy() on &first->info that caused the
-warning, so this makes perfect sense. It also seems reasonable to assume that
-it is the caller of __sigqueue_alloc() that has the responsibility of filling
-out (initializing) this member.
-
-But just which fields of the struct were uninitialized? Let's look at
-kmemcheck's report again:
-
-WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
-80000000000000000000000000000000000000000088ffff0000000000000000
- i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
- ^
-
-These first two lines are the memory dump of the memory object itself, and the
-shadow bytemap, respectively. The memory object itself is in this case
-&first->info. Just beware that the start of this dump is NOT the start of the
-object itself! The position of the caret (^) corresponds with the address of
-the read (ffff88003e4a2024).
-
-The shadow bytemap dump legend is as follows:
-
- i - initialized
- u - uninitialized
- a - unallocated (memory has been allocated by the slab layer, but has not
- yet been handed off to anybody)
- f - freed (memory has been allocated by the slab layer, but has been freed
- by the previous owner)
-
-In order to figure out where (relative to the start of the object) the
-uninitialized memory was located, we have to look at the disassembly. For
-that, we'll need the RIP address again:
-
-RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
-
- $ objdump -d --no-show-raw-insn vmlinux | grep -C 8 ffffffff8104ede8:
- ffffffff8104edc8: mov %r8,0x8(%r8)
- ffffffff8104edcc: test %r10d,%r10d
- ffffffff8104edcf: js ffffffff8104ee88 <__dequeue_signal+0x168>
- ffffffff8104edd5: mov %rax,%rdx
- ffffffff8104edd8: mov $0xc,%ecx
- ffffffff8104eddd: mov %r13,%rdi
- ffffffff8104ede0: mov $0x30,%eax
- ffffffff8104ede5: mov %rdx,%rsi
- ffffffff8104ede8: rep movsl %ds:(%rsi),%es:(%rdi)
- ffffffff8104edea: test $0x2,%al
- ffffffff8104edec: je ffffffff8104edf0 <__dequeue_signal+0xd0>
- ffffffff8104edee: movsw %ds:(%rsi),%es:(%rdi)
- ffffffff8104edf0: test $0x1,%al
- ffffffff8104edf2: je ffffffff8104edf5 <__dequeue_signal+0xd5>
- ffffffff8104edf4: movsb %ds:(%rsi),%es:(%rdi)
- ffffffff8104edf5: mov %r8,%rdi
- ffffffff8104edf8: callq ffffffff8104de60 <__sigqueue_free>
-
-As expected, it's the "rep movsl" instruction from the memcpy() that causes
-the warning. We know about REP MOVSL that it uses the register RCX to count
-the number of remaining iterations. By taking a look at the register dump
-again (from the kmemcheck report), we can figure out how many bytes were left
-to copy:
-
-RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
-
-By looking at the disassembly, we also see that %ecx is being loaded with the
-value $0xc just before (ffffffff8104edd8), so we are very lucky. Keep in mind
-that this is the number of iterations, not bytes. And since this is a "long"
-operation, we need to multiply by 4 to get the number of bytes. So this means
-that the uninitialized value was encountered at 4 * (0xc - 0x9) = 12 bytes
-from the start of the object.
-
-We can now try to figure out which field of the "struct siginfo" that was not
-initialized. This is the beginning of the struct:
-
-40 typedef struct siginfo {
-41 int si_signo;
-42 int si_errno;
-43 int si_code;
-44
-45 union {
-..
-92 } _sifields;
-93 } siginfo_t;
-
-On 64-bit, the int is 4 bytes long, so it must the union member that has
-not been initialized. We can verify this using gdb:
-
- $ gdb vmlinux
- ...
- (gdb) p &((struct siginfo *) 0)->_sifields
- $1 = (union {...} *) 0x10
-
-Actually, it seems that the union member is located at offset 0x10 -- which
-means that gcc has inserted 4 bytes of padding between the members si_code
-and _sifields. We can now get a fuller picture of the memory dump:
-
- _----------------------------=> si_code
- / _--------------------=> (padding)
- | / _------------=> _sifields(._kill._pid)
- | | / _----=> _sifields(._kill._uid)
- | | | /
--------|-------|-------|-------|
-80000000000000000000000000000000000000000088ffff0000000000000000
- i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
-
-This allows us to realize another important fact: si_code contains the value
-0x80. Remember that x86 is little endian, so the first 4 bytes "80000000" are
-really the number 0x00000080. With a bit of research, we find that this is
-actually the constant SI_KERNEL defined in include/asm-generic/siginfo.h:
-
-144 #define SI_KERNEL 0x80 /* sent by the kernel from somewhere */
-
-This macro is used in exactly one place in the x86 kernel: In send_signal()
-in kernel/signal.c:
-
-816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
-817 int group)
-818 {
-...
-828 pending = group ? &t->signal->shared_pending : &t->pending;
-...
-851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
-852 (is_si_special(info) ||
-853 info->si_code >= 0)));
-854 if (q) {
-855 list_add_tail(&q->list, &pending->list);
-856 switch ((unsigned long) info) {
-...
-865 case (unsigned long) SEND_SIG_PRIV:
-866 q->info.si_signo = sig;
-867 q->info.si_errno = 0;
-868 q->info.si_code = SI_KERNEL;
-869 q->info.si_pid = 0;
-870 q->info.si_uid = 0;
-871 break;
-...
-890 }
-
-Not only does this match with the .si_code member, it also matches the place
-we found earlier when looking for where siginfo_t objects are enqueued on the
-"shared_pending" list.
-
-So to sum up: It seems that it is the padding introduced by the compiler
-between two struct fields that is uninitialized, and this gets reported when
-we do a memcpy() on the struct. This means that we have identified a false
-positive warning.
-
-Normally, kmemcheck will not report uninitialized accesses in memcpy() calls
-when both the source and destination addresses are tracked. (Instead, we copy
-the shadow bytemap as well). In this case, the destination address clearly
-was not tracked. We can dig a little deeper into the stack trace from above:
-
- arch/x86/kernel/signal.c:805
- arch/x86/kernel/signal.c:871
- arch/x86/kernel/entry_64.S:694
-
-And we clearly see that the destination siginfo object is located on the
-stack:
-
-782 static void do_signal(struct pt_regs *regs)
-783 {
-784 struct k_sigaction ka;
-785 siginfo_t info;
-...
-804 signr = get_signal_to_deliver(&info, &ka, regs, NULL);
-...
-854 }
-
-And this &info is what eventually gets passed to copy_siginfo() as the
-destination argument.
-
-Now, even though we didn't find an actual error here, the example is still a
-good one, because it shows how one would go about to find out what the report
-was all about.
-
-
-3.4. Annotating false positives
-===============================
-
-There are a few different ways to make annotations in the source code that
-will keep kmemcheck from checking and reporting certain allocations. Here
-they are:
-
- o __GFP_NOTRACK_FALSE_POSITIVE
-
- This flag can be passed to kmalloc() or kmem_cache_alloc() (therefore
- also to other functions that end up calling one of these) to indicate
- that the allocation should not be tracked because it would lead to
- a false positive report. This is a "big hammer" way of silencing
- kmemcheck; after all, even if the false positive pertains to
- particular field in a struct, for example, we will now lose the
- ability to find (real) errors in other parts of the same struct.
-
- Example:
-
- /* No warnings will ever trigger on accessing any part of x */
- x = kmalloc(sizeof *x, GFP_KERNEL | __GFP_NOTRACK_FALSE_POSITIVE);
-
- o kmemcheck_bitfield_begin(name)/kmemcheck_bitfield_end(name) and
- kmemcheck_annotate_bitfield(ptr, name)
-
- The first two of these three macros can be used inside struct
- definitions to signal, respectively, the beginning and end of a
- bitfield. Additionally, this will assign the bitfield a name, which
- is given as an argument to the macros.
-
- Having used these markers, one can later use
- kmemcheck_annotate_bitfield() at the point of allocation, to indicate
- which parts of the allocation is part of a bitfield.
-
- Example:
-
- struct foo {
- int x;
-
- kmemcheck_bitfield_begin(flags);
- int flag_a:1;
- int flag_b:1;
- kmemcheck_bitfield_end(flags);
-
- int y;
- };
-
- struct foo *x = kmalloc(sizeof *x);
-
- /* No warnings will trigger on accessing the bitfield of x */
- kmemcheck_annotate_bitfield(x, flags);
-
- Note that kmemcheck_annotate_bitfield() can be used even before the
- return value of kmalloc() is checked -- in other words, passing NULL
- as the first argument is legal (and will do nothing).
-
-
-4. Reporting errors
-===================
-
-As we have seen, kmemcheck will produce false positive reports. Therefore, it
-is not very wise to blindly post kmemcheck warnings to mailing lists and
-maintainers. Instead, I encourage maintainers and developers to find errors
-in their own code. If you get a warning, you can try to work around it, try
-to figure out if it's a real error or not, or simply ignore it. Most
-developers know their own code and will quickly and efficiently determine the
-root cause of a kmemcheck report. This is therefore also the most efficient
-way to work with kmemcheck.
-
-That said, we (the kmemcheck maintainers) will always be on the lookout for
-false positives that we can annotate and silence. So whatever you find,
-please drop us a note privately! Kernel configs and steps to reproduce (if
-available) are of course a great help too.
-
-Happy hacking!
-
-
-5. Technical description
-========================
-
-kmemcheck works by marking memory pages non-present. This means that whenever
-somebody attempts to access the page, a page fault is generated. The page
-fault handler notices that the page was in fact only hidden, and so it calls
-on the kmemcheck code to make further investigations.
-
-When the investigations are completed, kmemcheck "shows" the page by marking
-it present (as it would be under normal circumstances). This way, the
-interrupted code can continue as usual.
-
-But after the instruction has been executed, we should hide the page again, so
-that we can catch the next access too! Now kmemcheck makes use of a debugging
-feature of the processor, namely single-stepping. When the processor has
-finished the one instruction that generated the memory access, a debug
-exception is raised. From here, we simply hide the page again and continue
-execution, this time with the single-stepping feature turned off.
-
-kmemcheck requires some assistance from the memory allocator in order to work.
-The memory allocator needs to
-
- 1. Tell kmemcheck about newly allocated pages and pages that are about to
- be freed. This allows kmemcheck to set up and tear down the shadow memory
- for the pages in question. The shadow memory stores the status of each
- byte in the allocation proper, e.g. whether it is initialized or
- uninitialized.
-
- 2. Tell kmemcheck which parts of memory should be marked uninitialized.
- There are actually a few more states, such as "not yet allocated" and
- "recently freed".
-
-If a slab cache is set up using the SLAB_NOTRACK flag, it will never return
-memory that can take page faults because of kmemcheck.
-
-If a slab cache is NOT set up using the SLAB_NOTRACK flag, callers can still
-request memory with the __GFP_NOTRACK or __GFP_NOTRACK_FALSE_POSITIVE flags.
-This does not prevent the page faults from occurring, however, but marks the
-object in question as being initialized so that no warnings will ever be
-produced for this object.
-
-Currently, the SLAB and SLUB allocators are supported by kmemcheck.
M: Vegard Nossum <vegardno@ifi.uio.no>
M: Pekka Enberg <penberg@kernel.org>
S: Maintained
-F: Documentation/kmemcheck.txt
+F: Documentation/dev-tools/kmemcheck.rst
F: arch/x86/include/asm/kmemcheck.h
F: arch/x86/mm/kmemcheck/
F: include/linux/kmemcheck.h
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