perf augmented_syscalls: Check probe_read_str() return separately

Using a value returned from probe_read_str() to tell how many bytes to
copy using perf_event_output() has issues in some older kernels, like
4.17.17-100.fc27.x86_64, so separate the bounds checking done on how
many bytes to copy to a separate variable, so that the next patch has
only what is being done to make the test pass on older BPF validators.

For reference, see the discussion in this thread:

  https://www.spinics.net/lists/netdev/msg480099.html

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-jtsapwibyxrnv1xjfsgzp0fj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/examples/bpf/augmented_syscalls.c b/tools/perf/examples/bpf/augmented_syscalls.c
index 1419a9186937264c57c29aea2f050b439768f683..0decbcfa8b9019dc58e4898d9b35fc24cec95017 100644 (file)
--- a/tools/perf/examples/bpf/augmented_syscalls.c
+++ b/tools/perf/examples/bpf/augmented_syscalls.c
@@ -33,9 +33,9 @@ struct syscall_exit_args {
 };
 
 struct augmented_filename {
-       int     size;
-       int     reserved;
-       char    value[256];
+       unsigned int    size;
+       int             reserved;
+       char            value[256];
 };
 
 #define augmented_filename_syscall(syscall)                                                    \
@@ -46,14 +46,15 @@ struct augmented_enter_##syscall##_args {                                                   \
 int syscall_enter(syscall)(struct syscall_enter_##syscall##_args *args)                                \
 {                                                                                              \
        struct augmented_enter_##syscall##_args augmented_args = { .filename.reserved = 0, };   \
+       unsigned int len = sizeof(augmented_args);                                              \
        probe_read(&augmented_args.args, sizeof(augmented_args.args), args);                    \
        augmented_args.filename.size = probe_read_str(&augmented_args.filename.value,           \
                                                      sizeof(augmented_args.filename.value),    \
                                                      args->filename_ptr);                      \
+       if (augmented_args.filename.size < sizeof(augmented_args.filename.value))               \
+               len -= sizeof(augmented_args.filename.value) - augmented_args.filename.size;    \
        perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU,                     \
-                         &augmented_args,                                                      \
-                         (sizeof(augmented_args) - sizeof(augmented_args.filename.value) +     \
-                          augmented_args.filename.size));                                      \
+                         &augmented_args, len);                                                \
        return 0;                                                                               \
 }                                                                                              \
 int syscall_exit(syscall)(struct syscall_exit_args *args)                                      \