perf machine: Adjust dso->long_name for offline module

perf machine: Adjust dso->long_name for offline module

Something unexpected may happen if copy statically linked perf to a
production environment:

  # ./perf probe -m ./mymodule.ko my_func
  [mymodule] with build id 326ab42550ef3d24944f53c817533728367effeb not found, continuing without symbols
  Failed to find symbol my_func in /home/wangnan/kmodule/mymodule.ko
    Error: Failed to add events.
  # ./perf buildid-cache -a ./mymodule.ko
  # ./perf probe -m ./mymodule.ko my_func
  Added new event:
    probe:my_func        (on my_func in /home/wangnan/kmodule/mymodule.ko)

  You can now use it in all perf tools, such as:

  	perf record -e probe:my_func -aR sleep 1

Where:

  # ldd ./perf
 	not a dynamic executable
  # strace -e open ./perf probe -m ./mymodule.ko my_func
  ...
  open("/home/wangnan/kmodule/mymodule.ko", O_RDONLY) = 3
  open("/home/wangnan/kmodule/../lib64/elfutils/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
  ...
  open("/lib64/tls/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
  open("/lib64/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
  open("/usr/lib64/tls/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
  open("/usr/lib64/libebl_x86_64.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
  open("[mymodule]", O_RDONLY)            = -1 ENOENT (No such file or directory)
  open("/home/wangnan/.debug/.build-id/32/6ab42550ef3d24944f53c817533728367effeb", O_RDONLY) = -1 ENOENT (No such file or directory)
  open("[mymodule]", O_RDONLY)            = -1 ENOENT (No such file or directory)

In the above example, probe fails before we put the module into
buildid-cache. However, user would expect it success in both case
because perf is able to find probe points actually.

The reason is because perf won't utilize module's full path if it failed
to open debuginfo. In:

     convert_to_probe_trace_events ->
        find_probe_trace_events_from_map ->
            get_target_map ->
                kernel_get_module_map ->
                    machine__findnew_module_map ->
                        map_groups__find_by_name

map_groups__find_by_name() is able to find the map of that module, but
this information is found from /proc/module before it knows the real
path of the offline module. Therefore, the map->dso->long_name is set to
something like '[mymodule]', which prevent dso__load() find the real
path of the module file.

In another aspect, if dso__load() can get the offline module through
buildid cache, it can read symble table from that ko. Even if debuginfo
is not available, 'perf probe' can success if the '.symtab' can be
found.

This patch improves machine__findnew_module_map(): when dso->long_name
is leading with '[' (doesn't find path of module when parsing
/proc/modules), fixes it by dso__set_long_name(), so following
dso__load() is possible to find the symbol table.

This patch won't interfere with buildid matching. Here is the test
result:

  # ./perf probe -m ./mymodule.ko my_func
  Added new event:
    probe:my_func        (on my_func in /home/wangnan/kmodule/mymodule.ko)

  You can now use it in all perf tools, such as:

  	perf record -e probe:my_func -aR sleep 1

  # ./perf probe -d '*'
  Removed event: probe:my_func
  # mv ./mymodule.{ko,.bak}
  # mv ./moduleb.ko mymodule.ko
  # ./perf probe -m ./mymodule.ko my_func
  /home/wangnan/kmodule/mymodule.ko with build id 326ab42550ef3d24944f53c817533728367effeb not found, continuing without symbols
  Failed to find symbol my_func in /home/wangnan/kmodule/mymodule.ko
    Error: Failed to add events.

  # ./perf probe -v -m ./mymodule.ko my_func
  probe-definition(0): my_func
  symbol:my_func file:(null) line:0 offset:0 return:0 lazy:(null)
  0 arguments
  Could not open debuginfo. Try to use symbols.
  symsrc__init: build id mismatch for /home/wangnan/kmodule/mymodule.ko.
  /home/wangnan/kmodule/mymodule.ko with build id 326ab42550ef3d24944f53c817533728367effeb not found, continuing without symbols
  Failed to find symbol my_func in /home/wangnan/kmodule/mymodule.ko
    Error: Failed to add events. Reason: No such file or directory (Code: -2)

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1448510397-187965-1-git-send-email-wangnan0@huawei.com
[ Renamed adjust_dso_long_name() do dso__adjust_kmod_long_name() ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>