perf probe: Fix to get correct modname from elf header

Since 'perf probe' supports cross-arch probes, it is possible to analyze
different arch kernel image which has different bits-per-long.

In that case, it fails to get the module name because it uses the
MOD_NAME_OFFSET macro based on the host machine bits-per-long, instead
of the target arch bits-per-long.

This fixes above issue by changing modname-offset based on the target
archs bit width. This is ok because linux kernel uses LP64 model on
64bit arch.

E.g. without this (on x86_64, and target module is arm32):

  $ perf probe -m build-arm/fs/configfs/configfs.ko -D configfs_lookup
  p:probe/configfs_lookup :configfs_lookup+0
                          ^-Here is an empty module name.

With this fix, you can see correct module name:

  $ perf probe -m build-arm/fs/configfs/configfs.ko -D configfs_lookup
  p:probe/configfs_lookup configfs:configfs_lookup+0

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/148337043836.6752.383495516397005695.stgit@devbox
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index d281ae2b54e8fef7b85574a8b799dc33c21e9e44..8f810961ec78b3448eb81573e9a3bb374762d48d 100644 (file)
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -267,21 +267,6 @@ static bool kprobe_warn_out_range(const char *symbol, unsigned long address)
        return true;
 }
 
-/*
- * NOTE:
- * '.gnu.linkonce.this_module' section of kernel module elf directly
- * maps to 'struct module' from linux/module.h. This section contains
- * actual module name which will be used by kernel after loading it.
- * But, we cannot use 'struct module' here since linux/module.h is not
- * exposed to user-space. Offset of 'name' has remained same from long
- * time, so hardcoding it here.
- */
-#ifdef __LP64__
-#define MOD_NAME_OFFSET 24
-#else
-#define MOD_NAME_OFFSET 12
-#endif
-
 /*
  * @module can be module name of module file path. In case of path,
  * inspect elf and find out what is actual module name.
@@ -296,6 +281,7 @@ static char *find_module_name(const char *module)
        Elf_Data *data;
        Elf_Scn *sec;
        char *mod_name = NULL;
+       int name_offset;
 
        fd = open(module, O_RDONLY);
        if (fd < 0)
@@ -317,7 +303,21 @@ static char *find_module_name(const char *module)
        if (!data || !data->d_buf)
                goto ret_err;
 
-       mod_name = strdup((char *)data->d_buf + MOD_NAME_OFFSET);
+       /*
+        * NOTE:
+        * '.gnu.linkonce.this_module' section of kernel module elf directly
+        * maps to 'struct module' from linux/module.h. This section contains
+        * actual module name which will be used by kernel after loading it.
+        * But, we cannot use 'struct module' here since linux/module.h is not
+        * exposed to user-space. Offset of 'name' has remained same from long
+        * time, so hardcoding it here.
+        */
+       if (ehdr.e_ident[EI_CLASS] == ELFCLASS32)
+               name_offset = 12;
+       else    /* expect ELFCLASS64 by default */
+               name_offset = 24;
+
+       mod_name = strdup((char *)data->d_buf + name_offset);
 
 ret_err:
        elf_end(elf);