[PATCH] perf tools: Fix build-id matching on vmlinux

[Namhyung Kim]

There's a problem on finding correct kernel symbols when perf report
runs on a different kernel.  Although a part of the problem was solved
by the prior commit 0a7e6d1b6844 ("perf tools: Check recorded kernel
version when finding vmlinux"), there's a remaining problem still.

When perf records samples, it synthesizes the kernel map using
machine__mmap_name() and ref_reloc_sym like "[kernel.kallsyms]_text".
You can easily see it using 'perf report -D' command.

After finishing record, it goes through the recorded events to find
maps/dsos actually used.  And then record build-id info of them.

During this process, it needs to load symbols in a dso and it'd call
dso__load_vmlinux() since the default value of the symbol_conf.try_
vmlinux_path is true.  However it changes dso->long_name to a real
path of the vmlinux file (e.g. /lib/modules/3.16.0-rc2+/build/vmlinux)
if one is running on a custom kernel.

It resulted in that perf report reads the build-id of the vmlinux, but
cannot use it since it only knows about the [kernel.kallsyms] map.  It
then falls back to possible vmlinux paths by using the recorded kernel
version (in case of a recent version) or a running kernel silently
(which might break the result).  I think it's worth going to the
stable tree.

I can think of a couple of ways to fix it.  In this patch, I changed
to use the name of "[kernel.kallsyms]" for the kernel build-id event
instead of not trying vmlinux paths.  This way we can provide maximum
info (like annotation) with minimum change IMHO.

Before:

  $ perf record -a usleep 1

  $ perf buildid-list
  00d5ff078efe1d30b8492854f259215fd877ce30 /lib/modules/3.16.0-rc2+/build/vmlinux
  78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
  4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
  1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so

  $ perf buildid-list -H
  0000000000000000000000000000000000000000 [kernel.kallsyms]
  78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
  4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
  1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
  0000000000000000000000000000000000000000 /tmp/perf-2523.map

After:

  $ perf record -a usleep 1

  $ perf buildid-list
  00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]
  78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
  4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
  1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so

  $ perf buildid-list -H
  00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]
  78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
  4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
  1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
  0000000000000000000000000000000000000000 /tmp/perf-2523.map

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 tools/perf/util/header.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 158c787ce0c4..5c4093dee467 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -263,6 +263,9 @@ static int __dsos__write_buildid_table(struct list_head *head,
 			machine__mmap_name(machine, nm, sizeof(nm));
 			name = nm;
 			name_len = strlen(nm) + 1;
+		} else if (dso__is_vmlinux(pos)) {
+			name = pos->name;
+			name_len = strlen(pos->name) + 1;
 		} else {
 			name = pos->long_name;
 			name_len = pos->long_name_len + 1;
-- 
2.0.0

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