[PATCH] perf: fix race in build_id_cache__add_s()

[Milos Vyletel <milos@redhat.com>]

int build_id_cache__add_s(const char *sbuild_id, const char *debugdir,
                          const char *name, bool is_kallsyms, bool is_vdso)
{
...
        if (access(filename, F_OK)) {
               ^--------------------------------------------------------- [1]
                if (is_kallsyms) {
                         if (copyfile("/proc/kallsyms", filename))
                                goto out_free;
                } else if (link(realname, filename) && copyfile(name, filename))
                             ^-----------------------------^------------- [2]
                                                            \------------ [3]
                        goto out_free;
        }
...

when multiple instances of perf record get to [1] at more or less same time and
run access() one or more may get failure because the file does not exist yet
(since the first instance did not have chance to link it yet). at this point the
race moves to link() at [2] where first thread to get there links file and goes
on but second one gets -EEXIST so it runs copyfile [3] which truncates the file.

recreator:
rm -rf /root/.debug
for cpu in $(awk '/processor/ {print $3}' /proc/cpuinfo); do
	perf record -a -v -T -F 1000 -C $cpu \
		-o perf-${cpu}.data sleep 5 2> /dev/null &
done
wait

and simply search for empty files by
find /lib/modules/`uname -r`/kernel/* -size 0

Signed-off-by: Milos Vyletel <milos@redhat.com>
---
 tools/perf/util/build-id.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/build-id.c b/tools/perf/util/build-id.c
index 0c72680..a9db5a1 100644
--- a/tools/perf/util/build-id.c
+++ b/tools/perf/util/build-id.c
@@ -293,7 +293,8 @@ int build_id_cache__add_s(const char *sbuild_id, const char *debugdir,
 		if (is_kallsyms) {
 			 if (copyfile("/proc/kallsyms", filename))
 				goto out_free;
-		} else if (link(realname, filename) && copyfile(name, filename))
+		} else if (link(realname, filename) && errno != EEXIST &&
+				copyfile(name, filename))
 			goto out_free;
 	}
 
-- 
2.1.0