Re: [PATCH] perf: fix race in build_id_cache__add_s()

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Mar 20, 2015 at 11:37:25AM +0100, Milos Vyletel wrote:
> 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.

nice.. :-\ 

Acked-by: Jiri Olsa <jolsa@kernel.org>

in addition we should use some inter-perf lock
on all .debug dir operations

jirka