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Date: Fri, 20 Mar 2015 13:18:07 +0100
From: Jiri Olsa <jolsa@redhat.com>
To: Milos Vyletel <milos@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, Paul Mackerras <paulus@samba.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
        Stephane Eranian <eranian@google.com>,
        "open list:PERFORMANCE EVENT..." <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] perf: fix race in build_id_cache__add_s()
Message-ID: <20150320121807.GB6304@krava.brq.redhat.com>
References: <1426847846-11112-1-git-send-email-milos@redhat.com>
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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