linux-kernel - Re: [PATCH] perf: fix race in build_id_cache__add

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

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@...nel.org>

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

jirka
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