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

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Date:	Fri, 20 Mar 2015 10:26:15 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Milos Vyletel <milos@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	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()

Em Fri, Mar 20, 2015 at 01:18:07PM +0100, Jiri Olsa escreveu:
> 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

Yeah, would be nice to have that improved.

Thanks, applied.

- Arnaldo
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