Re: [patch v2] perf_event_open.2 : document PERF_FLAG_FD_CLOEXEC flag

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 04/02/2014 05:22 PM, Vince Weaver wrote:
> On Wed, 2 Apr 2014, Michael Kerrisk (man-pages) wrote:
> 
>> I applied, and reworked the text to be:
>>
>>        PERF_FLAG_FD_CLOEXEC (since Linux 3.14).
>>               This flag enables the close-on-exec flag for the created
>>               event  file  descriptor,  so that the file descriptor is
>>               automatically closed on execve(2).  Setting  the  close-
>>               on-exec  flags at  creation time, rather than later with
>>               fcntl(2), avoids potential  race  conditions  where  the
>>               calling  thread  invokes  perf_event_open()  at the same
>>               time as another thread calls fork(2) then execve(2).
>>
>> Okay?
> 
> I think the actual race condition being worried about is this:
> 
> 
> thread1                   thread2
> 
> fd=perf_event_open()         |
>       |                    fork()
>       |                    exec()
>       |                      |
> fcntl(CLOEXEC)               |
>                           fd has escaped past exec because
>                           fcntl didn't happen fast enough

Yes, that's the one.

> so if you do a
> 
> fd=perf_event_open(...,PERF_FLAG_FD_CLOEXEC);
> 
> then fd is always CLOEXEC and you don't have a brief window of time
> before the fcntl() where the fd can escape.
> 
> 
> I'm not sure of a scuccint way to express that though.

I think the existing text suffices.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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