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
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.
Vince
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