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Date: Tue, 9 Apr 2013 11:39:36 -0400
From: Rich Felker <dalias@...ifal.cx>
To: Laurent Bercot <ska-skaware@...rnet.org>
Cc: musl@...ts.openwall.com
Subject: Re: musl signalfd (was: selfpipe + musl libc info)
On Mon, Apr 08, 2013 at 07:52:31AM +0200, Laurent Bercot wrote:
> Thanks David.
> And Rich, thanks for patching musl. However, I think your workaround for
> pre-2.6.27 kernels is unneeded, if not downright incorrect.
>
> The signalfd(2) manpage says: "In Linux up to version 2.6.26, the flags
> argument is unused, and must be specified as zero."
> And also:
> "ERRORS
> EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is nonzero."
>
> The current musl patch actually honors the flags with a race condition.
> Instead, returning -1 EINVAL if the signalfd4 syscall does not exist but
> flags is nonzero is the safer thing to do - and is what the manpage
> specifies.
>
> The skalibs signalfd autodetection test actually relies on this. It
> tests the signalfd() libc call with a SFD_NONBLOCK flags argument; if
> the call fails for any reason, skalibs will not use signalfd at all
> (and implement selfpipe via a real self-pipe).
I don't know if you got my last email, because somehow when I hit
reply-to-all it went to your list (where it was held as moderated) but
not your personal address. So I'm repeating it here:
----------------------------------------------------------------------
This makes no sense. For SFD_NONBLOCK, setting it atomically does not
matter; the application could just fallback to setting it with fcntl
if opening it with the nonblocking flag failed. Falling back to using
a pipe in that situation makes no sense. If the issue were
SFD_CLOEXEC, non-atomicity is a problem, but there is no way to
fallback anyway. Using a pipe would still have the same non-atomicity
issue.
If you think there are situations where the application really could
work around the lack of these kernel features, please explain. I'm
open to changing things but I want a good justification.
----------------------------------------------------------------------
I did think of one additional thing I want to ask about though: does
setting non-blocking mode even via fcntl after the fd is obtained fail
to work properly on old kernels? If so, then I agree we should fail
the call if SFD_NONBLOCK was specified and signalfd4 returns -ENOSYS.
Can you let me know what the situation is and whether this issue, or
any other issue I might not be aware of, applies?
Rich
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