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Message-ID: <20130408144006.GK20323@brightrain.aerifal.cx>
Date: Mon, 8 Apr 2013 10:40:06 -0400
From: Rich Felker <dalias@...ifal.cx>
To: skaware@...t.skarnet.org, dalias@...ifal.cx
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).
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.
Rich
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