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Date: Wed, 5 Dec 2018 22:17:56 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: sem_wait and EINTR

On Thu, Dec 06, 2018 at 02:43:40AM +0000, Orivej Desh wrote:
> * Rich Felker <dalias@...c.org> [2018-12-05]
> > On Wed, Dec 05, 2018 at 10:27:16PM +0100, Ondřej Jirman wrote:
> > > On Wed, Dec 05, 2018 at 08:47:59PM +0100, Markus Wichmann wrote:  
> > > 
> > > It's specified by POSIX:
> > > 
> > > https://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_wait.html
> > > 
> > > Sates: "The sem_wait() function is interruptible by the delivery of a signal."  
> > 
> > This seems contradictory with EINTR being a "may fail" error, and, if
> > interpreted the way you want to interpret it, seems to be
> > contradictory with SA_RESTART semantics, since it doesn't say anything
> > about whether that signal is an interrupting one. I think we should
> > attempt to obtain a clarification on what the intent is here. Does "is
> > interruptible" mean that it needs to fail on signals (only without
> > SA_RESTART?) or simply that signal handlers must be permitted to run
> > (i.e. the wait can't happen with signals blocked)?
> 
> There is a definition of interruptible functions on the sigaction page:
> 
>     SA_RESTART
> 
>     This flag affects the behavior of interruptible functions; that is, those
>     specified to fail with errno set to [EINTR].
> 
>     If set, and a function specified as interruptible is interrupted by this
>     signal, the function shall restart and shall not fail with [EINTR] unless
>     otherwise specified.
> 
>     If the flag is not set, interruptible functions interrupted by this signal
>     shall fail with errno set to [EINTR].
> 
> https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaction.html

OK, this seems correct. I still don't understand why EINTR is a "may
fail" error; it's been that way at least back to SUSv2:

http://pubs.opengroup.org/onlinepubs/7908799/xsh/sem_wait.html

I'd like it if we could avoid the pre-linux-2.6.22 bug of spurious
EINTR from SYS_futex, but I don't see any way to do so except possibly
wrapping all signal handlers and implementing restart-vs-EINTR
ourselves. So if we need to change this, it might just be a case where
we say "well, sorry, your kernel is broken" if someone is using a
broken kernel.

Thoughts?

Rich

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