linux-kernel - Re: [patch/rfc] Make poll/select report error (POLLNVAL and EBADF) for unsupported files

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Message-ID: <1266255759.8262.26.camel@edumazet-laptop>
Date:	Mon, 15 Feb 2010 18:42:39 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stephane.thiell@....fr
Subject: Re: [patch/rfc] Make poll/select report error (POLLNVAL and EBADF)
 for unsupported files

Le dimanche 14 février 2010 à 14:27 -0800, Davide Libenzi a écrit :
> Currently poll and select consider a non poll-supported file as one with 
> full event mask set, instead of reporting proper error to the caller.
> This behavior can fool the caller of proper functionality being returned, 
> while instead no valid event was processed/read from the device.
> This came out linked to this bug report:
> 
> http://bugzilla.kernel.org/show_bug.cgi?id=15272
> 
> IMHO, it'd be more adequate to report proper error code, for files that do 
> not support f_op->poll(), but then I am also not sure how much breakage 
> can this bring to existing (already broken "in just the right way") 
> applications.
> Untested, discussion-only, patch.
> 
> 
> Signed-off-by: Davide Libenzi <davidel@...ilserver.org>
> 
> 
> - Davide

Hmm, according to POSIX :

	The poll() function shall support regular files, terminal and
pseudo-terminal devices, FIFOs, pipes, sockets ...

	Regular files shall always poll TRUE for reading and writing.


So unless I missed something, this patch could break some conformant
applications.

In particular, if an application is polling() on stdin (usually a tty),
and other 'files', what's happening if we do :

cat replay_file | application

Either it wont read stdin, or application exits without reading its
input.



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