linux-kernel - Re: [PATCH 1/1] Fix seq_file mishandling of consecutive pread() invocations.

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Message-Id: <20120126160038.b5bb63be.akpm@linux-foundation.org>
Date:	Thu, 26 Jan 2012 16:00:38 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Earl Chew <echew@...acom.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<adobriyan@...il.com>, <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/1] Fix seq_file mishandling of consecutive pread()
 invocations.

On Sun, 22 Jan 2012 11:01:21 -0800
Earl Chew <echew@...acom.com> wrote:

> [ Added Maintainers; Added reference to bugzilla.kernel.org in commit log ]
> 
> Also reported in:
> 
>     https://bugzilla.kernel.org/show_bug.cgi?id=11856
> 
> The following program illustrates the problem:
> 
>     char buf[8192];
> 
>     int fd = open("/proc/self/maps", O_RDONLY);
> 
>     n = pread(fd, buf, sizeof(buf), 0);
>     printf("%d\n", n);
> 
>     /* lseek(fd, 0, SEEK_CUR); */ /* Uncomment to work around */
> 
>     n = pread(fd, buf, sizeof(buf), 0);
>     printf("%d\n", n);
> 
> The second printf() prints zero, but uncommenting the lseek()
> corrects its behaviour.

I'm stunned and confused.  That sequence of operations is the only sane
way in which to poll the contents of a procfs file.

Surely there are many applications which open a procfs file then
repeatedly read it with pread(fd, ..., 0).  How can this problem not
have been noticed in the first five minutes??
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