linux-kernel - Re: 2.6.19 file content corruption on ext3

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Message-ID: <87d56fv9bz.fsf@mid.deneb.enyo.de>
Date:	Tue, 19 Dec 2006 22:56:16 +0100
From:	Florian Weimer <fw@...eb.enyo.de>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...l.org>, andrei.popa@...eo.ro,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hugh@...itas.com>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	Martin Michlmayr <tbm@...ius.com>
Subject: Re: 2.6.19 file content corruption on ext3

* Linus Torvalds:

> Now, this should _matter_ only for user processes that are buggy,
> and that have written to the page _before_ extending it with
> ftruncate().

APT seems to properly extend the file before mapping it, by writing a
zero byte at the desired position (creating a hole).

24986 open("/var/cache/apt/pkgcache.bin", O_RDWR|O_CREAT|O_TRUNC, 0666) = 6

24986 lseek(6, 12582911, SEEK_SET)      = 12582911
24986 write(6, "\0", 1)                 = 1

24986 mmap(NULL, 12582912, PROT_READ|PROT_WRITE, MAP_SHARED, 6, 0) = 0x2b6578636000

24986 msync(0x2b6578636000, 7464112, MS_SYNC) = 0
24986 msync(0x2b6578636000, 8656, MS_SYNC) = 0
24986 munmap(0x2b6578636000, 12582912)  = 0
24986 ftruncate(6, 7464112)             = 0
24986 fstat(6, {st_mode=S_IFREG|0644, st_size=7464112, ...}) = 0
24986 mmap(NULL, 7464112, PROT_READ, MAP_SHARED, 6, 0) = 0x2b6578636000

APT's code is pretty convoluted, though, and there might be some code
path in it that gets it wrong. 8-P
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