GNU bug report logs - #21790
[PATCH] coreutils/cp: handle EOF extents correctly

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Message #16 received at 21790-done <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Dmitry Monakhov <dmonakhov <at> openvz.org>, 21790-done <at> debbugs.gnu.org
Cc: vvs <at> parallels.com, devel <at> openvz.org
Subject: Re: bug#21790: [PATCH] coreutils/cp: handle EOF extents correctly
Date: Fri, 20 Nov 2015 18:26:57 +0000

[Message part 1 (text/plain, inline)]

On 30/10/15 18:54, Pádraig Brady wrote:
> On 30/10/15 16:57, Pádraig Brady wrote:
>> On 30/10/15 09:02, Dmitry Monakhov wrote:
>>> fallocate can allocate extens beyond EOF via FALLOC_FL_KEEP_SIZE.
>>> Currenly sparse engine tries to copy such extents which is wrong and
>>> result in silent data corruption (leave file with incorrect size).
>>>
>>> ##TESTCASE
>>> echo blabla > sparse_falloc.in
>>> truncate -s 2M sparse_falloc.in
>>> fallocate -n -o 4M -l 1M sparse_falloc.in
>>> cp sparse_falloc.in sparse_falloc.out
>>> cmp sparse_falloc.in sparse_falloc.out
>>
>> Ouch.  Thanks for the analysis and patch.
>> It looks correct.  I'll analyze further before applying.
> 
> This doesn't handle the --sparse==never case
> (which is broken with and without the patch).
> 
> Also one might have an extent spanning the file size boundary,
> in which this patch could miss the remaining data?
> 
> Also currently if the source file is being extended
> while being copied, we continue to read, whereas we now wont.
> Theoretically this is an issue when st_size doesn't match
> what's available to be read, though maybe not a practical issue
> since we don't use this path for a zero st_size, nor
> sources that don't support fiemap anyway.
> 
> This might be an opportune time to rip out the fiemap stuff
> in favor of SEEK{DATA,HOLE} anyway, which I intended to do
> during this cycle.  For fix backporting sake though,
> we should apply the minimal fix to the fiemap code first.
> 
> Attached is the current minimally tested patch.

I'll apply the attached in your name soon.
Please review.

thanks,
Pádraig.

[cp-sparse-extents.patch (text/x-patch, attachment)]

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