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Re: [Qemu-devel] [PATCH for-2.11] qcow2: Fix overly broad madvise()

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH for-2.11] qcow2: Fix overly broad madvise()
Date: Wed, 15 Nov 2017 14:51:01 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 2017-11-15 10:09, Alberto Garcia wrote:
> On Tue 14 Nov 2017 07:41:27 PM CET, Max Reitz wrote:
>> @mem_size and @offset are both size_t, thus subtracting them from one
>> another will just return a big size_t if mem_size < offset -- even more
>> obvious here because the result is stored in another size_t.
>> Checking that result to be positive is therefore not sufficient to
>> excluse the case that offset > mem_size.  Thus, we currently sometimes
>> issue an madvise() over a very large address range.
>> This is triggered by iotest 163, but with -m64, this does not result in
>> tangible problems.  But with -m32, this test produces three segfaults,
>> all of which are fixed by this patch.
>> Signed-off-by: Max Reitz <address@hidden>
> Oh, I guess this happens when the page size is larger than the cluster
> size? Otherwise I don't see how...
> Reviewed-by: Alberto Garcia <address@hidden>

Yes, the test uses 512 byte clusters.


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