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From: | Max Reitz |
Subject: | Re: [Qemu-devel] [PATCH] qcow2: Unset zero_beyond_eof in save_vmstate |
Date: | Wed, 23 Oct 2013 19:07:54 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.0 |
On 2013-10-21 22:37, Eric Blake wrote:
On 10/20/2013 08:52 PM, Max Reitz wrote:Saving the VM state is done using bdrv_pwrite. This function may perform a read-modify-write, which in this case results in data being read from beyond the end of the virtual disk. Since we are actually trying to access an area which is not a part of the virtual disk, zero_beyond_eof has to be set to false before performing the partial write, otherwise the VM state may become corrupted. Signed-off-by: Max Reitz <address@hidden> --- Follow-up to (depends on): - qcow2: Restore total_sectors value in save_vmstateReviewed-by: Eric Blake <address@hidden> Do you have test cases that demonstrate the corruption pre-patch?
I could write a test case for the other patch, but for this one it would probably be rather difficult. What I did to bisect the bug was just starting a VM over and over while saving a snapshot at some time during boot-up and trying to load that snapshot again. Sometimes, qemu itself would report a corrupted VM state, but most of the time, the guest simply hang or paniced. This is something I can detect interactively, but I don't know if I could write a test for this (at least not for hanging).
Max
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