Re: [PATCH] erofs: fix wrong primary bvec selection on deduplicated extents

[Yue Hu]

On Wed, 19 Jul 2023 14:54:59 +0800
Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:

> When handling deduplicated compressed data, there can be multiple
> decompressed extents pointing to the same compressed data in one shot.
> 
> In such cases, the bvecs which belong to the longest extent will be
> selected as the primary bvecs for real decompressors to decode and the
> other duplicated bvecs will be directly copied from the primary bvecs.
> 
> Previously, only relative offsets of the longest extent was checked to
> decompress the primary bvecs.  On rare occasions, it can be incorrect
> if there are several extents with the same start relative offset.
> As a result, some short bvecs could be selected for decompression and
> then cause data corruption.
> 
> For example, as Shijie Sun reported off-list, considering the following
> extents of a file:
>  117:   903345..  915250 |   11905 :     385024..    389120 |    4096
> ...
>  119:   919729..  930323 |   10594 :     385024..    389120 |    4096
> ...
>  124:   968881..  980786 |   11905 :     385024..    389120 |    4096
> 
> The start relative offset is the same: 2225, but extent 119 (919729..
> 930323) is shorter than the others.
> 
> Let's restrict the bvec length in addition to the start offset if bvecs
> are not full.
> 
> Reported-by: Shijie Sun <sunshijie@xxxxxxxxxx>
> Fixes: 5c2a64252c5d ("erofs: introduce partial-referenced pclusters")
> Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>

Reviewed-by: Yue Hu <huyue2@xxxxxxxxxxx>