Re: [PATCH] erofs: ensure the extra temporary copy is valid for shortened bvecs

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2025/5/7 09:40, Hongbo Li wrote:
> 
> 
> On 2025/5/6 18:18, Gao Xiang wrote:
>> When compressed data deduplication is enabled, multiple logical extents
>> may reference the same compressed physical cluster.
>>
>> The previous commit 94c43de73521 ("erofs: fix wrong primary bvec
>> selection on deduplicated extents") already avoids using shortened
>> bvecs.  However, in such cases, the extra temporary buffers also
>> need to be preserved for later use in z_erofs_fill_other_copies() to
>> to prevent data corruption.
>>
>> IOWs, extra temporary buffers have to be retained not only due to
>> varying start relative offsets (`pageofs_out`, as indicated by
>> `pcl->multibases`) but also because of shortened bvecs.
>>
>> android.hardware.graphics.composer@2.1.so : 270696 bytes
>>     0:        0..  204185 |  204185 :  628019200.. 628084736 |   65536
>> -> 1:   204185..  225536 |   21351 :  544063488.. 544129024 |   65536
>>     2:   225536..  270696 |   45160 :          0..         0 |       0
>>
>> com.android.vndk.v28.apex : 93814897 bytes
>> ...
>>     364: 53869896..54095257 |  225361 :  543997952.. 544063488 |   65536
>> -> 365: 54095257..54309344 |  214087 :  544063488.. 544129024 |   65536
>>     366: 54309344..54514557 |  205213 :  544129024.. 544194560 |   65536
>> ...
>>
>> Both 204185 and 54095257 have the same start relative offset of 3481,
>> but the logical page 55 of `android.hardware.graphics.composer@2.1.so`
>> ranges from 225280 to 229632, forming a shortened bvec [225280, 225536)
>> that cannot be used for decompressing the range from 54095257 to
>> 54309344 of `com.android.vndk.v28.apex`.
>>
>> Since `pcl->multibases` is already meaningless, just mark `keepxcpy`
>> on demand for simplicity.
>>
>> Again, this issue can only lead to data corruption if `-Ededupe` is on.
>>
>> Fixes: 94c43de73521 ("erofs: fix wrong primary bvec selection on deduplicated extents")
>> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
>> ---
>>   fs/erofs/zdata.c | 31 ++++++++++++++-----------------
>>   1 file changed, 14 insertions(+), 17 deletions(-)
>>
>> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
>> index 5c061aaeeb45..b8e6b76c23d5 100644
>> --- a/fs/erofs/zdata.c
>> +++ b/fs/erofs/zdata.c
>> @@ -79,9 +79,6 @@ struct z_erofs_pcluster {
>>       /* L: whether partial decompression or not */
>>       bool partial;
>> -    /* L: indicate several pageofs_outs or not */
>> -    bool multibases;
>> -
>>       /* L: whether extra buffer allocations are best-effort */
>>       bool besteffort;
>> @@ -1046,8 +1043,6 @@ static int z_erofs_scan_folio(struct z_erofs_frontend *f,
>>                   break;
>>               erofs_onlinefolio_split(folio);
>> -            if (f->pcl->pageofs_out != (map->m_la & ~PAGE_MASK))
>> -                f->pcl->multibases = true;
>>               if (f->pcl->length < offset + end - map->m_la) {
>>                   f->pcl->length = offset + end - map->m_la;
>>                   f->pcl->pageofs_out = map->m_la & ~PAGE_MASK;
>> @@ -1093,7 +1088,6 @@ struct z_erofs_backend {
>>       struct page *onstack_pages[Z_EROFS_ONSTACK_PAGES];
>>       struct super_block *sb;
>>       struct z_erofs_pcluster *pcl;
>> -
>>       /* pages with the longest decompressed length for deduplication */
>>       struct page **decompressed_pages;
>>       /* pages to keep the compressed data */
>> @@ -1102,6 +1096,8 @@ struct z_erofs_backend {
>>       struct list_head decompressed_secondary_bvecs;
>>       struct page **pagepool;
>>       unsigned int onstack_used, nr_pages;
>> +    /* indicate if temporary copies should be preserved for later use */
>> +    bool keepxcpy;
>>   };
>>   struct z_erofs_bvec_item {
>> @@ -1112,18 +1108,20 @@ struct z_erofs_bvec_item {
>>   static void z_erofs_do_decompressed_bvec(struct z_erofs_backend *be,
>>                        struct z_erofs_bvec *bvec)
>>   {
>> +    int poff = bvec->offset + be->pcl->pageofs_out;
> 
> Looks good!
> 
> Reviewed-by: Hongbo Li <lihongbo22@huawei.com>

Thanks, and the stress test also passes. Will submit it soon.

Thanks,
Gao Xiang