7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Binglei Wang [ Upstream commit 135d84c66f85426299db01a09d93a79a87af18ba ] erofs_bread() locates the target folio with index = (buf->off + offset) >> PAGE_SHIFT; but computes the in-folio offset without taking buf->off into account: return buf->base + (offset & ~PAGE_MASK); If buf->off is not page-aligned, the returned pointer misses the in-page component of buf->off, so callers end up fetching data from a wrong offset. buf->off is set to sbi->dif0.fsoff in erofs_init_metabuf(), and fsoff can be specified via the "fsoffset=" mount option, which only requires block-size alignment. Therefore, on an image with a sub-page block size (e.g. 512 bytes), a non-page-aligned fsoff (e.g. 512) triggers the issue, since 512 is a multiple of the block size but not of PAGE_SIZE. It can be reproduced by mounting an image that is placed at a non-page-aligned offset: mkfs.erofs -b512 -zlz4hc sub.erofs src/ # prepend 512 bytes of padding to the image mount -t erofs -o loop,fsoffset=512 padded.erofs /mnt which fails with erofs (device loop0): cannot find valid erofs superblock because the on-disk superblock (at offset 1024 within the image, i.e. 1536 within the padded file) is read from a wrong in-folio offset. With this fixed, the very same image mounts successfully and its file contents match those read from the unpadded image. Fix it by including buf->off in the in-folio offset calculation, so that it is consistent with the folio index calculation. Fixes: c36ec00d7f67 ("erofs: add 'fsoffset' mount option to specify filesystem offset") Signed-off-by: Binglei Wang Reviewed-by: Gao Xiang Signed-off-by: Gao Xiang Signed-off-by: Sasha Levin --- fs/erofs/data.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/erofs/data.c b/fs/erofs/data.c index 9aa48c8d67d12..b6cdffd0d8936 100644 --- a/fs/erofs/data.c +++ b/fs/erofs/data.c @@ -61,7 +61,7 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap) return NULL; if (!buf->base) buf->base = kmap_local_page(buf->page); - return buf->base + (offset & ~PAGE_MASK); + return buf->base + ((buf->off + offset) & ~PAGE_MASK); } int erofs_init_metabuf(struct erofs_buf *buf, struct super_block *sb, -- 2.53.0