erofs: add missing buf->off in erofs_bread()

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: c36ec00d7f ("erofs: add 'fsoffset' mount option to specify filesystem offset")
Signed-off-by: Binglei Wang <l3b2w1@gmail.com>
Reviewed-by: Gao Xiang <xiang@kernel.org>
Signed-off-by: Gao Xiang <xiang@kernel.org>

fs/erofs/data.c

@@ -48,7 +48,7 @@ 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,