Re: [PATCH] erofs: fix unbalanced buf->off handling in erofs_bread()

[Gao Xiang <xiang@kernel.org>]

Hi Binglei,

On Fri, Sep 11, 2026 at 12:01:48PM +0800, binglei wang wrote:
> 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")
> Cc: <stable@vger.kernel.org> # 6.16+
> Signed-off-by: Binglei Wang <l3b2w1@gmail.com>

The fix looks fine, but the patch format is broken:
Reviewed-by: Gao Xiang <xiang@kernel.org>

I applied the following version manually with an updated
patch subject.

From 135d84c66f85426299db01a09d93a79a87af18ba Mon Sep 17 00:00:00 2001
From: Binglei Wang <l3b2w1@gmail.com>
Date: Fri, 11 Sep 2026 12:11:33 +0800
Subject: [PATCH] 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: c36ec00d7f67 ("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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 0885b1f2fc92..be63b89f0862 100644
--- a/fs/erofs/data.c
+++ b/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,
-- 
2.47.3