Commit a5242d37c83a ("erofs: error out obviously illegal extents in
advance") changed the per-extent algorithm presence check from "is the
bit set" to "is the only bit set":
  -		     !(sbi->available_compr_algs & (1 << map->m_algorithmformat))
  +		(sbi->available_compr_algs ^ BIT(map->m_algorithmformat))

`available_compr_algs` is a bitmap of every compression algorithm
available in the image (z_erofs_parse_cfgs() iterates it with
for_each_set_bit()), so an image that enables more than one algorithm
has multiple bits set.  XOR is zero only when the bitmap is exactly
BIT(map->m_algorithmformat); for any image with two or more algorithms
the test is non-zero for every extent and the read fails with
-EFSCORRUPTED ("inconsistent algorithmtype %u").

Reproducer (mkfs.erofs from erofs-utils 1.7.1):
  $ mkdir src
  $ yes A | head -c 100K > src/a
  $ head -c 64K /dev/zero > src/b
  $ mkfs.erofs -zlz4:deflate multi.erofs src
  $ mount -t erofs -o loop multi.erofs /mnt
  $ cat /mnt/a >/dev/null
  cat: /mnt/a: Structure needs cleaning
  $ dmesg | tail
    erofs (device loop0): inconsistent algorithmtype 0 for nid 46
    erofs (device loop0): read error -117 @ 0 of nid 46

The erofs on-disk format (Z_EROFS_COMPRESSION_MAX = 4 with LZ4, LZMA,
DEFLATE, ZSTD) and the kernel parser explicitly support
multi-algorithm images, and erofs-utils 1.7.1 generates them via the
"-z X:Y" syntax.

Restore the original per-bit presence check.

Fixes: a5242d37c83a ("erofs: error out obviously illegal extents in advance")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
 fs/erofs/zmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
index a72db36096ca..e1a02a2c8406 100644
--- a/fs/erofs/zmap.c
+++ b/fs/erofs/zmap.c
@@ -716,7 +716,7 @@ static int z_erofs_map_sanity_check(struct inode *inode,
 	}
 
 	if (map->m_algorithmformat < Z_EROFS_COMPRESSION_MAX) {
-		if (sbi->available_compr_algs ^ BIT(map->m_algorithmformat)) {
+		if (!(sbi->available_compr_algs & BIT(map->m_algorithmformat))) {
 			erofs_err(inode->i_sb, "inconsistent algorithmtype %u for nid %llu",
 				  map->m_algorithmformat, EROFS_I(inode)->nid);
 			return -EFSCORRUPTED;
-- 
2.43.0

On 2026/6/1 16:51, Zhan Xusheng wrote:
> Commit a5242d37c83a ("erofs: error out obviously illegal extents in
> advance") changed the per-extent algorithm presence check from "is the
> bit set" to "is the only bit set":
>    -		     !(sbi->available_compr_algs & (1 << map->m_algorithmformat))
>    +		(sbi->available_compr_algs ^ BIT(map->m_algorithmformat))
> 
> `available_compr_algs` is a bitmap of every compression algorithm
> available in the image (z_erofs_parse_cfgs() iterates it with
> for_each_set_bit()), so an image that enables more than one algorithm
> has multiple bits set.  XOR is zero only when the bitmap is exactly
> BIT(map->m_algorithmformat); for any image with two or more algorithms
> the test is non-zero for every extent and the read fails with
> -EFSCORRUPTED ("inconsistent algorithmtype %u").
> 
> Reproducer (mkfs.erofs from erofs-utils 1.7.1):
>    $ mkdir src
>    $ yes A | head -c 100K > src/a
>    $ head -c 64K /dev/zero > src/b
>    $ mkfs.erofs -zlz4:deflate multi.erofs src
>    $ mount -t erofs -o loop multi.erofs /mnt
>    $ cat /mnt/a >/dev/null
>    cat: /mnt/a: Structure needs cleaning
>    $ dmesg | tail
>      erofs (device loop0): inconsistent algorithmtype 0 for nid 46
>      erofs (device loop0): read error -117 @ 0 of nid 46
> 
> The erofs on-disk format (Z_EROFS_COMPRESSION_MAX = 4 with LZ4, LZMA,
> DEFLATE, ZSTD) and the kernel parser explicitly support
> multi-algorithm images, and erofs-utils 1.7.1 generates them via the
> "-z X:Y" syntax.
> 
> Restore the original per-bit presence check.
> 
> Fixes: a5242d37c83a ("erofs: error out obviously illegal extents in advance")
> Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>

Thanks, that is an awkward regression:
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Thanks,
Gao Xiang