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erofs: fix EFSCORRUPTED on multi-algorithm images in z_erofs_map_sanity_check()

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Commit a5242d37c8 ("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: a5242d37c8 ("erofs: error out obviously illegal extents in advance")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
This commit is contained in:
Zhan Xusheng 2026-06-01 16:51:36 +08:00 • committed by Gao Xiang
commit 27f2d085bd
1 changed files with 1 additions and 1 deletions
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2
fs/erofs/zmap.c
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@ -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;