Re: [f2fs-dev] [PATCH] f2fs-tools: fix casefolded dentry hash for names that fold beyond F2FS_NAME_LEN

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On 10/6/26 04:17, Juhyung Park wrote:
> Since kernel commit 43c780ba2624 ("f2fs: rework filename handling",
> v5.8), the kernel casefolds a name into an F2FS_NAME_LEN buffer, and if
> the folded name doesn't fit, it falls back to hashing the raw on-disk
> name (f2fs_init_casefolded_name()).
> 
> f2fs_dentry_hash() still folds into a PATH_MAX buffer, as the original
> kernel implementation did, so it hashes the folded name instead. NFD
> decomposition can grow a name considerably (each 3-byte Hangul syllable
> becomes 6-9 bytes), so a valid <= 255-byte name can fold to more than
> 255 bytes. For such a name, fsck.f2fs computes a different hash than the
> kernel. It then "fixes" the kernel-written hash_code to a wrong value,
> and the file becomes unreachable via lookup (ENOENT) unless the kernel
> falls back to linear lookup (e.g. lookup_mode=compat). Later fsck runs
> consider the broken hash correct and report nothing.
> 
> This silently makes user files unreachable on Android, where /data/media
> is casefolded and fsck is triggered on a full OTA and a kernel version
> change. This personally happened to me on my daily driver unit. Since
> the buggy fsck considers its own hash correct afterwards, the damage is
> never reported, and it repeats for every new such file the kernel
> creates.
> 
> Minimal reproducer:
> 
>    NAME=$(printf '가%.0s' $(seq 43))  # 129 bytes raw, 258 bytes folded
>    truncate -s 128M cf.img
>    mkfs.f2fs -q -O extra_attr,casefold -C utf8 cf.img
>    mount -o loop cf.img mnt
>    mkdir mnt/d && chattr +F mnt/d && touch "mnt/d/$NAME"
>    umount mnt
>    fsck.f2fs -f -y cf.img
>    mount -o loop,ro cf.img mnt
>    stat "mnt/d/$NAME"
> 
> Before this patch, fsck.f2fs wrongfully modifies the hash and breaks
> kernel lookup:
> 
>    stat: cannot statx 'mnt/d/가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가가': No such file or directory
> 
> Use an F2FS_NAME_LEN buffer, as the kernel does. utf8_casefold() already
> returns -ENAMETOOLONG on overflow, which falls back to the raw-name hash.
> With this, fsck.f2fs leaves the reproducer image untouched, and it also
> repairs the hashes that the buggy version previously rewrote.
> 
> Fixes: ce64ea0815bf ("f2fs-tools: Add support for Casefolding")
> Signed-off-by: Juhyung Park <qkrwngud825@gmail.com>

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,


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