linux-kernel - Re: [PATCH] fat: avoid parent link count underflow in rmdir

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Message-ID: <CALf2hKu=M8TALyqv=Tv9Vu98UKUcFjWix1n5D9raMKYqqZtY5A@mail.gmail.com>
Date: Tue, 13 Jan 2026 01:45:18 +0800
From: Zhiyu Zhang <zhiyuzhang999@...il.com>
To: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc: Andrew Morton <akpm@...ux-foundation.org>, viro@...iv.linux.org.uk, brauner@...nel.org, 
	jack@...e.cz, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	gregkh@...uxfoundation.org
Subject: Re: [PATCH] fat: avoid parent link count underflow in rmdir

Hi OGAWA,

Sorry, I thought the further merge request would be done by the maintainers.

What should I do then?

Best,
Zhiyu Zhang

OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> 于2026年1月13日周二 00:21写道：
>
> Ping?
>
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> writes:
>
> > Zhiyu Zhang <zhiyuzhang999@...il.com> writes:
> >
> >> Corrupted FAT images can leave a directory inode with an incorrect
> >> i_nlink (e.g. 2 even though subdirectories exist). rmdir then
> >> unconditionally calls drop_nlink(dir) and can drive i_nlink to 0,
> >> triggering the WARN_ON in drop_nlink().
> >>
> >> Add a sanity check in vfat_rmdir() and msdos_rmdir(): only drop the
> >> parent link count when it is at least 3, otherwise report a filesystem
> >> error.
> >>
> >> Fixes: 9a53c3a783c2 ("[PATCH] r/o bind mounts: unlink: monitor i_nlink")
> >> Reported-by: Zhiyu Zhang <zhiyuzhang999@...il.com>
> >> Closes: https://lore.kernel.org/linux-fsdevel/aVN06OKsKxZe6-Kv@casper.infradead.org/T/#t
> >> Tested-by: Zhiyu Zhang <zhiyuzhang999@...il.com>
> >> Signed-off-by: Zhiyu Zhang <zhiyuzhang999@...il.com>
> >
> > Looks good. Thanks.
> >
> > Acked-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
> >
> >> ---
> >>  fs/fat/namei_msdos.c | 7 ++++++-
> >>  fs/fat/namei_vfat.c  | 7 ++++++-
> >>  2 files changed, 12 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/fs/fat/namei_msdos.c b/fs/fat/namei_msdos.c
> >> index 0b920ee40a7f..262ec1b790b5 100644
> >> --- a/fs/fat/namei_msdos.c
> >> +++ b/fs/fat/namei_msdos.c
> >> @@ -325,7 +325,12 @@ static int msdos_rmdir(struct inode *dir, struct dentry *dentry)
> >>      err = fat_remove_entries(dir, &sinfo);  /* and releases bh */
> >>      if (err)
> >>              goto out;
> >> -    drop_nlink(dir);
> >> +    if (dir->i_nlink >= 3)
> >> +            drop_nlink(dir);
> >> +    else {
> >> +            fat_fs_error(sb, "parent dir link count too low (%u)",
> >> +                    dir->i_nlink);
> >> +    }
> >>
> >>      clear_nlink(inode);
> >>      fat_truncate_time(inode, NULL, S_CTIME);
> >> diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
> >> index 5dbc4cbb8fce..47ff083cfc7e 100644
> >> --- a/fs/fat/namei_vfat.c
> >> +++ b/fs/fat/namei_vfat.c
> >> @@ -803,7 +803,12 @@ static int vfat_rmdir(struct inode *dir, struct dentry *dentry)
> >>      err = fat_remove_entries(dir, &sinfo);  /* and releases bh */
> >>      if (err)
> >>              goto out;
> >> -    drop_nlink(dir);
> >> +    if (dir->i_nlink >= 3)
> >> +            drop_nlink(dir);
> >> +    else {
> >> +            fat_fs_error(sb, "parent dir link count too low (%u)",
> >> +                    dir->i_nlink);
> >> +    }
> >>
> >>      clear_nlink(inode);
> >>      fat_truncate_time(inode, NULL, S_ATIME|S_MTIME);
>
> --
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>