Re: [PATCH] xfs: introduce protection for drop nlink

[Darrick J. Wong]

On Fri, Aug 25, 2023 at 04:32:22PM +0800, cheng.lin130@xxxxxxxxxx wrote:
> > On Thu, Aug 24, 2023 at 03:43:52PM +0800, cheng.lin130@xxxxxxxxxx wrote:
> >> From: Cheng Lin <cheng.lin130@xxxxxxxxxx>
> >> An dir nlinks overflow which down form 0 to 0xffffffff, cause the
> >> directory to become unusable until the next xfs_repair run.
> >>
> >> Introduce protection for drop nlink to reduce the impact of this.
> >> And produce a warning for directory nlink error during remove.
> >>
> >> Signed-off-by: Cheng Lin <cheng.lin130@xxxxxxxxxx>
> >> ---
> >>  fs/xfs/xfs_inode.c | 16 +++++++++++++++-
> >>  1 file changed, 15 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> >> index 9e62cc5..536dbe4 100644
> >> --- a/fs/xfs/xfs_inode.c
> >> +++ b/fs/xfs/xfs_inode.c
> >> @@ -919,6 +919,15 @@ STATIC int xfs_iunlink_remove(struct xfs_trans *tp, struct xfs_perag *pag,
> >>      xfs_trans_t *tp,
> >>      xfs_inode_t *ip)
> >>  {
> >> +    xfs_mount_t     *mp;
> >> +
> >> +    if (VFS_I(ip)->i_nlink == 0) {
> >> +        mp = ip->i_mount;
> >> +        xfs_warn(mp, "%s: Deleting inode %llu with no links.",
> >> +             __func__, ip->i_ino);
> >> +        return 0;
> >> +    }
> >> +
> >>      xfs_trans_ichgtime(tp, ip, XFS_ICHGTIME_CHG);
> >>
> >>      drop_nlink(VFS_I(ip));
> > I'm not sure how nlink would ever get to 0xFFFFFFFF since the VFS won't
> > let a link count exceed s_max_links, and XFS sets that to 0x7FFFFFFF.
> > Unless, of course, you did that outside of Linux.
> In VFS drop_nlink() only produce a warning, when (inode->i_nlink == 0),
> not prevent its self-reduce(inode->__i_nlink--), cause it underflow
> from 0 to 0xffffffff.

It is interesting that vfs_unlink doesn't check the link counts of
either the parent or the child.  Maybe it should, since the VFS
link/mkdir/rename functions check.

I wonder if this is a historical leftover from the days when the VFS
did no checking at all?

> In the old kernel version, this situation was
> encountered, but I don't know how it happened. It was already a scene
> with directory errors: "Too many links".
> 
>  kernel: WARNING: CPU: 12 PID: 12928 at fs/inode.c:286 drop_nlink+0x3e/0x50
>  kernel: CPU: 12 PID: 12928 Comm: gbased Tainted: G        W  OE  ------------ T 3.10.0-693.21.1.el7.x86_64 #1
>  kernel: Hardware name: HPE ProLiant BL460c Gen10/ProLiant BL460c Gen10, BIOS I41 01/23/2021
>  kernel: Call Trace:-------------------
>  kernel: [<ffffffff816c5fce>] dump_stack+0x19/0x1b
>  kernel: [<ffffffff8108dfa8>] __warn+0xd8/0x100/*
>  kernel: [<ffffffff8108e0ed>] warn_slowpath_null+0x1d/0x20
>  kernel: [<ffffffff8122cdfe>] drop_nlink+0x3e/0x50
>  kernel: [<ffffffffc03cdc78>] xfs_droplink+0x28/0x60 [xfs]
>  kernel: [<ffffffffc03cf87a>] xfs_remove+0x2aa/0x320 [xfs]
>  kernel: [<ffffffffc03c9f7a>] xfs_vn_unlink+0x5a/0xa0 [xfs]
>  kernel: [<ffffffff8121f19c>] vfs_rmdir+0xdc/0x150
>  kernel: [<ffffffff81221e41>] do_rmdir+0x1f1/0x220
>  kernel: [<ffffffff81223046>] SyS_rmdir+0x16/0x20
>  kernel: [<ffffffff816d86d5>] system_call_fastpath+0x1c/0x21
> > That said, why wouldn't you /pin/ the link count at -1U instead of
> > allowing it to overflow to zero?
> > Could you please take a look at this patch that's waiting in my
> > submission queue?
> > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?h=inode-repair-improvements&id=05f5a82efa6395c92038e18e008aaf7154238f27
> I think the XFS_NLINK_PINNEED(~0U) can be used prevent Overflow in inc_nlink().
> Is it better to compare i_nlink with (0U) in drop_nlink() to prevent Underflow?
> (like this patch does, do not make i_nlink underflow from 0 to 0xffffffff)

Is it a problem if a directory i_nlink underflows to XFS_NLINK_PINNED?
At that point the directory will never be freed, and xfs_repair/scrub
get to figure out the correct link count.

--D

> 
> Thanks.
> > --D
> >> @@ -2442,7 +2451,12 @@ STATIC int xfs_iunlink_remove(struct xfs_trans *tp, struct xfs_perag *pag,
> >>       */
> >>      if (is_dir) {
> >>          ASSERT(VFS_I(ip)->i_nlink >= 2);
> >> -        if (VFS_I(ip)->i_nlink != 2) {
> >> +        if (VFS_I(ip)->i_nlink < 2) {
> >> +            xfs_warn(ip->i_mount,
> >> +            "%s: Remove dir (inode %llu) with invalid links.",
> >> +                 __func__, ip->i_ino);
> >> +        }
> >> +        if (VFS_I(ip)->i_nlink > 2) {
> >>              error = -ENOTEMPTY;
> >>              goto out_trans_cancel;
> >>          }
> >> --
> >> 1.8.3.1