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SubjectRe: [PATCH 2 2/2] xfs: make sure link path does not go away at access
On Mon, Nov 15, 2021 at 10:24:28AM +0800, Ian Kent wrote:
> When following an inline symlink in rcu-walk mode it's possible to
> succeed in getting the ->get_link() method pointer but the link path
> string be deallocated while it's being used.
>
> This is becuase of the xfs inode reclaim mechanism. While rcu freeing
> the link path can prevent it from being freed during use the inode
> reclaim could assign a new value to the field at any time outside of
> the path walk and result in an invalid link path pointer being
> returned. Admittedly a very small race window but possible.
>
> The best way to mitigate this risk is to return -ECHILD to the VFS
> if the inline symlink method, ->get_link(), is called in rcu-walk mode
> so the VFS can switch to ref-walk mode or redo the walk if the inode
> has become invalid.
>
> If it's discovered that staying in rcu-walk mode gives a worth while
> performance improvement (unlikely) then the link path could be freed
> under rcu once potential side effects of the xfs inode reclaim
> sub-system have been analysed and dealt with if needed.
>
> Signed-off-by: Ian Kent <raven@themaw.net>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

> fs/xfs/xfs_iops.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index a607d6aca5c4..0a96183c5381 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -520,6 +520,9 @@ xfs_vn_get_link_inline(
> struct xfs_inode *ip = XFS_I(inode);
> char *link;
>
> + if (!dentry)
> + return ERR_PTR(-ECHILD);
> +
> ASSERT(ip->i_df.if_format == XFS_DINODE_FMT_LOCAL);
>
> /*
>
>

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