Re: [PATCH v6 12/15] Add start_renaming_two_dentries()

On Wed, Nov 12, 2025 at 7:42 PM NeilBrown <[email protected]> wrote:
>
> From: NeilBrown <[email protected]>
>
> A few callers want to lock for a rename and already have both dentries.
> Also debugfs does want to perform a lookup but doesn't want permission
> checking, so start_renaming_dentry() cannot be used.
>
> This patch introduces start_renaming_two_dentries() which is given both
> dentries.  debugfs performs one lookup itself.  As it will only continue
> with a negative dentry and as those cannot be renamed or unlinked, it is
> safe to do the lookup before getting the rename locks.
>
> overlayfs uses start_renaming_two_dentries() in three places and  selinux
> uses it twice in sel_make_policy_nodes().
>
> In sel_make_policy_nodes() we now lock for rename twice instead of just
> once so the combined operation is no longer atomic w.r.t the parent
> directory locks.  As selinux_state.policy_mutex is held across the whole
> operation this does not open up any interesting races.
>
> Reviewed-by: Amir Goldstein <[email protected]>
> Reviewed-by: Jeff Layton <[email protected]>
> Signed-off-by: NeilBrown <[email protected]>
>
> ---
> changes since v5:
>  - sel_make_policy_nodes now uses "goto out" on error from start_renaming_two_dentries()
>
> changes since v3:
>  added missing assignment to rd.mnt_idmap in ovl_cleanup_and_whiteout
> ---
>  fs/debugfs/inode.c           | 48 ++++++++++++--------------
>  fs/namei.c                   | 65 ++++++++++++++++++++++++++++++++++++
>  fs/overlayfs/dir.c           | 43 ++++++++++++++++--------
>  include/linux/namei.h        |  2 ++
>  security/selinux/selinuxfs.c | 15 +++++++--
>  5 files changed, 131 insertions(+), 42 deletions(-)

...

> diff --git a/fs/namei.c b/fs/namei.c
> index 4b740048df97..7f0384ceb976 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3877,6 +3877,71 @@ int start_renaming_dentry(struct renamedata *rd, int lookup_flags,
>  }
>  EXPORT_SYMBOL(start_renaming_dentry);
>
> +/**
> + * start_renaming_two_dentries - Lock to dentries in given parents for rename

I'm guessing you meant this to read "Lock *two* dentries ...".

Otherwise the SELinux changes look fine to me.

Acked-by: Paul Moore <[email protected]> (SELinux)

> + * @rd:           rename data containing parent
> + * @old_dentry:   dentry of name to move
> + * @new_dentry:   dentry to move to
> + *
> + * Ensure locks are in place for rename and check parentage is still correct.
> + *
> + * On success the two dentries are stored in @rd.old_dentry and
> + * @rd.new_dentry and @rd.old_parent and @rd.new_parent are confirmed to
> + * be the parents of the dentries.
> + *
> + * References and the lock can be dropped with end_renaming()
> + *
> + * Returns: zero or an error.
> + */

-- 
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