Re: [PATCH 02/13] rcu pathwalk: prevent bogus hard errors from may_lookup()

On Sun, Feb 04, 2024 at 02:17:28AM +0000, Al Viro wrote:

If lazy call of ->permission() returns a hard error, check that
try_to_unlazy() succeeds before returning it.  That both makes
life easier for ->permission() instances and closes the race
in ENOTDIR handling - it is possible that positive d_can_lookup()
seen in link_path_walk() applies to the state *after* unlink() +
mkdir(), while nd->inode matches the state prior to that.

Normally seeing e.g. EACCES from permission check in rcu pathwalk
means that with some timings non-rcu pathwalk would've run into
the same; however, running into a non-executable regular file
in the middle of a pathname would not get to permission check -
it would fail with ENOTDIR instead.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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Reviewed-by: Christian Brauner <brauner@kernel.org>