lookup(): ensure that openat("/", "..", O_RESOLVE_BENEATH) fails

[kib]

Summary

PR: 269780

[emaste]

sys/kern/vfs_lookup.c

1087

Incidentally I think STRICTRELATIVE is slightly confusing now (as of 7359fdcf5ffa). Previously ni_strictrelative meant /* relative lookup only; no '..' */ - i.e., the "strict" part referred to disallowing ... (Unrelated to this review.)

[markj]

sys/kern/vfs_lookup.c

1086–1095

Do we really want to prohibit lookups in the (v_flag & VV_ROOT) && (cn_flags & NOCROSSMOUNT) case?

1090

This is redundant. If we break, then pr != NULL, and the next if-statement will handle it.

[emaste]

sys/kern/vfs_lookup.c

1087

This thought seemed familiar to me, and I found my forgotten D28445 again. Will take another look.

[kib]

sys/kern/vfs_lookup.c

1086–1095

I think it does not matter. NOCROSSMOUNT is strictly nfs server option, which does not use BENEATH. But ok, let me restructure code some more.

[kib]

Drop the jail checks.
Exclude NOCROSSMOUNT lookups.

[markj]

sys/kern/vfs_lookup.c

1088

The indentation here is wrong.