CVE-2026-99084: readdir: take no inode lock on an immutable directory

In the Linux kernel, the following vulnerability has been resolved:

readdir: take no inode lock on an immutable directory

iteratedir() takes the directory's irwsem shared and holds it across ->iterateshared(). For a directory that never has an entry and is never removed the lock keeps nothing still, it only orders every reader and every writer of that inode behind each other.

For the directory of a nullfs instance that matters. The instance of the initial mount namespace is the root of every empty mount namespace and the private instance is the root of every kernel thread, so one inode is shared across users who have nothing else in common. And a reader can hold the lock for as long as it likes: back the getdents() buffer with a mapping of a file on a FUSE mount of your own, let the copy of "." and ".." fault and let the server wait. Queue an exclusive taker behind it, a mkdir() in that directory goes through startdirop() before the read-only mount is reported, and from then on every lookup that misses the dcache in that directory, every create and every mount on it waits until the server answers. One user of an empty mount namespace stalls all the others.

Add FOPIMMUTABLE for the file operations of a directory that never changes and is never removed and let iteratedir() skip the lock for it. The flag never changes for a file, ->fpos is protected by fposlock since directories are FMODEATOMICPOS, ISDEADDIR can't be set on such a directory and neither touchatime() nor fsnotify take irwsem. Set it on the nullfs directory. The placeholder directories of libfs never have an entry either but their owners remove them, so they keep the lock.