Attempting to set a POSIX ACL on a tmpfs mount, where such tmpfs mount is made in a mount namespace owned by a non-initial user namespace, results in an "Invalid argument" error.


Steps to reproduce:

# unshare -U -m
(In another shell, write as root "0 1000 1\n100 10000 100\n" to both uid_map and gid_map of the resulting shell, where the 1000 in 0 1000 1 is the effective user/group ID of the original shell)
# mount -t tmpfs -o mode=0755 none /run
# mkdir -p -m 700 /run/test
# setfacl -m u:100:rx /run/test

Expected result: POSIX ACL set correctly on /run/test (mapped to UID 10000 in the init_user_ns)

Actual result: "setfacl: /run/test: Invalid argument"

Worked in Linux 5.16, broken in 5.17.