linux.git/tools/testing/selftests/filesystems/idmapped_tmpfile.c, branch master Linux kernel source tree selftests/filesystems: test O_TMPFILE creation on idmapped mounts 2026-07-01T13:26:18+00:00 Christian Brauner brauner@kernel.org 2026-06-15T12:52:19+00:00 d943e68edc5cb98192d38e31373bb6b6a73230c6 Add a regression test for the fsuidgid_has_mapping() check in vfs_tmpfile(). It idmaps a detached tmpfs mount so that the caller-visible id range [0, 10000) maps onto the on-disk range [10000, 20000) and checks that: - a caller whose fsuid/fsgid fall outside that range cannot create an O_TMPFILE through the mount and gets -EOVERFLOW instead of an inode owned by (uid_t)-1; - a mapped caller can create an O_TMPFILE, link it into the namespace, and the ownership round-trips through the mount idmap: it is reported as 0 through the mount and stored as 10000 on the underlying tmpfs. The test runs entirely as root and uses setfsuid()/setfsgid() to become the unmapped caller, so it needs no helper user. The layer directory is world-writable so that an unmapped caller still clears the directory permission check and reaches the fsuidgid_has_mapping() test. Link: https://patch.msgid.link/20260615-work-idmapped-tmpfile-v1-2-754a94d81f83@kernel.org Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org> 
Add a regression test for the fsuidgid_has_mapping() check in
vfs_tmpfile().  It idmaps a detached tmpfs mount so that the
caller-visible id range [0, 10000) maps onto the on-disk range
[10000, 20000) and checks that:

  - a caller whose fsuid/fsgid fall outside that range cannot create an
    O_TMPFILE through the mount and gets -EOVERFLOW instead of an inode
    owned by (uid_t)-1;

  - a mapped caller can create an O_TMPFILE, link it into the namespace,
    and the ownership round-trips through the mount idmap: it is reported
    as 0 through the mount and stored as 10000 on the underlying tmpfs.

The test runs entirely as root and uses setfsuid()/setfsgid() to become
the unmapped caller, so it needs no helper user.  The layer directory is
world-writable so that an unmapped caller still clears the directory
permission check and reaches the fsuidgid_has_mapping() test.

Link: https://patch.msgid.link/20260615-work-idmapped-tmpfile-v1-2-754a94d81f83@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>