selftests/fuse: add test for FUSE_HAS_SYNCFS privilege gating

Add a selftest that talks the raw FUSE protocol over /dev/fuse (rather
than via libfuse, which negotiates INIT internally) so it can both choose
whether to advertise FUSE_HAS_SYNCFS and directly observe whether a
FUSE_SYNCFS opcode is forwarded by the kernel.

Three cases are covered:

  T1: host-root mount, server sets FUSE_HAS_SYNCFS
      -> FUSE_SYNCFS must reach the server.
  T2: host-root mount, server does not opt in
      -> FUSE_SYNCFS must not be sent (back-compat).
  T3: server opts in but opened /dev/fuse without CAP_SYS_ADMIN while still
      in the initial user namespace
      -> FUSE_SYNCFS must be withheld.  This is the case that distinguishes
         gating on the server's privilege from gating on the mount's user
         namespace.

Signed-off-by: Jimmy Zuber <jamz@amazon.com>
Assisted-by: Claude:claude-opus-4-8 [Claude-Code]
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

0 files changed, 0 insertions, 0 deletions