selftests/fuse: add the missing FUSE_HAS_SYNCFS privilege test

Commit a6ca1a573e1f ("selftests/fuse: add test for FUSE_HAS_SYNCFS
privilege gating") registered test_syncfs in the fuse selftest Makefile
and .gitignore, but the test source was not part of the applied commit,
so building the fuse selftests fails:

  make[1]: *** No rule to make target '.../test_syncfs', needed by 'all'.

Add test_syncfs.c as posted in Jimmy Zuber's v3 submission. The test
talks the raw FUSE protocol over /dev/fuse, so it can choose whether to
advertise FUSE_HAS_SYNCFS and directly observe whether the kernel
forwards a FUSE_SYNCFS request. It covers three cases:

  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.

Fixes: a6ca1a573e1f ("selftests/fuse: add test for FUSE_HAS_SYNCFS privilege gating")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202609021808.5OxsIter-lkp@intel.com/
Closes: https://lore.kernel.org/r/202609230913.G4keWCG7-lkp@intel.com/
Link: https://lore.kernel.org/r/20260820130158.254808-3-jamz@amazon.com
Originally-by: Jimmy Zuber <jamz@amazon.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

0 files changed, 0 insertions, 0 deletions