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selftests/fuse: add test for FUSE_HAS_SYNCFS privilege gating

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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>
This commit is contained in:
Jimmy Zuber 2026-08-20 13:01:58 +00:00 committed by Miklos Szeredi
parent 989db66818
commit a6ca1a573e
2 changed files with 2 additions and 1 deletions
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1
tools/testing/selftests/filesystems/fuse/.gitignore vendored
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@ -1,4 +1,5 @@
# SPDX-License-Identifier: GPL-2.0-only
fuse_mnt
fusectl_test
test_syncfs
write_extend_eof_test

2
tools/testing/selftests/filesystems/fuse/Makefile
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@ -2,7 +2,7 @@
CFLAGS += -Wall -O2 -g $(KHDR_INCLUDES)
TEST_GEN_PROGS := fusectl_test
TEST_GEN_PROGS := fusectl_test test_syncfs
TEST_GEN_PROGS += write_extend_eof_test
TEST_GEN_FILES := fuse_mnt