export/fuse: Pass default_permissions for mount

We do not do any permission checks in fuse_open(), so let the kernel do
them.  We already let fuse_getattr() report the proper UNIX permissions,
so this should work the way we want.

This causes a change in 308's reference output, because now opening a
non-writable export with O_RDWR fails already, instead of only actually
attempting to write to it.  (That is an improvement.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210625142317.271673-2-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/fuse.c

@@ -153,8 +153,12 @@ static int setup_fuse_export(FuseExport *exp, const char *mountpoint,
     struct fuse_args fuse_args;
     int ret;
 
-    /* Needs to match what fuse_init() sets.  Only max_read must be supplied. */
-    mount_opts = g_strdup_printf("max_read=%zu", FUSE_MAX_BOUNCE_BYTES);
+    /*
+     * max_read needs to match what fuse_init() sets.
+     * max_write need not be supplied.
+     */
+    mount_opts = g_strdup_printf("max_read=%zu,default_permissions",
+                                 FUSE_MAX_BOUNCE_BYTES);
 
     fuse_argv[0] = ""; /* Dummy program name */
     fuse_argv[1] = "-o";

tests/qemu-iotests/308

@@ -215,7 +215,8 @@ echo '=== Writable export ==='
 fuse_export_add 'export-mp' "'mountpoint': '$EXT_MP', 'writable': true"
 
 # Check that writing to the read-only export fails
-$QEMU_IO -f raw -c 'write -P 42 1M 64k' "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -f raw -c 'write -P 42 1M 64k' "$TEST_IMG" 2>&1 \
+    | _filter_qemu_io | _filter_testdir | _filter_imgfmt
 
 # But here it should work
 $QEMU_IO -f raw -c 'write -P 42 1M 64k' "$EXT_MP" | _filter_qemu_io

tests/qemu-iotests/308.out

@@ -91,7 +91,7 @@ virtual size: 0 B (0 bytes)
               'mountpoint': 'TEST_DIR/t.IMGFMT.fuse', 'writable': true
           } }
 {"return": {}}
-write failed: Permission denied
+qemu-io: can't open device TEST_DIR/t.IMGFMT: Could not open 'TEST_DIR/t.IMGFMT': Permission denied
 wrote 65536/65536 bytes at offset 1048576
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 1048576