main, fusefrontend: enable suid functionality

FUSE filesystems are mounted with "nosuid" by default. If we run as root,
we can use device files by passing the opposite mount option, "suid".

Also we have to use syscall.Chmod instead of os.Chmod because the
portability translation layer "syscallMode" messes up the sgid
and suid bits.

Fixes 70% of the failures in xfstests generic/193. The remaining are
related to truncate, but we err on the safe side:

    $ diff -u tests/generic/193.out /home/jakob/src/fuse-xfstests/results//generic/193.out.bad
    [...]
     check that suid/sgid bits are cleared after successful truncate...
     with no exec perm
     before: -rwSr-Sr--
    -after:  -rw-r-Sr--
    +after:  -rw-r--r--

1 files changed, 2 insertions, 0 deletions

diff --git a/main.go b/main.go
index 8b2f41a..68dfb0a 100644
--- a/main.go
+++ b/main.go
@@ -406,6 +406,8 @@ func initFuseFrontend(key []byte, args argContainer, confFile *configfile.ConfFi
 		// FUSE filesystems are mounted with "nodev" by default. If we run as root,
 		// we can use device files by passing the opposite mount option, "dev".
 		mOpts.Options = append(mOpts.Options, "dev")
+		// Same thing for "nosuid". If we run as root, we can pass "suid".
+		mOpts.Options = append(mOpts.Options, "suid")
 	}
 	// Set values shown in "df -T" and friends
 	// First column, "Filesystem"