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--

2 files changed, 9 insertions, 3 deletions

diff --git a/internal/fusefrontend/file.go b/internal/fusefrontend/file.go
index 780a792..546526b 100644
--- a/internal/fusefrontend/file.go
+++ b/internal/fusefrontend/file.go
@@ -459,7 +459,10 @@ func (f *file) Chmod(mode uint32) fuse.Status {
 	f.fdLock.RLock()
 	defer f.fdLock.RUnlock()
 
-	return fuse.ToStatus(f.fd.Chmod(os.FileMode(mode)))
+	// os.File.Chmod goes through the "syscallMode" translation function that messes
+	// up the suid and sgid bits. So use syscall.Fchmod directly.
+	err := syscall.Fchmod(f.intFd(), mode)
+	return fuse.ToStatus(err)
 }
 
 func (f *file) Chown(uid uint32, gid uint32) fuse.Status {
diff --git a/internal/fusefrontend/fs.go b/internal/fusefrontend/fs.go
index 9ae57fa..49bd031 100644
--- a/internal/fusefrontend/fs.go
+++ b/internal/fusefrontend/fs.go
@@ -158,11 +158,14 @@ func (fs *FS) Chmod(path string, mode uint32, context *fuse.Context) (code fuse.
 	if fs.isFiltered(path) {
 		return fuse.EPERM
 	}
-	cPath, err := fs.encryptPath(path)
+	cPath, err := fs.getBackingPath(path)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
-	return fs.FileSystem.Chmod(cPath, mode, context)
+	// os.Chmod goes through the "syscallMode" translation function that messes
+	// up the suid and sgid bits. So use syscall.Chmod directly.
+	err = syscall.Chmod(cPath, mode)
+	return fuse.ToStatus(err)
 }
 
 func (fs *FS) Chown(path string, uid uint32, gid uint32, context *fuse.Context) (code fuse.Status) {