fusefrontend: run acl Setxattr in user context

The result of setting an acl depends on who runs the
operation!

Fixes fuse-xfstests generic/375
(see https://github.com/rfjakob/fuse-xfstests/wiki/results_2021-05-19)

internal/fusefrontend/node_xattr.go

@@ -7,6 +7,8 @@ import (
 	"strings"
 	"syscall"
 
+	"github.com/hanwen/go-fuse/v2/fuse"
+
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
@@ -90,7 +92,12 @@ func (n *Node) Setxattr(ctx context.Context, attr string, data []byte, flags uin
 
 	// ACLs are passed through without encryption
 	if isAcl(attr) {
-		return n.setXAttr(attr, data, flags)
+		// result of setting an acl depends on the user doing it
+		var context *fuse.Context
+		if rn.args.PreserveOwner {
+			context = toFuseCtx(ctx)
+		}
+		return n.setXAttr(context, attr, data, flags)
 	}
 
 	cAttr, err := rn.encryptXattrName(attr)
@@ -98,7 +105,7 @@ func (n *Node) Setxattr(ctx context.Context, attr string, data []byte, flags uin
 		return syscall.EINVAL
 	}
 	cData := rn.encryptXattrValue(data)
-	return n.setXAttr(cAttr, cData, flags)
+	return n.setXAttr(nil, cAttr, cData, flags)
 }
 
 // RemoveXAttr - FUSE call.

internal/fusefrontend/node_xattr_darwin.go

@@ -6,6 +6,7 @@ import (
 	"golang.org/x/sys/unix"
 
 	"github.com/hanwen/go-fuse/v2/fs"
+	"github.com/hanwen/go-fuse/v2/fuse"
 
 	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 )
@@ -40,7 +41,7 @@ func (n *Node) getXAttr(cAttr string) (out []byte, errno syscall.Errno) {
 	return cData, 0
 }
 
-func (n *Node) setXAttr(cAttr string, cData []byte, flags uint32) (errno syscall.Errno) {
+func (n *Node) setXAttr(context *fuse.Context, cAttr string, cData []byte, flags uint32) (errno syscall.Errno) {
 	dirfd, cName, errno := n.prepareAtSyscall("")
 	if errno != 0 {
 		return

internal/fusefrontend/node_xattr_linux.go

@@ -7,6 +7,7 @@ import (
 	"golang.org/x/sys/unix"
 
 	"github.com/hanwen/go-fuse/v2/fs"
+	"github.com/hanwen/go-fuse/v2/fuse"
 
 	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 )
@@ -30,7 +31,7 @@ func (n *Node) getXAttr(cAttr string) (out []byte, errno syscall.Errno) {
 	return cData, 0
 }
 
-func (n *Node) setXAttr(cAttr string, cData []byte, flags uint32) (errno syscall.Errno) {
+func (n *Node) setXAttr(context *fuse.Context, cAttr string, cData []byte, flags uint32) (errno syscall.Errno) {
 	dirfd, cName, errno := n.prepareAtSyscall("")
 	if errno != 0 {
 		return
@@ -38,7 +39,8 @@ func (n *Node) setXAttr(cAttr string, cData []byte, flags uint32) (errno syscall
 	defer syscall.Close(dirfd)
 
 	procPath := fmt.Sprintf("/proc/self/fd/%d/%s", dirfd, cName)
-	return fs.ToErrno(unix.Lsetxattr(procPath, cAttr, cData, int(flags)))
+
+	return fs.ToErrno(syscallcompat.LsetxattrUser(procPath, cAttr, cData, int(flags), context))
 }
 
 func (n *Node) removeXAttr(cAttr string) (errno syscall.Errno) {

internal/syscallcompat/sys_linux.go

@@ -130,6 +130,9 @@ func asUser(f func() (int, error), context *fuse.Context) (int, error) {
 //
 // It switches the current thread to the new user, performs the syscall,
 // and switches back.
+//
+// If `context` is nil, this function behaves like ordinary Openat (no
+// user switching).
 func OpenatUser(dirfd int, path string, flags int, mode uint32, context *fuse.Context) (fd int, err error) {
 	f := func() (int, error) {
 		return Openat(dirfd, path, flags, mode)
@@ -143,6 +146,7 @@ func Mknodat(dirfd int, path string, mode uint32, dev int) (err error) {
 }
 
 // MknodatUser runs the Mknodat syscall in the context of a different user.
+// If `context` is nil, this function behaves like ordinary Mknodat.
 //
 // See OpenatUser() for how this works.
 func MknodatUser(dirfd int, path string, mode uint32, dev int, context *fuse.Context) (err error) {
@@ -195,6 +199,7 @@ func FchmodatNofollow(dirfd int, path string, mode uint32) (err error) {
 }
 
 // SymlinkatUser runs the Symlinkat syscall in the context of a different user.
+// If `context` is nil, this function behaves like ordinary Symlinkat.
 //
 // See OpenatUser() for how this works.
 func SymlinkatUser(oldpath string, newdirfd int, newpath string, context *fuse.Context) (err error) {
@@ -207,6 +212,7 @@ func SymlinkatUser(oldpath string, newdirfd int, newpath string, context *fuse.C
 }
 
 // MkdiratUser runs the Mkdirat syscall in the context of a different user.
+// If `context` is nil, this function behaves like ordinary Mkdirat.
 //
 // See OpenatUser() for how this works.
 func MkdiratUser(dirfd int, path string, mode uint32, context *fuse.Context) (err error) {
@@ -218,6 +224,20 @@ func MkdiratUser(dirfd int, path string, mode uint32, context *fuse.Context) (er
 	return err
 }
 
+// LsetxattrUser runs the Lsetxattr syscall in the context of a different user.
+// This is useful when setting ACLs, as the result depends on the user running
+// the operation (see fuse-xfstests generic/375).
+//
+// If `context` is nil, this function behaves like ordinary Lsetxattr.
+func LsetxattrUser(path string, attr string, data []byte, flags int, context *fuse.Context) (err error) {
+	f := func() (int, error) {
+		err := unix.Lsetxattr(path, attr, data, flags)
+		return -1, err
+	}
+	_, err = asUser(f, context)
+	return err
+}
+
 func timesToTimespec(a *time.Time, m *time.Time) []unix.Timespec {
 	ts := make([]unix.Timespec, 2)
 	ts[0] = unix.Timespec(fuse.UtimeToTimespec(a))