Revert "syscallcompat: drop Faccessat AT_SYMLINK_NOFOLLOW helper"

Breaks mounting on MacOS: unix.Faccessat on Darwin does NOT (yet)
support AT_SYMLINK_NOFOLLOW. See d44fe89ba4 .

This reverts commit 0805a63df1.

internal/fusefrontend/fs.go

@@ -626,7 +626,7 @@ func (fs *FS) Access(relPath string, mode uint32, context *fuse.Context) (code f
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
-	err = unix.Faccessat(dirfd, cName, mode, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, mode)
 	syscall.Close(dirfd)
 	return fuse.ToStatus(err)
 }

internal/fusefrontend/openbackingdir_test.go

@@ -7,6 +7,7 @@ import (
 
 	"golang.org/x/sys/unix"
 
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/tests/test_helpers"
 )
 
@@ -45,11 +46,11 @@ func TestOpenBackingDir(t *testing.T) {
 		t.Fatal("cName should be .")
 	}
 
-	err = unix.Faccessat(dirfd, cName, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
-	err = unix.Faccessat(dirfd, ".", unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, ".", unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
@@ -62,7 +63,7 @@ func TestOpenBackingDir(t *testing.T) {
 	if cName == "" {
 		t.Fatal("cName should not be empty")
 	}
-	err = unix.Faccessat(dirfd, cName, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
@@ -75,7 +76,7 @@ func TestOpenBackingDir(t *testing.T) {
 	if cName == "" {
 		t.Fatal("cName should not be empty")
 	}
-	err = unix.Faccessat(dirfd, cName, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
@@ -94,7 +95,7 @@ func TestOpenBackingDir(t *testing.T) {
 	if len(cName) >= 255 {
 		t.Fatalf("cName is too long: %q", cName)
 	}
-	err = unix.Faccessat(dirfd, cName, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
@@ -125,11 +126,11 @@ func TestOpenBackingDirPlaintextNames(t *testing.T) {
 	if cName != "." {
 		t.Fatal("cName should be .")
 	}
-	err = unix.Faccessat(dirfd, cName, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
-	err = unix.Faccessat(dirfd, ".", unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, ".", unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
@@ -142,7 +143,7 @@ func TestOpenBackingDirPlaintextNames(t *testing.T) {
 	if cName != "dir1" {
 		t.Fatalf("wrong cName: %q", cName)
 	}
-	err = unix.Faccessat(dirfd, cName, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}
@@ -155,7 +156,7 @@ func TestOpenBackingDirPlaintextNames(t *testing.T) {
 	if cName != "dir2" {
 		t.Fatalf("wrong cName: %q", cName)
 	}
-	err = unix.Faccessat(dirfd, cName, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
 	if err != nil {
 		t.Error(err)
 	}

internal/fusefrontend_reverse/rfs.go

@@ -255,7 +255,7 @@ func (rfs *ReverseFS) Access(relPath string, mode uint32, context *fuse.Context)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
-	err = unix.Faccessat(dirfd, name, mode, unix.AT_SYMLINK_NOFOLLOW)
+	err = syscallcompat.Faccessat(dirfd, name, mode)
 	syscall.Close(dirfd)
 	return fuse.ToStatus(err)
 }

internal/syscallcompat/sys_common.go

@@ -29,6 +29,21 @@ func Readlinkat(dirfd int, path string) (string, error) {
 	}
 }
 
+// Faccessat exists both in Linux and in MacOS 10.10+, but the Linux version
+// DOES NOT support any flags. Emulate AT_SYMLINK_NOFOLLOW like glibc does.
+func Faccessat(dirfd int, path string, mode uint32) error {
+	var st unix.Stat_t
+	err := Fstatat(dirfd, path, &st, unix.AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		return err
+	}
+	if st.Mode&syscall.S_IFMT == syscall.S_IFLNK {
+		// Pretend that a symlink is always accessible
+		return nil
+	}
+	return unix.Faccessat(dirfd, path, mode, 0)
+}
+
 // Openat wraps the Openat syscall.
 func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error) {
 	if flags&syscall.O_CREAT != 0 {

tests/reverse/correctness_test.go

@@ -11,6 +11,7 @@ import (
 	"golang.org/x/sys/unix"
 
 	"github.com/rfjakob/gocryptfs/internal/ctlsock"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/tests/test_helpers"
 )
 
@@ -158,12 +159,12 @@ func TestAccess(t *testing.T) {
 	}
 	for _, n := range names {
 		// Check if file exists - this should never fail
-		err = unix.Faccessat(unix.AT_FDCWD, dirB+"/"+n, unix.F_OK, unix.AT_SYMLINK_NOFOLLOW)
+		err = syscallcompat.Faccessat(unix.AT_FDCWD, dirB+"/"+n, unix.F_OK)
 		if err != nil {
 			t.Errorf("%s: %v", n, err)
 		}
 		// Check if file is readable
-		err = unix.Faccessat(unix.AT_FDCWD, dirB+"/"+n, unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
+		err = syscallcompat.Faccessat(unix.AT_FDCWD, dirB+"/"+n, unix.R_OK)
 		if err != nil {
 			t.Logf("%s: %v", n, err)
 		}