darwin: syscallcompat: Openat: use O_SYMLINK

Also add tests that opening a symlink (using unix.O_PATH | unix.O_NOFOLLOW)
works.

https://github.com/rfjakob/gocryptfs/issues/993

3 files changed, 12 insertions, 4 deletions

diff --git a/internal/syscallcompat/sys_common.go b/internal/syscallcompat/sys_common.go
index 70ee633..3cb9ffa 100644
--- a/internal/syscallcompat/sys_common.go
+++ b/internal/syscallcompat/sys_common.go
@@ -54,10 +54,10 @@ func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error)
 			flags |= syscall.O_EXCL
 		}
 	} else {
-		// If O_CREAT is not used, we should use O_NOFOLLOW
-		if flags&syscall.O_NOFOLLOW == 0 {
-			tlog.Warn.Printf("Openat: O_NOFOLLOW missing: flags = %#x", flags)
-			flags |= syscall.O_NOFOLLOW
+		// If O_CREAT is not used, we should use O_NOFOLLOW or O_SYMLINK
+		if flags&(unix.O_NOFOLLOW|OpenatFlagNofollowSymlink) == 0 {
+			tlog.Warn.Printf("Openat: O_NOFOLLOW/O_SYMLINK missing: flags = %#x", flags)
+			flags |= unix.O_NOFOLLOW
 		}
 	}
 
diff --git a/internal/syscallcompat/sys_darwin.go b/internal/syscallcompat/sys_darwin.go
index 0ebdd3b..ef19f24 100644
--- a/internal/syscallcompat/sys_darwin.go
+++ b/internal/syscallcompat/sys_darwin.go
@@ -27,6 +27,10 @@ const (
 	// Only exists on Linux. Define here to fix build failure, even though
 	// we will never see this flag.
 	RENAME_WHITEOUT = 1 << 30
+
+	// On Darwin we use O_SYMLINK which allows opening a symlink itself.
+	// On Linux, we only have O_NOFOLLOW.
+	OpenatFlagNofollowSymlink = unix.O_SYMLINK
 )
 
 // Unfortunately fsetattrlist does not have a syscall wrapper yet.
diff --git a/internal/syscallcompat/sys_linux.go b/internal/syscallcompat/sys_linux.go
index 19d2c56..71478af 100644
--- a/internal/syscallcompat/sys_linux.go
+++ b/internal/syscallcompat/sys_linux.go
@@ -28,6 +28,10 @@ const (
 	RENAME_NOREPLACE = unix.RENAME_NOREPLACE
 	RENAME_WHITEOUT  = unix.RENAME_WHITEOUT
 	RENAME_EXCHANGE  = unix.RENAME_EXCHANGE
+
+	// On Darwin we use O_SYMLINK which allows opening a symlink itself.
+	// On Linux, we only have O_NOFOLLOW.
+	OpenatFlagNofollowSymlink = unix.O_NOFOLLOW
 )
 
 var preallocWarn sync.Once