syscallcompat: check that we get NOFOLLOW wherever possible

...and fix the instances where the AT_SYMLINK_NOFOLLOW /
O_NOFOLLOW / O_EXCL flag was missing.

2 files changed, 19 insertions, 2 deletions

diff --git a/internal/syscallcompat/emulate.go b/internal/syscallcompat/emulate.go
index 59d0ea5..3af45f8 100644
--- a/internal/syscallcompat/emulate.go
+++ b/internal/syscallcompat/emulate.go
@@ -136,12 +136,12 @@ func emulateFchmodat(dirfd int, path string, mode uint32, flags int) (err error)
 	}
 	defer syscall.Fchdir(cwd)
 	// We also don't have Lchmod, so emulate it (poorly).
-	if flags&unix.AT_SYMLINK_NOFOLLOW > 0 {
+	if flags&unix.AT_SYMLINK_NOFOLLOW != 0 {
 		fi, err := os.Lstat(path)
 		if err != nil {
 			return err
 		}
-		if fi.Mode()&os.ModeSymlink > 0 {
+		if fi.Mode()&os.ModeSymlink != 0 {
 			return nil
 		}
 	}
diff --git a/internal/syscallcompat/sys_linux.go b/internal/syscallcompat/sys_linux.go
index 1ea56b5..e9ca7cb 100644
--- a/internal/syscallcompat/sys_linux.go
+++ b/internal/syscallcompat/sys_linux.go
@@ -6,6 +6,8 @@ import (
 	"syscall"
 	"unsafe"
 
+	"golang.org/x/sys/unix"
+
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
@@ -46,6 +48,11 @@ func Fallocate(fd int, mode uint32, off int64, len int64) (err error) {
 
 // Openat wraps the Openat syscall.
 func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error) {
+	// Why would we ever want to call this without O_NOFOLLOW and O_EXCL?
+	if !(flags&syscall.O_CREAT != 0 && flags&syscall.O_EXCL != 0) && flags&syscall.O_NOFOLLOW == 0 {
+		tlog.Warn.Printf("Openat: adding missing O_NOFOLLOW flag")
+		flags |= syscall.O_NOFOLLOW
+	}
 	return syscall.Openat(dirfd, path, flags, mode)
 }
 
@@ -82,11 +89,21 @@ func Dup3(oldfd int, newfd int, flags int) (err error) {
 
 // Fchmodat syscall.
 func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
+	// Why would we ever want to call this without AT_SYMLINK_NOFOLLOW?
+	if flags&unix.AT_SYMLINK_NOFOLLOW == 0 {
+		tlog.Warn.Printf("Fchmodat: adding missing AT_SYMLINK_NOFOLLOW flag")
+		flags |= unix.AT_SYMLINK_NOFOLLOW
+	}
 	return syscall.Fchmodat(dirfd, path, mode, flags)
 }
 
 // Fchownat syscall.
 func Fchownat(dirfd int, path string, uid int, gid int, flags int) (err error) {
+	// Why would we ever want to call this without AT_SYMLINK_NOFOLLOW?
+	if flags&unix.AT_SYMLINK_NOFOLLOW == 0 {
+		tlog.Warn.Printf("Fchownat: adding missing AT_SYMLINK_NOFOLLOW flag")
+		flags |= unix.AT_SYMLINK_NOFOLLOW
+	}
 	return syscall.Fchownat(dirfd, path, uid, gid, flags)
 }