syscall: support Faccessat flags argument

The Linux kernel faccessat system call does not take a flags parameter.
The flag parameter to the C library faccessat function is implemented in C.
The syscall.Faccessat function takes a flags parameter. In older releases
we have passed the flags parameter to the kernel, which ignored it.
In CL 120015 we started returning an error if any flags were set.
That seems clearly better than ignoring them, but it turns out that some
code was using the flags. The code was previously subtly broken.
Now it is obviously broken. That is better, but we can do better still:
we can implement the flags as the C library does. That is what this CL does.

Change-Id: I259bd6f240c3951e939b81c3032dead3d9c567b4
Reviewed-on: https://go-review.googlesource.com/126415
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 32 insertions, 8 deletions

diff --git a/src/syscall/syscall_linux_test.go b/src/syscall/syscall_linux_test.go
index 24158bb8b8..99de6ebaf2 100644
--- a/src/syscall/syscall_linux_test.go
+++ b/src/syscall/syscall_linux_test.go
@@ -57,25 +57,27 @@ const (
 	_AT_SYMLINK_NOFOLLOW = 0x100
 	_AT_FDCWD            = -0x64
 	_AT_EACCESS          = 0x200
+	_F_OK                = 0
+	_R_OK                = 4
 )
 
 func TestFaccessat(t *testing.T) {
 	defer chtmpdir(t)()
 	touch(t, "file1")
 
-	err := syscall.Faccessat(_AT_FDCWD, "file1", syscall.O_RDONLY, 0)
+	err := syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, 0)
 	if err != nil {
 		t.Errorf("Faccessat: unexpected error: %v", err)
 	}
 
-	err = syscall.Faccessat(_AT_FDCWD, "file1", syscall.O_RDONLY, 2)
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, 2)
 	if err != syscall.EINVAL {
 		t.Errorf("Faccessat: unexpected error: %v, want EINVAL", err)
 	}
 
-	err = syscall.Faccessat(_AT_FDCWD, "file1", syscall.O_RDONLY, _AT_EACCESS)
-	if err != syscall.EOPNOTSUPP {
-		t.Errorf("Faccessat: unexpected error: %v, want EOPNOTSUPP", err)
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, _AT_EACCESS)
+	if err != nil {
+		t.Errorf("Faccessat: unexpected error: %v", err)
 	}
 
 	err = os.Symlink("file1", "symlink1")
@@ -83,9 +85,31 @@ func TestFaccessat(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	err = syscall.Faccessat(_AT_FDCWD, "symlink1", syscall.O_RDONLY, _AT_SYMLINK_NOFOLLOW)
-	if err != syscall.EOPNOTSUPP {
-		t.Errorf("Faccessat: unexpected error: %v, want EOPNOTSUPP", err)
+	err = syscall.Faccessat(_AT_FDCWD, "symlink1", _R_OK, _AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		t.Errorf("Faccessat SYMLINK_NOFOLLOW: unexpected error %v", err)
+	}
+
+	// We can't really test _AT_SYMLINK_NOFOLLOW, because there
+	// doesn't seem to be any way to change the mode of a symlink.
+	// We don't test _AT_EACCESS because such tests are only
+	// meaningful if run as root.
+
+	err = syscall.Fchmodat(_AT_FDCWD, "file1", 0, 0)
+	if err != nil {
+		t.Errorf("Fchmodat: unexpected error %v", err)
+	}
+
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _F_OK, _AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		t.Errorf("Faccessat: unexpected error: %v", err)
+	}
+
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, _AT_SYMLINK_NOFOLLOW)
+	if err != syscall.EACCES {
+		if syscall.Getuid() != 0 {
+			t.Errorf("Faccessat: unexpected error: %v, want EACCES", err)
+		}
 	}
 }