syscall, internal/syscall/unix: add Openat support for wasip1

The syscall package is mostly frozen, but wasip1 file syscall
support was added to syscall and the Open and Openat
implementations overlap. Implement Openat in syscall for
overall simplicity.

We already have syscall.Openat for some platforms, so this
doesn't add any new functions to syscall.

For #67002

Change-Id: Ia34b12ef11fc7a3b7832e07b3546a760c23efe5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/617378
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/internal/syscall/unix/at_wasip1.go

@@ -19,6 +19,10 @@ const (
 	UTIME_OMIT = -0x2
 )
 
+func Openat(dirfd int, path string, flags int, perm uint32) (int, error) {
+	return syscall.Openat(dirfd, path, flags, perm)
+}
+
 func Readlinkat(dirfd int, path string, buf []byte) (int, error) {
 	var nwritten size
 	errno := path_readlink(

src/syscall/fs_wasip1.go

@@ -520,7 +520,14 @@ func Open(path string, openmode int, perm uint32) (int, error) {
 		return -1, EINVAL
 	}
 	dirFd, pathPtr, pathLen := preparePath(path)
+	return openat(dirFd, pathPtr, pathLen, openmode, perm)
+}
 
+func Openat(dirFd int, path string, openmode int, perm uint32) (int, error) {
+	return openat(int32(dirFd), stringPointer(path), size(len(path)), openmode, perm)
+}
+
+func openat(dirFd int32, pathPtr unsafe.Pointer, pathLen size, openmode int, perm uint32) (int, error) {
 	var oflags oflags
 	if (openmode & O_CREATE) != 0 {
 		oflags |= OFLAG_CREATE
@@ -558,10 +565,15 @@ func Open(path string, openmode int, perm uint32) (int, error) {
 		fdflags |= FDFLAG_SYNC
 	}
 
+	var lflags lookupflags
+	if openmode&O_NOFOLLOW == 0 {
+		lflags = LOOKUP_SYMLINK_FOLLOW
+	}
+
 	var fd int32
 	errno := path_open(
 		dirFd,
-		LOOKUP_SYMLINK_FOLLOW,
+		lflags,
 		pathPtr,
 		pathLen,
 		oflags,

src/syscall/syscall_wasip1.go

@@ -223,6 +223,7 @@ const (
 	O_EXCL      = 0200
 	O_SYNC      = 010000
 	O_DIRECTORY = 020000
+	O_NOFOLLOW  = 0400
 
 	O_CLOEXEC = 0
 )