syscall: reduce memory allocated by UTF16FromString

The function allocated a buffer larger than needed.
Fixes #59967.

Signed-off-by: Frediano Ziglio <frediano.ziglio@nextdlp.com>
Change-Id: I5f30a135acf5f27d6c2ef4bc4abef5144da4dc94
Reviewed-on: https://go-review.googlesource.com/c/go/+/492575
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/syscall/syscall_windows.go

@@ -42,9 +42,13 @@ func UTF16FromString(s string) ([]uint16, error) {
 	if bytealg.IndexByteString(s, 0) != -1 {
 		return nil, EINVAL
 	}
-	// In the worst case all characters require two uint16.
-	// Also account for the terminating NULL character.
-	buf := make([]uint16, 0, len(s)*2+1)
+	// Valid UTF-8 characters between 1 and 3 bytes require one uint16.
+	// Valid UTF-8 characters of 4 bytes require two uint16.
+	// Bytes with invalid UTF-8 encoding require maximum one uint16 per byte.
+	// So the number of UTF-8 code units (len(s)) is always greater or
+	// equal than the number of UTF-16 code units.
+	// Also account for the terminating NUL character.
+	buf := make([]uint16, 0, len(s)+1)
 	for _, r := range s {
 		buf = utf16.AppendRune(buf, r)
 	}

src/syscall/syscall_windows_test.go

@@ -169,3 +169,31 @@ int main(int argc, char *argv[])
 		t.Fatalf("c program output is wrong: got %q, want %q", have, want)
 	}
 }
+
+func FuzzUTF16FromString(f *testing.F) {
+	f.Add("hi")           // ASCII
+	f.Add("â")            // latin1
+	f.Add("ねこ")           // plane 0
+	f.Add("😃")            // extra Plane 0
+	f.Add("\x90")         // invalid byte
+	f.Add("\xe3\x81")     // truncated
+	f.Add("\xe3\xc1\x81") // invalid middle byte
+
+	f.Fuzz(func(t *testing.T, tst string) {
+		res, err := syscall.UTF16FromString(tst)
+		if err != nil {
+			if strings.Contains(tst, "\x00") {
+				t.Skipf("input %q contains a NUL byte", tst)
+			}
+			t.Fatalf("UTF16FromString(%q): %v", tst, err)
+		}
+		t.Logf("UTF16FromString(%q) = %04x", tst, res)
+
+		if len(res) < 1 || res[len(res)-1] != 0 {
+			t.Fatalf("missing NUL terminator")
+		}
+		if len(res) > len(tst)+1 {
+			t.Fatalf("len(%04x) > len(%q)+1", res, tst)
+		}
+	})
+}