runtime: separate error result for mmap

Currently mmap returns an unsafe.Pointer that encodes OS errors as
values less than 4096. In practice this is okay, but it borders on
being really unsafe: for example, the value has to be checked
immediately after return and if stack copying were ever to observe
such a value, it would panic. It's also not remotely idiomatic.

Fix this by making mmap return a separate pointer value and error,
like a normal Go function.

Updates #22218.

Change-Id: Iefd965095ffc82cc91118872753a5d39d785c3a6
Reviewed-on: https://go-review.googlesource.com/71270
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 10 insertions, 16 deletions

diff --git a/src/runtime/runtime_mmap_test.go b/src/runtime/runtime_mmap_test.go
index 2eca6b9e88..57c38bc5dc 100644
--- a/src/runtime/runtime_mmap_test.go
+++ b/src/runtime/runtime_mmap_test.go
@@ -16,16 +16,10 @@ import (
 // what the code in mem_bsd.go, mem_darwin.go, and mem_linux.go expects.
 // See the uses of ENOMEM in sysMap in those files.
 func TestMmapErrorSign(t *testing.T) {
-	p := runtime.Mmap(nil, ^uintptr(0)&^(runtime.GetPhysPageSize()-1), 0, runtime.MAP_ANON|runtime.MAP_PRIVATE, -1, 0)
+	p, err := runtime.Mmap(nil, ^uintptr(0)&^(runtime.GetPhysPageSize()-1), 0, runtime.MAP_ANON|runtime.MAP_PRIVATE, -1, 0)
 
-	// The runtime.mmap function is nosplit, but t.Errorf is not.
-	// Reset the pointer so that we don't get an "invalid stack
-	// pointer" error from t.Errorf if we call it.
-	v := uintptr(p)
-	p = nil
-
-	if v != runtime.ENOMEM {
-		t.Errorf("mmap = %v, want %v", v, runtime.ENOMEM)
+	if p != nil || err != runtime.ENOMEM {
+		t.Errorf("mmap = %v, %v, want nil, %v", p, err, runtime.ENOMEM)
 	}
 }
 
@@ -35,20 +29,20 @@ func TestPhysPageSize(t *testing.T) {
 	ps := runtime.GetPhysPageSize()
 
 	// Get a region of memory to play with. This should be page-aligned.
-	b := uintptr(runtime.Mmap(nil, 2*ps, 0, runtime.MAP_ANON|runtime.MAP_PRIVATE, -1, 0))
-	if b < 4096 {
-		t.Fatalf("Mmap: %v", b)
+	b, err := runtime.Mmap(nil, 2*ps, 0, runtime.MAP_ANON|runtime.MAP_PRIVATE, -1, 0)
+	if err != 0 {
+		t.Fatalf("Mmap: %v", err)
 	}
 
 	// Mmap should fail at a half page into the buffer.
-	err := uintptr(runtime.Mmap(unsafe.Pointer(uintptr(b)+ps/2), ps, 0, runtime.MAP_ANON|runtime.MAP_PRIVATE|runtime.MAP_FIXED, -1, 0))
-	if err >= 4096 {
+	_, err = runtime.Mmap(unsafe.Pointer(uintptr(b)+ps/2), ps, 0, runtime.MAP_ANON|runtime.MAP_PRIVATE|runtime.MAP_FIXED, -1, 0)
+	if err == 0 {
 		t.Errorf("Mmap should have failed with half-page alignment %d, but succeeded: %v", ps/2, err)
 	}
 
 	// Mmap should succeed at a full page into the buffer.
-	err = uintptr(runtime.Mmap(unsafe.Pointer(uintptr(b)+ps), ps, 0, runtime.MAP_ANON|runtime.MAP_PRIVATE|runtime.MAP_FIXED, -1, 0))
-	if err < 4096 {
+	_, err = runtime.Mmap(unsafe.Pointer(uintptr(b)+ps), ps, 0, runtime.MAP_ANON|runtime.MAP_PRIVATE|runtime.MAP_FIXED, -1, 0)
+	if err != 0 {
 		t.Errorf("Mmap at full-page alignment %d failed: %v", ps, err)
 	}
 }