runtime: fix system memory allocator on plan9

The following line in sysFree:
n += (n + memRound) &^ memRound
doubles value of n (n += n).
Which is wrong and can lead to memory corruption.

Fixes #9712

Change-Id: I3c141b71da11e38837c09408cf4f1d22e8f7f36e
Reviewed-on: https://go-review.googlesource.com/3602Reviewed-by: David du Colombier <0intro@gmail.com>

src/runtime/mem_plan9.go

@@ -9,21 +9,24 @@ import "unsafe"
 var bloc uintptr
 var memlock mutex
 
-const memRound = _PAGESIZE - 1
+func memRound(p uintptr) uintptr {
+	return (p + _PAGESIZE - 1) &^ (_PAGESIZE - 1)
+}
 
 func initBloc() {
-	bloc = uintptr(unsafe.Pointer(&end))
+	bloc = memRound(uintptr(unsafe.Pointer(&end)))
 }
 
 func sbrk(n uintptr) unsafe.Pointer {
 	lock(&memlock)
 	// Plan 9 sbrk from /sys/src/libc/9sys/sbrk.c
-	bl := (bloc + memRound) &^ memRound
+	bl := bloc
+	n = memRound(n)
 	if brk_(unsafe.Pointer(bl+n)) < 0 {
 		unlock(&memlock)
 		return nil
 	}
-	bloc = bl + n
+	bloc += n
 	unlock(&memlock)
 	return unsafe.Pointer(bl)
 }
@@ -42,7 +45,7 @@ func sysFree(v unsafe.Pointer, n uintptr, stat *uint64) {
 	// from tiny/mem.c
 	// Push pointer back if this is a free
 	// of the most recent sysAlloc.
-	n += (n + memRound) &^ memRound
+	n = memRound(n)
 	if bloc == uintptr(v)+n {
 		bloc -= n
 	}

src/runtime/pprof/trace_test.go

@@ -20,8 +20,6 @@ func skipTraceTestsIfNeeded(t *testing.T) {
 	switch runtime.GOOS {
 	case "solaris":
 		t.Skip("skipping: solaris timer can go backwards (http://golang.org/issue/8976)")
-	case "plan9":
-		t.Skip("skipping: plan9 tests fail with out of memory (http://golang.org/issue/9712")
 	}
 
 	switch runtime.GOARCH {
@@ -234,7 +232,7 @@ eventLoop:
 		for _, f := range ev.stk {
 			if strings.HasSuffix(f.file, "trace_test.go") &&
 				strings.HasSuffix(f.fn, "pprof_test.TestTraceSymbolize") &&
-				f.line == 218 {
+				f.line == 216 {
 				found = true
 				break eventLoop
 			}