From: Ian Lance Taylor <iant@golang.org>
Date: Tue, 16 Apr 2013 16:08:06 +0000 (-0700)
Subject: runtime: if span was scavenged, mark it as needing to be zeroed
X-Git-Tag: go1.1rc2~93
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=9ad236ab7215b406e867028ef295445a2c4b8b5d;p=gostls13.git

runtime: if span was scavenged, mark it as needing to be zeroed

Update #4979.

R=dvyukov, r, bradfitz
CC=golang-dev
https://golang.org/cl/8697045
---

diff --git a/src/pkg/runtime/mheap.c b/src/pkg/runtime/mheap.c
index 177f406596..f4fbbee7a4 100644
--- a/src/pkg/runtime/mheap.c
+++ b/src/pkg/runtime/mheap.c
@@ -121,6 +121,25 @@ HaveSpan:
 	s->state = MSpanInUse;
 	mstats.heap_idle -= s->npages<<PageShift;
 	mstats.heap_released -= s->npreleased<<PageShift;
+	if(s->npreleased > 0) {
+		// We have called runtime·SysUnused with these pages, and on
+		// Unix systems it called madvise.  At this point at least
+		// some BSD-based kernels will return these pages either as
+		// zeros or with the old data.  For our caller, the first word
+		// in the page indicates whether the span contains zeros or
+		// not (this word was set when the span was freed by
+		// MCentral_Free or runtime·MCentral_FreeSpan).  If the first
+		// page in the span is returned as zeros, and some subsequent
+		// page is returned with the old data, then we will be
+		// returning a span that is assumed to be all zeros, but the
+		// actual data will not be all zeros.  Avoid that problem by
+		// explicitly marking the span as not being zeroed, just in
+		// case.  The beadbead constant we use here means nothing, it
+		// is just a unique constant not seen elsewhere in the
+		// runtime, as a clue in case it turns up unexpectedly in
+		// memory or in a stack trace.
+		*(uintptr*)(s->start<<PageShift) = (uintptr)0xbeadbeadbeadbeadULL;
+	}
 	s->npreleased = 0;
 
 	if(s->npages > npage) {