runtime: inform arena placement using sbrk(0)

On 32-bit architectures (or if we fail to map a 64-bit-style arena),
we try to map the heap arena just above the end of the process image.
While we can accept any address, using lower addresses is preferable
because lower addresses cause us to map less of the heap bitmap.

However, if a program is linked against C code that has global
constructors, those constructors may call brk/sbrk to allocate memory
(e.g., many C malloc implementations do this for small allocations).
The brk also starts just above the process image, so this may adjust
the brk past the beginning of where we want to put the heap arena. In
this case, the kernel will pick a different address for the arena and
it will usually be very high (at least, as these things go in a 32-bit
address space).

Fix this by consulting the current value of the brk and using this in
addition to the end of the process image to compute the initial arena
placement.

This is implemented only on Linux currently, since we have no evidence
that it's an issue on any other OSes.

Fixes #19831.

Change-Id: Id64b45d08d8c91e4f50d92d0339146250b04f2f8
Reviewed-on: https://go-review.googlesource.com/39810
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, 15 insertions, 0 deletions

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index 7517f1284e..2e6c3aca0a 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -312,6 +312,15 @@ func mallocinit() {
 		// When that gets used up, we'll start asking the kernel
 		// for any memory anywhere.
 
+		// We want to start the arena low, but if we're linked
+		// against C code, it's possible global constructors
+		// have called malloc and adjusted the process' brk.
+		// Query the brk so we can avoid trying to map the
+		// arena over it (which will cause the kernel to put
+		// the arena somewhere else, likely at a high
+		// address).
+		procBrk := sbrk0()
+
 		// If we fail to allocate, try again with a smaller arena.
 		// This is necessary on Android L where we share a process
 		// with ART, which reserves virtual memory aggressively.
@@ -336,6 +345,12 @@ func mallocinit() {
 			// to a MB boundary.
 			p = round(firstmoduledata.end+(1<<18), 1<<20)
 			pSize = bitmapSize + spansSize + arenaSize + _PageSize
+			if p <= procBrk && procBrk < p+pSize {
+				// Move the start above the brk,
+				// leaving some room for future brk
+				// expansion.
+				p = round(procBrk+(1<<20), 1<<20)
+			}
 			p = uintptr(sysReserve(unsafe.Pointer(p), pSize, &reserved))
 			if p != 0 {
 				break