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runtime: ensure that searchAddr always refers to inUse memory

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This change formalizes an assumption made by the page allocator, which
is that (*pageAlloc).searchAddr should never refer to memory that is not
represented by (*pageAlloc).inUse. The portion of address space covered
by (*pageAlloc).inUse reflects the parts of the summary arrays which are
guaranteed to mapped, and so looking at any summary which is not
reflected there may cause a segfault.

In fact, this can happen today. This change thus also removes a
micro-optimization which is the only case which may cause
(*pageAlloc).searchAddr to point outside of any region covered by
(*pageAlloc).inUse, and adds a test verifying that the current segfault
can no longer occur.

Change-Id: I98b534f0ffba8656d3bd6d782f6fc22549ddf1c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/216697
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
Michael Anthony Knyszek 2020-01-28 19:59:19 +00:00 • committed by Michael Knyszek
parent b13ce14c4a
commit e7f9e17b79
4 changed files with 53 additions and 13 deletions
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7
src/runtime/mpagealloc.go
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@ -225,7 +225,9 @@ type pageAlloc struct {
// the bitmaps align better on zero-values.
chunks [1 << pallocChunksL1Bits]*[1 << pallocChunksL2Bits]pallocData
// The address to start an allocation search with.
// The address to start an allocation search with. It must never
// point to any memory that is not contained in inUse, i.e.
// inUse.contains(searchAddr) must always be true.
//
// When added with arenaBaseOffset, we guarantee that
// all valid heap addresses (when also added with
@ -237,7 +239,8 @@ type pageAlloc struct {
// space on architectures with segmented address spaces.
searchAddr uintptr
// The address to start a scavenge candidate search with.
// The address to start a scavenge candidate search with. It
// need not point to memory contained in inUse.
scavAddr uintptr
// The amount of memory scavenged since the last scavtrace print.