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runtime: "fatal error: unexpected signal" 0xC0000005 on Windows for a small program with a large allocation [1.14 backport] #37480

gopherbot opened this issue Feb 26, 2020 · 3 comments
CherryPickApproved FrozenDueToAge


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@gopherbot gopherbot commented Feb 26, 2020

@aclements requested issue #37470 to be considered for backport to the next 1.14 minor release.

@gopherbot, please open a backport to Go 1.14 and Go 1.15.

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@dmitshur dmitshur commented Mar 5, 2020

Approving because this is a serious problem (crash in rare but specific cases) with no workaround.

@dmitshur dmitshur added CherryPickApproved and removed CherryPickCandidate labels Mar 5, 2020
@cagedmantis cagedmantis removed this from the Go1.14.1 milestone Mar 19, 2020
@cagedmantis cagedmantis added this to the Go1.14.2 milestone Mar 19, 2020
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@gopherbot gopherbot commented Mar 20, 2020

Change mentions this issue: [release-branch.go1.14] runtime: fix rounding in materializeGCProg

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@gopherbot gopherbot commented Apr 7, 2020

Closed by merging c8d1e4c to release-branch.go1.14.

gopherbot pushed a commit that referenced this issue Apr 7, 2020
materializeGCProg allocates a temporary buffer for unrolling a GC
program. Unfortunately, when computing the size of the buffer, it
rounds *down* the number of bytes needed to store bitmap before
rounding up the number of pages needed to store those bytes. The fact
that it rounds up to pages usually mitigates the rounding down, but
the type from #37470 exists right on the boundary where this doesn't

type Sequencer struct {
	htable [1 << 17]uint32
	buf    []byte

On 64-bit, this GC bitmap is exactly 8 KiB of zeros, followed by three
one bits. Hence, this needs 8193 bytes of storage, but the current
math in materializeGCProg rounds *down* the three one bits to 8192
bytes. Since this is exactly pageSize, the next step of rounding up to
the page size doesn't mitigate this error, and materializeGCProg
allocates a buffer that is one byte too small. runGCProg then writes
one byte past the end of this buffer, causing either a segfault (if
you're lucky!) or memory corruption.

Updates #37470.
Fixes #37480.

Change-Id: Iad24c463c501cd9b1dc1924bc2ad007991a094a0
Run-TryBot: Austin Clements <>
Reviewed-by: Cherry Zhang <>
TryBot-Result: Gobot Gobot <>
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CherryPickApproved FrozenDueToAge
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