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cmd/link: incorrect GC bitmap when global's type is in another shared object [1.14 backport] #39955

gopherbot opened this issue Jun 30, 2020 · 6 comments


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@gopherbot gopherbot commented Jun 30, 2020

@aclements requested issue #39927 to be considered for backport to the next 1.14 minor release.

@gopherbot, please backport to Go 1.14 and 1.15.

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@dmitshur dmitshur commented Jun 30, 2020

@aclements Can you please include a short rationale about why the backport might be needed? (Per MinorReleases.) Thanks.


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@gopherbot gopherbot commented Jul 1, 2020

Change mentions this issue: [release-branch.go1.14] cmd/link: fix GC data reading from shared library (attempt 2)


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@aclements aclements commented Jul 1, 2020

Rationale for backport: This causes the garbage collector to incorrectly free reachable objects when using shared build mode, leading to memory corruption and crashes. There is no workaround (other than not using shared build mode).


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@dmitshur dmitshur commented Jul 10, 2020

Backport approved for Go 1.14 (this issue) and 1.13 (#39956) based on that rationale.



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@andybons andybons removed this from the Go1.14.5 milestone Jul 14, 2020
@andybons andybons added this to the Go1.14.6 milestone Jul 14, 2020
@andybons andybons removed this from the Go1.14.6 milestone Jul 16, 2020
@andybons andybons added this to the Go1.14.7 milestone Jul 16, 2020
@toothrot toothrot removed this from the Go1.14.7 milestone Aug 6, 2020
@toothrot toothrot added this to the Go1.14.8 milestone Aug 6, 2020
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@gopherbot gopherbot commented Aug 21, 2020

Closed by merging 73268be to release-branch.go1.14.


@gopherbot gopherbot closed this Aug 21, 2020
gopherbot pushed a commit that referenced this issue Aug 21, 2020
…rary (attempt 2)

This is a backport of CL 240621. This is not a clean cherry-pick,
as Go 1.15 switches to the new linker while it is still the old
linker here. Backporting is straightforward, though.

When linking against a Go shared library, when a global variable
in the main module has a type defined in the shared library, the
linker needs to pull the GC data from the shared library to build
the GC program for the global variable. Currently, this fails
silently, as the shared library file is closed too early and the
read failed (with no error check), causing a zero GC map emitted
for the variable, which in turn causes the runtime to treat the
variable as pointerless.

For now, fix this by keeping the file open. In the future we may
want to use mmap to read from the shared library instead.

Also add error checking. And fix a (mostly harmless) mistake in
size caluculation.

Also remove an erroneous condition for ARM64. ARM64 has a special
case to get the addend from the relocation on the gcdata field.
But that doesn't actually work. And it's no longer necessary to
have any special case, since the addend is now applied directly
to the gcdata field on ARM64, like on all the other platforms.

Fixes #39955.
Updates #39927.

Change-Id: I01c82422b9f67e872d833336885935bc509bc91b
Run-TryBot: Cherry Zhang <>
TryBot-Result: Gobot Gobot <>
Reviewed-by: Than McIntosh <>
(cherry picked from commit 7799756)
Reviewed-by: Austin Clements <>
@dmitshur dmitshur removed this from the Go1.14.8 milestone Sep 1, 2020
@dmitshur dmitshur added this to the Go1.14.9 milestone Sep 1, 2020
@golang golang locked and limited conversation to collaborators Sep 1, 2021
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5 participants