This change finally fully fixes mallocgc for asan after the recent
refactoring. Here is everything that changed:
Fix the accounting for the alloc header; large objects don't have them.
Mask out extra bits set from unrolling the bitmap for slice backing
stores in writeHeapBitsSmall. The redzone in asan mode makes it so that
dataSize is no longer an exact multiple of typ.Size_ in this case (a
new assumption I have recently discovered) but we didn't mask out any
extra bits, so we'd accidentally set bits in other allocations. Oops.
Move the initHeapBits optimization for the 8-byte scan sizeclass on
64-bit platforms up to mallocgc, out from writeHeapBitsSmall. So, this
actually caused a problem with asan when the optimization first landed,
but we missed it. The issue was then masked once we started passing the
redzone down into writeHeapBitsSmall, since the optimization would no
longer erroneously fire on asan. What happened was that dataSize would
be 8 (because that was the user-provided alloc size) so we'd skip
writing heap bits, but it would turn out the redzone bumped the size
class, so we'd actually *have* to write the heap bits for that size
class. This is not really a problem now *but* it caused problems for me
when debugging, since I would try to remove the red zone from dataSize
and this would trigger this bug again. Ultimately, this whole situation
is confusing because the check in writeHeapBitsSmall is *not* the same
as the check in initHeapBits. By moving this check up to mallocgc, we
can make the checks align better by matching on the sizeclass, so this
should be less error-prone in the future.
Change-Id: I1e9819223be23f722f3bf21e63e812f5fb557194
Reviewed-on: https://go-review.googlesource.com/c/go/+/622041 Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>