825ae71e56
newdefer() actually adds the new defer to the current g's defer chain. That happens even if we are on the system stack, in which case the g will be the g0 stack. For open-coded defers, we call newdefer() (only during panic processing) while on the system stack, so the new defer is unintentionally added to the g0._defer defer list. The code later correctly adds the defer to the user g's defer list. The g0._defer list is never used. However, that pointer on the g0._defer list can keep a defer struct alive that is intended to be garbage-collected (smaller defers use a defer pool, but larger-sized defer records are just GC'ed). freedefer() does not zero out pointers when it intends that a defer become garbage-collected. So, we can have the pointers in a defer that is held alive by g0._defer become invalid (in particular d.link). This is the cause of the bad pointer bug in this issue The fix is to change newdefer (only used in two places) to not add the new defer to the gp._defer list. We just do it after the call with the correct gp pointer. (As mentioned above, this code was already there after the newdefer in addOneOpenDeferFrame.) That ensures that defers will be correctly garbage-collected and eliminate the bad pointer. This fix definitely fixes the original repro. I added a test and tried hard to reproduce the bug (based on the original repro code), but awasn't actually able to cause the bug. However, the test is still an interesting mix of heap-allocated, stack-allocated, and open-coded defers. Fixes #37688 Change-Id: I1a481b9d9e9b9ba4e8726ef718a1f4512a2d6faf Reviewed-on: https://go-review.googlesource.com/c/go/+/224581 Run-TryBot: Dan Scales <danscales@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org> |
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