cmd/compile: panic: runtime error: invalid memory address or nil pointer dereference [dev.regabi] #43701

ALTree · 2021-01-14T20:17:05Z

dev.regabi branch

$ gotip version
go version devel +35b9c66601 Thu Jan 14 17:35:39 2021 +0000 linux/amd64

package p

func f() {
	var st struct {
		s string
		i int16
	}
	_ = func() {
		var m map[int16]int
		m[st.i] = 0
	}
}

$ gotip tool compile crash.go 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x51bd59]

goroutine 21 [running]:
cmd/internal/obj.(*LSym).prepwrite(0x0, 0xc00031c000, 0x0, 0x8)
	/home/alberto/go/src/cmd/internal/obj/data.go:71 +0x119
cmd/internal/obj.(*LSym).WriteInt(0x0, 0xc00031c000, 0x0, 0x8, 0x1)
	/home/alberto/go/src/cmd/internal/obj/data.go:100 +0x4d
cmd/compile/internal/objw.UintN(0x0, 0x0, 0x1, 0x8, 0x1)
	/home/alberto/go/src/cmd/compile/internal/objw/objw.go:34 +0x6e
cmd/compile/internal/objw.Uintptr(...)
	/home/alberto/go/src/cmd/compile/internal/objw/objw.go:27
cmd/compile/internal/ssagen.emitStackObjects(0xc000389da0, 0xc0000ea150)
	/home/alberto/go/src/cmd/compile/internal/ssagen/ssa.go:6492 +0x22a
cmd/compile/internal/ssagen.genssa(0xc000083500, 0xc0000ea150)
	/home/alberto/go/src/cmd/compile/internal/ssagen/ssa.go:6525 +0xf2
cmd/compile/internal/ssagen.Compile(0xc0000fa420, 0x0)
	/home/alberto/go/src/cmd/compile/internal/ssagen/pgen.go:178 +0x2a5
cmd/compile/internal/gc.compileFunctions.func2.1(0xc000308310, 0xc0000fa420, 0xc0000b8550, 0xc0000b6ce0)
	/home/alberto/go/src/cmd/compile/internal/gc/compile.go:143 +0x65
created by cmd/compile/internal/gc.compileFunctions.func2
	/home/alberto/go/src/cmd/compile/internal/gc/compile.go:141 +0x8e

cc @mdempsky @cuonglm

The text was updated successfully, but these errors were encountered:

cuonglm · 2021-01-15T01:24:35Z

git bisect points to https://go-review.googlesource.com/c/go/+/281352

mdempsky · 2021-01-15T02:54:49Z

Thanks. I'm guessing the test case tickles some mismatch between the logic in gc.prepareFunc for deciding whether to create the stkobj symbol, and the code in ssagen.emitStackObjects.

I wonder if we can just move that code from gc.prepareFunc to ssagen.emitStackObjects? The comments say that code isn't safe to run during SSA generation, but I don't think that's true anymore. At least looking at them briefly, I see mutexes being used. I'd recommend trying to relocate the code and see if it passes the racecompile trybot.

It would also be interesting to know what variables are being caught by ssagen.emitStackObjects but missed by gc.prepareFunc. Not because it's necessary for the fix per se, but to double-check that we understand the failure.

@cuonglm Do you want to look into fixing this?

cuonglm · 2021-01-15T03:05:09Z

@mdempsky I did some investigation already, the problem is after CL 281352, the closure var &st isn't pushed to fn.Dcl in frontend anymore, so gc.prepareFunc won't see it. In SSA, we did add it to fn.Dcl in buildssa, then calling emitStackObjects inside genssa.

I will be AFK for couple of hours, I can try a fix when I'm back.

mdempsky · 2021-01-15T03:23:14Z

@cuonglm Thanks. No rush on the fix if you're interested in it.

Hm, I can believe that's the issue, but I'm not immediately seeing how it led to this issue. Do you mind getting the output from ir.Dump("dcl", fn.Dcl) in both gc.prepareFunc and ssagen.emitStackObjects, both before and after that CL and maybe at tip of dev.regabi too? (I've made further closure changes since then, so it's possible there are multiple CLs in play here.)

Sorry for asking so much. Also, if simply moving the sym generation to emitStackObjects fixes the issue and passes the racecompile slowbot, I think it's reasonable to go ahead and commit that while we continue understanding why it failed in the first place.

mdempsky · 2021-01-15T03:23:57Z

Oh, and finally: are we crashing trying to write out stack objects for f or for f.func1?

mdempsky · 2021-01-15T03:26:12Z

@ALTree Thanks for the issue report and the reproducer. I'm assuming that's minimized down from some real world code? If so, are you able to share that? (Not important, just curious.)

cuonglm · 2021-01-15T03:35:33Z

Oh, and finally: are we crashing trying to write out stack objects for f or for f.func1?

it's f.func1.

mdempsky · 2021-01-15T03:55:47Z

Oh, that's surprising. I didn't expect f.func1 needed stack objects. I assumed f was crashing.

gopherbot · 2021-01-15T04:21:19Z

Change https://golang.org/cl/284112 mentions this issue: [dev.regabi] cmd/compile: create stkobj symbol inside emitStackObjects

ALTree · 2021-01-15T07:41:50Z

Thanks for the issue report and the reproducer. I'm assuming that's minimized down from some real world code? If so, are you able to share that? (Not important, just curious.)

@mdempsky Sorry, there's no real world code; the crasher was found by a fuzzer.

mdempsky · 2021-01-15T08:17:33Z

@ALTree No apology needed. I'd rather a fuzzer discover compiler issues than an end user. :)

gopherbot · 2021-01-15T09:03:04Z

Change https://golang.org/cl/284075 mentions this issue: [dev.regabi] cmd/compile: don't promote Byval CaptureVars if Addrtaken

gopherbot · 2021-01-15T09:03:05Z

Change https://golang.org/cl/284076 mentions this issue: [dev.regabi] cmd/compile: move stkobj symbol generation to SSA

gopherbot · 2021-01-15T09:03:06Z

Change https://golang.org/cl/284077 mentions this issue: [dev.regabi] cmd/compile: unexport reflectdata.WriteType

We decide during escape analysis whether to pass closure variables by value or reference. One of the factors that's considered is whether a variable has had its address taken. However, this analysis is based only on the user-written source code, whereas order+walk may introduce rewrites that take the address of a variable (e.g., passing a uint16 key by reference to the size-generic map runtime builtins). Typically this would be harmless, albeit suboptimal. But in #43701 it manifested as needing a stack object for a function where we didn't realize we needed one up front when we generate symbols. Probably we should just generate symbols on demand, now that those routines are all concurrent-safe, but this is a first fix. Thanks to Alberto Donizetti for reporting the issue, and Cuong Manh Le for initial investigation. Fixes #43701. Change-Id: I16d87e9150723dcb16de7b43f2a8f3cd807a9437 Reviewed-on: https://go-review.googlesource.com/c/go/+/284075 Trust: Matthew Dempsky <mdempsky@google.com> Run-TryBot: Matthew Dempsky <mdempsky@google.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

The code for allocating linksyms and recording that we need runtime type descriptors is now concurrent-safe, so move it to where those symbols are actually needed to reduce complexity and risk of failing to generate all needed symbols in advance. For #43701. Change-Id: I759d2508213ac9a4e0b504b51a75fa10dfa37a8d Reviewed-on: https://go-review.googlesource.com/c/go/+/284076 Run-TryBot: Matthew Dempsky <mdempsky@google.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com> Trust: Matthew Dempsky <mdempsky@google.com>

WriteType isn't safe for direct concurrent use, and users should instead use TypeLinksym or another higher-level API provided by reflectdata. After the previous CL, there are no remaining uses of WriteType elsewhere in the compiler, so unexport it to keep it that way. For #43701. [git-generate] cd src/cmd/compile/internal/reflectdata rf ' mv WriteType writeType ' Change-Id: I294a78be570a47feb38a1ad4eaae7723653d5991 Reviewed-on: https://go-review.googlesource.com/c/go/+/284077 Trust: Matthew Dempsky <mdempsky@google.com> Run-TryBot: Matthew Dempsky <mdempsky@google.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

cuonglm · 2021-01-15T17:55:27Z

@mdempsky @ALTree we can close this issue now.

ALTree added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jan 14, 2021

ALTree closed this as completed Jan 15, 2021

mdempsky mentioned this issue Jan 26, 2021

all: merge dev.typeparams to master during Go 1.17 #43931

Closed

golang locked and limited conversation to collaborators Jan 15, 2022

gopherbot added the FrozenDueToAge label Jan 15, 2022

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cmd/compile: panic: runtime error: invalid memory address or nil pointer dereference [dev.regabi] #43701

cmd/compile: panic: runtime error: invalid memory address or nil pointer dereference [dev.regabi] #43701

ALTree commented Jan 14, 2021

cuonglm commented Jan 15, 2021 •

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mdempsky commented Jan 15, 2021

cuonglm commented Jan 15, 2021

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cuonglm commented Jan 15, 2021

mdempsky commented Jan 15, 2021

gopherbot commented Jan 15, 2021

ALTree commented Jan 15, 2021

mdempsky commented Jan 15, 2021

gopherbot commented Jan 15, 2021

gopherbot commented Jan 15, 2021

gopherbot commented Jan 15, 2021

cuonglm commented Jan 15, 2021

cmd/compile: panic: runtime error: invalid memory address or nil pointer dereference [dev.regabi] #43701

cmd/compile: panic: runtime error: invalid memory address or nil pointer dereference [dev.regabi] #43701

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ALTree commented Jan 14, 2021

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cuonglm commented Jan 15, 2021 •

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