cmd/compile: crash in type loop detection

[fhs]

What version of Go are you using (go version)?

$ go version
go version devel +33d72fd412 Mon Feb 15 01:54:22 2021 +0000 linux/amd64

Does this issue reproduce with the latest release?

No (tested with go1.15.8)

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/fhs/.cache/go-build"
GOENV="/home/fhs/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/fhs/go/pkg/mod"
GOOS="linux"
GOPATH="/home/fhs/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/big/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/big/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel +33d72fd412 Mon Feb 15 01:54:22 2021 +0000"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2015226142=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version devel +33d72fd412 Mon Feb 15 01:54:22 2021 +0000 linux/amd64
GOROOT/bin/go tool compile -V: compile version devel +33d72fd412 Mon Feb 15 01:54:22 2021 +0000
uname -sr: Linux 5.10.11-arch1-1
LSB Version:	1.4
Distributor ID:	Arch
Description:	Arch Linux
Release:	rolling
Codename:	n/a
/usr/lib/libc.so.6: GNU C Library (GNU libc) release release version 2.33.
gdb --version: GNU gdb (GDB) 10.1

What did you do?

Run this program: https://play.golang.org/p/JghmS05HSAj

What did you expect to see?

The compiler shouldn't crash

What did you see instead?

$ go run crash.go 
# command-line-arguments
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xa42da5]

goroutine 1 [running]:
cmd/compile/internal/gc.findTypeLoop(0xc0000d8e40, 0xc0001088d8, 0xc00009c160)
	/home/big/go/src/cmd/compile/internal/gc/align.go:202 +0x105
cmd/compile/internal/gc.findTypeLoop(0xc0000d8d80, 0xc0001088d8, 0x1)
	/home/big/go/src/cmd/compile/internal/gc/align.go:223 +0x225
cmd/compile/internal/gc.findTypeLoop(0xc0000d8d20, 0xc0001088d8, 0xc0000980d8)
	/home/big/go/src/cmd/compile/internal/gc/align.go:202 +0x117
cmd/compile/internal/gc.findTypeLoop(0xc0000d91a0, 0xc0001088d8, 0x0)
	/home/big/go/src/cmd/compile/internal/gc/align.go:216 +0x2c5
cmd/compile/internal/gc.findTypeLoop(0xc0000d9140, 0xc0001088d8, 0x11)
	/home/big/go/src/cmd/compile/internal/gc/align.go:202 +0x117
cmd/compile/internal/gc.reportTypeLoop(0xc0000d9140)
	/home/big/go/src/cmd/compile/internal/gc/align.go:240 +0x76
cmd/compile/internal/gc.dowidth(0xc0000d9140)
	/home/big/go/src/cmd/compile/internal/gc/align.go:282 +0x1105
cmd/compile/internal/gc.widstruct(0xc0000d9140, 0xc0000d9140, 0x0, 0x1, 0x203000)
	/home/big/go/src/cmd/compile/internal/gc/align.go:114 +0xd3
cmd/compile/internal/gc.dowidth(0xc0000d9140)
	/home/big/go/src/cmd/compile/internal/gc/align.go:435 +0x4cf
cmd/compile/internal/gc.widstruct(0xc0000d91a0, 0xc0000d91a0, 0x0, 0x1, 0x8)
	/home/big/go/src/cmd/compile/internal/gc/align.go:114 +0xd3
cmd/compile/internal/gc.dowidth(0xc0000d91a0)
	/home/big/go/src/cmd/compile/internal/gc/align.go:435 +0x4cf
cmd/compile/internal/gc.resumecheckwidth()
	/home/big/go/src/cmd/compile/internal/gc/align.go:526 +0x4c
cmd/compile/internal/gc.typecheckdef(0xc0000c9b30)
	/home/big/go/src/cmd/compile/internal/gc/typecheck.go:3695 +0x8e5
cmd/compile/internal/gc.typecheck1(0xc0000c9b30, 0x4, 0x0)
	/home/big/go/src/cmd/compile/internal/gc/typecheck.go:343 +0xbace
cmd/compile/internal/gc.typecheck(0xc0000c9b30, 0x4, 0x0)
	/home/big/go/src/cmd/compile/internal/gc/typecheck.go:299 +0x785
cmd/compile/internal/gc.typecheck1(0xc0000ce300, 0x1, 0x0)
	/home/big/go/src/cmd/compile/internal/gc/typecheck.go:2077 +0x4e65
cmd/compile/internal/gc.typecheck(0xc0000ce300, 0x1, 0x0)
	/home/big/go/src/cmd/compile/internal/gc/typecheck.go:299 +0x785
cmd/compile/internal/gc.Main(0xcc7d50)
	/home/big/go/src/cmd/compile/internal/gc/main.go:603 +0x2aa5
main.main()
	/home/big/go/src/cmd/compile/main.go:52 +0xb1

[tpaschalis]

Might be obvious, but I ran a git bisect, this behavior was there all along in 0e85fd7.

Changing this line to also check for p.Ntype != nil fixes the issue; tests pass and we get the correct error message

./main.go:9:6: invalid recursive type T2
	/Users/paschalistsilias/44266-golang/main.go:9:6: T2 refers to
	/Users/paschalistsilias/44266-golang/main.go:9:6: T2

I can prepare and send a CL with a test if there's no objection, or could I be missing something in the bigger picture?

[ALTree]

cc @mdempsky

[gopherbot]

Change https://golang.org/cl/292149 mentions this issue: cmd/compile: fix ICE in reporting of recursive types with interfaces

[gopherbot]

Change https://golang.org/cl/292469 mentions this issue: cmd/compile: fix ICE in findTypeLoop with symbols from imported packages

[toothrot]

It looks like @mdempsky and @randall77 are cc'd on the CLs, also cc'ing @griesemer as an owner.

This is currently labeled as a release-blocking issue, and is currently blocking the Go 1.16 release today.

[randall77]

Removed release blocker. This is a compiler crash on an invalid program, which shouldn't block the release.

[odeke-em]

Thanks Keith for the status report. In that case I’ll also move this issue to Go1.17, and out of the Go1.16 milestone.

[mdempsky]

I believe this issue is already fixed on dev.regabi with golang.org/cl/274294. If this isn't a blocker for Go 1.16, I suggest we wait until dev.regabi is merged to fix this on the main branch. And we can just backport a fix to the Go 1.16 release branch when appropriate.

[gopherbot]

Change https://golang.org/cl/292470 mentions this issue: [dev.regabi] test: add test for findTypeLoop with symbols from other packages