reflect: TestStructOfWithInterface failure with cgocheck=2

[sternix]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.10 freebsd/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/sternix/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="freebsd"
GOOS="freebsd"
GOPATH="/home/sternix/go"
GORACE=""
GOROOT="/opt/go/1_10/go"
GOTMPDIR=""
GOTOOLDIR="/opt/go/1_10/go/pkg/tool/freebsd_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build875902433=/tmp/go-build -gno-record-gcc-switches"

What did you do?

i install go to the directory /opt/go/1_10
and set env variable GODEBUG cgocheck=2

cd /opt/go/1_10/go/src/reflect
go test -v .

....
=== RUN TestStructOfWithInterface
write of Go pointer 0xc4203a0000 to non-Go memory 0x800bd15d0
fatal error: Go pointer stored into non-Go memory

runtime stack:
runtime.throw(0x6337d9, 0x24)
/opt/go/1_10/go/src/runtime/panic.go:619 +0x81
runtime.cgoCheckWriteBarrier.func1()
/opt/go/1_10/go/src/runtime/cgocheck.go:48 +0xa9
runtime.systemstack(0x0)
/opt/go/1_10/go/src/runtime/asm_amd64.s:409 +0x79
runtime.mstart()
/opt/go/1_10/go/src/runtime/proc.go:1170

goroutine 58 [running]:
runtime.systemstack_switch()
/opt/go/1_10/go/src/runtime/asm_amd64.s:363 fp=0xc420071920 sp=0xc420071918 pc=0x455e90
runtime.cgoCheckWriteBarrier(0x800bd15d0, 0xc4203a0000)
/opt/go/1_10/go/src/runtime/cgocheck.go:46 +0xaa fp=0xc420071958 sp=0xc420071920 pc=0x40299a
runtime.wbBufFlush(0x800bd15d0, 0xc4203a0000)
/opt/go/1_10/go/src/runtime/mwbbuf.go:169 +0x6b fp=0xc420071978 sp=0xc420071958 pc=0x426d7b
runtime.gcWriteBarrier(0x20, 0x0, 0x9b0e60, 0x800bd15c8, 0xc420466fc0, 0x199, 0xc4203a0000, 0xc420466fc8, 0x4af056, 0x5e6bc0, ...)
/opt/go/1_10/go/src/runtime/asm_amd64.s:2442 +0xb4 fp=0xc420071a00 sp=0xc420071978 pc=0x458a34
runtime.getitab(0x5ef920, 0xc4203a0000, 0xc4203a0001, 0xc420466fc8)
/opt/go/1_10/go/src/runtime/iface.go:68 +0x43b fp=0xc420071a78 sp=0xc420071a00 pc=0x40d17b
runtime.assertE2I2(0x5ef920, 0xc4203a0000, 0xc420466fc8, 0xc4203a0000, 0xc420466fc8, 0x82)
/opt/go/1_10/go/src/runtime/iface.go:592 +0x43 fp=0xc420071aa8 sp=0xc420071a78 pc=0x40e5c3
reflect_test.TestStructOfWithInterface(0xc4200ac4b0)
/opt/go/1_10/go/src/reflect/all_test.go:4890 +0x742 fp=0xc420071fa8 sp=0xc420071aa8 pc=0x56f2e2
testing.tRunner(0xc4200ac4b0, 0x639640)
/opt/go/1_10/go/src/testing/testing.go:777 +0xd0 fp=0xc420071fd0 sp=0xc420071fa8 pc=0x4dd920
runtime.goexit()
/opt/go/1_10/go/src/runtime/asm_amd64.s:2361 +0x1 fp=0xc420071fd8 sp=0xc420071fd0 pc=0x458971
created by testing.(*T).Run
/opt/go/1_10/go/src/testing/testing.go:824 +0x2e0

goroutine 1 [chan receive]:
testing.(*T).Run(0xc4200ac4b0, 0x62fb4f, 0x19, 0x639640, 0x1)
/opt/go/1_10/go/src/testing/testing.go:825 +0x301
testing.runTests.func1(0xc4200ac000)
/opt/go/1_10/go/src/testing/testing.go:1063 +0x64
testing.tRunner(0xc4200ac000, 0xc42003ddf8)
/opt/go/1_10/go/src/testing/testing.go:777 +0xd0
testing.runTests(0xc42000a540, 0x98c240, 0x8e, 0x8e, 0x40fc01)
/opt/go/1_10/go/src/testing/testing.go:1061 +0x2c4
testing.(*M).Run(0xc4200a8000, 0x0)
/opt/go/1_10/go/src/testing/testing.go:978 +0x171
main.main()
_testmain.go:354 +0x151

goroutine 45 [sleep]:
time.Sleep(0x3b9aca00)
/opt/go/1_10/go/src/runtime/time.go:102 +0x166
reflect_test.selectWatcher()
/opt/go/1_10/go/src/reflect/all_test.go:1541 +0x40
created by reflect_test.TestSelect.func1
/opt/go/1_10/go/src/reflect/all_test.go:1271 +0x35
FAIL reflect 1.594s

What did you expect to see?

SUCCESS

What did you see instead?

FAIL

[ianlancetaylor]

This is because getitab is storing a Go pointer into memory allocated by persistentalloc. The code is fine but it's confusing the cgo check.

It may work to simply add a //go:notinheap comment to itab. CC @aclements

[gopherbot]

Change https://golang.org/cl/95676 mentions this issue: runtime: mark itab go:notinheap

[ianlancetaylor]

I'm concerned that this bug indicates a deeper problem when using types generated by reflect. We can create itab entries for such types, but the itabs are allocated using persistentalloc and as far as I can tell the GC never examines itabTable. That suggests that it is possible for the runtime to create an itab that points to types allocated by reflect, for those types to be garbage collected, and thus for the runtime to wind up with dangling pointers in itabTable.

[ianlancetaylor]

I no longer think that the previous comment is a concern, because the reflect package keeps a cache of generated types, so they will never be garbage collected.

[gopherbot]

Change https://golang.org/cl/155697 mentions this issue: runtime: skip writes to persistent memory in cgo checker

[FiloSottile]

Assigning to @aclements for review of CL 155697.