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cmd/compile: REFLECTMETHOD attribute is not set correctly #44207

cherrymui opened this issue Feb 10, 2021 · 2 comments

cmd/compile: REFLECTMETHOD attribute is not set correctly #44207

cherrymui opened this issue Feb 10, 2021 · 2 comments


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@cherrymui cherrymui commented Feb 10, 2021

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

tip (e9c9683)

Does this issue reproduce with the latest release?

Not in Go 1.15.

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


What did you do?

Run this program.

package main

import "reflect"

type S int

func (s S) M() { println(s) }

func main() {
	t := reflect.TypeOf(S(0))
	fn, _ := reflect.PtrTo(t).MethodByName("M")

What did you expect to see?

Print 0.

What did you see instead?


unexpected fault address 0xffffffffffffffff
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x1 addr=0xffffffffffffffff pc=0xffffffffffffffff]

goroutine 1 [running]:
runtime.throw(0x109ea7c, 0x5)
	/Users/cherryyz/src/go-tip/src/runtime/panic.go:1117 +0x72 fp=0xc000092b50 sp=0xc000092b20 pc=0x1030b12
	/Users/cherryyz/src/go-tip/src/runtime/signal_unix.go:741 +0x276 fp=0xc000092b88 sp=0xc000092b50 pc=0x10458f6
runtime.call16(0xc000098180, 0xc0000ae018, 0xc0000ae030, 0x800000008)
	/Users/cherryyz/src/go-tip/src/runtime/asm_amd64.s:550 +0x3e fp=0xc000092ba8 sp=0xc000092b88 pc=0x105ffbe, 0xc0000ae018, 0x13, 0x109e902, 0x4, 0xc000092ec0, 0x1, 0x1, 0xc000092e20, 0x107c50b, ...)
	/Users/cherryyz/src/go-tip/src/reflect/value.go:476 +0x8e7 fp=0xc000092db0 sp=0xc000092ba8 pc=0x1079ee7
reflect.Value.Call(0xc0000aa000, 0xc0000ae018, 0x13, 0xc000092ec0, 0x1, 0x1, 0x0, 0x10bb440, 0xc0000aa000)
	/Users/cherryyz/src/go-tip/src/reflect/value.go:337 +0xb9 fp=0xc000092e30 sp=0xc000092db0 pc=0x1079539
	/tmp/r.go:12 +0x1d6 fp=0xc000092f88 sp=0xc000092e30 pc=0x107fbb6
	/Users/cherryyz/src/go-tip/src/runtime/proc.go:225 +0x256 fp=0xc000092fe0 sp=0xc000092f88 pc=0x1033336
	/Users/cherryyz/src/go-tip/src/runtime/asm_amd64.s:1371 +0x1 fp=0xc000092fe8 sp=0xc000092fe0 pc=0x10618a1

What happens is that the main function used to (as in Go 1.15) have REFLECTMETHOD attribute set, so the linker knows it and marks all exported methods, but not now.

        0x0000 00000 (r.go:9)   TEXT    "".main(SB), ABIInternal, $336-0

(tip) vs.

        0x0000 00000 (r.go:9)   TEXT    "".main(SB), REFLECTMETHOD|ABIInternal, $256-0


It is because in cmd/compile/internal/gc/walk.go, it only marks reflect.Type.Method/MethodByName as interface calls (i.e. expecting a CALLINTER). But now it devirtualizes to a concrete call (CALLMETH).

.   AS2FUNC l(11) colas(true) tc(1)
.   .   CALLMETH l(11) tc(1) STRUCT-(reflect.Method, bool)
.   .   .   DOTMETH l(11) tc(1) reflect.(*rtype).MethodByName FUNC-method(*reflect.rtype) func(string) (reflect.Method, bool)
.   .   .   .   DOTTYPE l(11) tc(1) PTR-*reflect.rtype
.   .   .   .   .   CONVNOP l(11) tc(1) hascall reflect.Type
.   .   .   .   .   .   NAME-main.~R0 l(11) x(0) class(PAUTO) esc(no) tc(1) used reflect.Type
.   .   CALLMETH-list
.   .   .   LITERAL-"M" l(11) tc(1) string

(tip) vs.

.   AS2FUNC l(11) colas(true) tc(1)
.   .   CALLINTER l(11) tc(1) STRUCT-@0
.   .   .   DOTINTER l(11) x(152) tc(1) main.MethodByName FUNC-@0
.   .   .   .   CONVNOP l(11) tc(1) hascall reflect.Type
.   .   .   .   .   NAME-main.~R0 l(11) x(0) class(PAUTO) esc(no) tc(1) assigned used reflect.Type
.   .   CALLINTER-list
.   .   .   LITERAL-"M" l(11) tc(1) string


Without the REFLECTMETHOD attribute set, the linker prunes methods more aggressively. In this case, S.M is discarded.

@cherrymui cherrymui added this to the Go1.16 milestone Feb 10, 2021
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@mdempsky mdempsky commented Feb 10, 2021

Nice find.

It looks like just expanding walk to call usemethod for both ODOTINTER and ODOTMETH nodes should work. usemethod doesn't look at the receiver type at all, just the name and signature of the method called.


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@gopherbot gopherbot commented Feb 10, 2021

Change mentions this issue: cmd/compile: mark concrete call of reflect.(*rtype).Method as REFLECTMETHOD


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