runtime: checkptrAlignment crash on nil pointer

[mdempsky]

With Go 1.17rc1 (and probably since 4bb0847), this program crashes when run with checkptr enabled (e.g., with -race):

package main

import (
	"runtime"
	"time"
	"unsafe"
)

func main() {
	go func() {
		for {
			runtime.GC()
		}
	}()

	go func() {
		for i := 0; ; i++ {
			recurse(i % 1024)
		}
	}()

	time.Sleep(time.Second)
}

func recurse(n int) {
	// Inflate the stack so runtime.shrinkstack gets called during GC
	if n > 0 {
		recurse(n - 1)
	}

	var p unsafe.Pointer
	_ = (*int)(p)
}

Easy fix is adding an explicit nil check at the start of checkptrAlignment.

[gopherbot]

Change https://golang.org/cl/338029 mentions this issue: runtime: don't crash on nil pointers in checkptrAlignment

[mdempsky]

@golang/release Do I need to do anything to make sure b39e0f4 is included in Go 1.17?

It's a runtime bug that causes valid programs to sporadically fail, albeit only when checkptr is enabled (e.g., when the race detector or msan are used).

[cagedmantis]

@mdempsky It's marked as a release blocker which means Go 1.17 will not be released until the issue is resolved. This will probably block a second release candidate as well.

[cagedmantis]

I realize that I didn't answer the question explicitly. The @golang/release will ensure it is included in the Go 1.17 release.

[toothrot]

@mdempsky Is there more work to do on this issue, or is it resolved by the linked CL?

[mdempsky]

@cagedmantis Thanks!

@toothrot The issue is fixed by b39e0f4. I just reopened for discussion purposes to make sure it wasn't missed. I think we can close it if there's no procedural reason to keep it open any more.

[toothrot]

@mdempsky We've got it, thanks!