runtime: unaligned atomic access to m.procid on 32-bit (arm, 386, ...)

[bradfitz]

Repro, on Go 1.18.1:

package main

import (
        "fmt"
        "runtime"
        "syscall"
)

func main() {
        fmt.Println(runtime.Version())
        err := syscall.Setgroups([]int{1, 2, 3})
        fmt.Println(err)
}

Run on a Raspberry Pi (where it was first found) or really any 32-bit device or GOARCH=386 on amd64 and:

go1.18.1
panic: unaligned 64-bit atomic operation
preempt off reason: doAllThreadsSyscall
fatal error: panic during preemptoff

goroutine 1 [running]:
runtime.throw({0xb29fd, 0x17})
	/home/bradfitz/sdk/go1.18.1/src/runtime/panic.go:992 +0x5c fp=0xc9ae4c sp=0xc9ae38 pc=0x45b14
panic({0xa6658, 0xccb24})
	/home/bradfitz/sdk/go1.18.1/src/runtime/panic.go:780 +0x710 fp=0xc9aeac sp=0xc9ae4c pc=0x4595c
runtime/internal/atomic.panicUnaligned()
	/home/bradfitz/sdk/go1.18.1/src/runtime/internal/atomic/unaligned.go:8 +0x24 fp=0xc9aeb8 sp=0xc9aeac pc=0x11f54
runtime/internal/atomic.Load64(0xc80024)
	/home/bradfitz/sdk/go1.18.1/src/runtime/internal/atomic/atomic_arm.s:280 +0x14 fp=0xc9aebc sp=0xc9aeb8 pc=0x122a8
syscall.runtime_doAllThreadsSyscall(0xce, 0x3, 0xcb8020, 0x0, 0x0, 0x0, 0x0)
	/home/bradfitz/sdk/go1.18.1/src/runtime/os_linux.go:816 +0x360 fp=0xc9af2c sp=0xc9aebc pc=0x72e50
syscall.AllThreadsSyscall(0xce, 0x3, 0xcb8020, 0x0)
	/home/bradfitz/sdk/go1.18.1/src/syscall/syscall_linux.go:987 +0x6c fp=0xc9af58 sp=0xc9af2c pc=0x904c4
syscall.Setgroups({0xc9afa4, 0x3, 0x3})
	/home/bradfitz/sdk/go1.18.1/src/syscall/syscall_linux.go:307 +0x2cc fp=0xc9af80 sp=0xc9af58 pc=0x90388
main.main()
	/home/bradfitz/hack/setgroups/setgroups.go:11 +0xb0 fp=0xc9afc0 sp=0xc9af80 pc=0x9bfe8
runtime.main()
	/home/bradfitz/sdk/go1.18.1/src/runtime/proc.go:250 +0x26c fp=0xc9afec sp=0xc9afc0 pc=0x48764
runtime.goexit()
	/home/bradfitz/sdk/go1.18.1/src/runtime/asm_arm.s:824 +0x4 fp=0xc9afec sp=0xc9afec pc=0x75a64

That os_linux.go:816 is:

                for atomic.Load64(&mp.procid) == 0 {                                 |

The m type is:

type m struct {
        g0      *g     // goroutine with scheduling stack
        morebuf gobuf  // gobuf arg to morestack
        divmod  uint32 // div/mod denominator for arm - known to liblink

        // Fields not known to debuggers.
        procid        uint64            // for debuggers, but offset not hard-coded
        gsignal       *g                // signal-handling g

Looks like gobuf is 7 words wide, which puts procid at 9 words offset? (not 64-bit aligned)

Didn't look too closely.

/cc @maisem @raggi @golang/runtime

[bradfitz]

Maybe a regression from 8ac23a1 which added a word to gobuf? 8ac23a1#diff-2fee911f97df63422c5ecf0a75227796f174f1a8e0ae9200fe0acdbd29ffc29b ?

Edit: nope. 8ac23a1 passes, as does its parent, 678568a.

[bradfitz]

It broke as of 0a5fae2 (https://golang.org/cl/383434) on Feb 15. It's fine at 0a5fae2's parent (0b321c9).

bradfitz@tsdev:~/hack/setgroups$ GOARCH=386 ~/hack/go/bin/go run -exec=sudo setgroups.go
devel go1.18-0b321c9a7c Tue Feb 15 15:40:29 2022 +0000
<nil>
bradfitz@tsdev:~/hack/setgroups$ GOARCH=386 ~/hack/go/bin/go run -exec=sudo setgroups.go
devel go1.18-0a5fae2a0e Tue Feb 15 15:40:35 2022 +0000
panic: unaligned 64-bit atomic operation
preempt off reason: doAllThreadsSyscall
fatal error: panic during preemptoff

goroutine 1 [running]:
runtime.throw({0x80d8c54, 0x17})
        /home/bradfitz/hack/go/src/runtime/panic.go:992 +0x6a fp=0xa05ce54 sp=0xa05ce40 pc=0x80786ca
panic({0x80cbb00, 0x80f2170})
        /home/bradfitz/hack/go/src/runtime/panic.go:780 +0x606 fp=0xa05ceb0 sp=0xa05ce54 pc=0x8078526
runtime/internal/atomic.panicUnaligned()
        /home/bradfitz/hack/go/src/runtime/internal/atomic/unaligned.go:8 +0x2d fp=0xa05cebc sp=0xa05ceb0 pc=0x8049b1d
runtime/internal/atomic.Load64(0xa080024)
        /home/bradfitz/hack/go/src/runtime/internal/atomic/atomic_386.s:225 +0x10 fp=0xa05cec0 sp=0xa05cebc pc=0x8049d80
syscall.runtime_doAllThreadsSyscall(0xce, 0x3, 0xa010080, 0x0, 0x0, 0x0, 0x0)
        /home/bradfitz/hack/go/src/runtime/os_linux.go:816 +0x367 fp=0xa05cf30 sp=0xa05cec0 pc=0x809f097
syscall.AllThreadsSyscall(0xce, 0x3, 0xa010080, 0x0)
        /home/bradfitz/hack/go/src/syscall/syscall_linux.go:987 +0x7b fp=0xa05cf5c sp=0xa05cf30 pc=0x80b989b
syscall.Setgroups({0xa05cfa4, 0x3, 0x3})
        /home/bradfitz/hack/go/src/syscall/syscall_linux.go:307 +0x262 fp=0xa05cf84 sp=0xa05cf5c pc=0x80b97a2
main.main()
        /home/bradfitz/hack/setgroups/setgroups.go:11 +0xcf fp=0xa05cfc4 sp=0xa05cf84 pc=0x80c3b7f
runtime.main()
        /home/bradfitz/hack/go/src/runtime/proc.go:250 +0x24d fp=0xa05cff0 sp=0xa05cfc4 pc=0x807accd
runtime.goexit()
        /home/bradfitz/hack/go/src/runtime/asm_386.s:1326 +0x1 fp=0xa05cff4 sp=0xa05cff0 pc=0x80a1681
exit status 2

/cc @prattmic

[bradfitz]

Sigh. And now I found dup bug #51776.

Closing.