os: spurious SIGCHILD on running child process

[mohitaron]

Go version

1.23.6

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/aron/.cache/go-build'
GOENV='/home/aron/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/aron/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/aron/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/aron/junk/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/aron/junk/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.6'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/aron/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1713836783=/tmp/go-build -gno-record-gcc-switches'

What did you do?

The below program reproduces the bug. It panics when run with go 1.23.6 and runs fine with go 1.22.5:

What did you see happen?

// This program is used to test the SIGCHLD signal.
//
// It runs a child process and waits for it to exit. When the child process
// exits, it sends a SIGCHLD signal to the parent process. The parent process
// then waits for the SIGCHLD signal and prints a message. The parent should
// receive the SIGCHLD signal only once.
//
// Go 1.23.6 seems to have a bug where it receives the SIGCHLD signal twice.

package main

import (
        "flag"
        "fmt"
        "os"
        "os/exec"
        "os/signal"
        "syscall"
        "time"
)

var (
    isChild bool
)

func init() {
    flag.BoolVar(&isChild, "child", false, "if true, run as a child process")
}

func child() {
    fmt.Println("I am a child process")

    time.Sleep(2 * time.Second)

    fmt.Println("Child process exiting")

    os.Exit(0)
}

func main() {
    flag.Parse()

    if isChild {
        child()
        return
    }

    fmt.Println("I am a parent process")

    sigChan := make(chan os.Signal, 1)
        signal.Notify(sigChan, syscall.SIGCHLD)

    go func () {
        numSigchld := 0
        for {
            <-sigChan
            numSigchld++
            fmt.Printf("Received SIGCHLD %d\n", numSigchld)
            if numSigchld > 1 {
                panic("Received more than one SIGCHLD")
            }
        }
    }()

    // Start the child process.
    cmd := exec.Command(os.Args[0], "-child")
    err := cmd.Start()
    if err != nil {
        panic(err)
    }

    // Wait for the child process to exit.
    cmd.Wait()

    // Wait for any SIGCHLD signals.
    time.Sleep(1 * time.Second)

    os.Exit(0)
}

What did you expect to see?

In the above program, there's only one child process. Yet in go 1.23.6, two SIGCHLD signals are received by the parent process.

[gabyhelp]

Related Issues

• os/signal: not all notifications about SIGCHLD are sent #15437 (closed)
• cmd/go: run does not relay signals to child process #40467
• runtime: signal handling: c-shared libraries intercepts SIGCHLD and crashes #13042 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[varun-scifin]

@prattmic @cherrymui It seems to have been added between Go 1.23.2 and 1.23.3. It might be related to the pidfd changes in this commit, where we added some pidfd feature checking (though I may be completely wrong -- this is just my best guess).

[seankhliao]

The instructions don't seem sufficient to reproduce the issue, what's the environment it runs in? (os, distro, kernel, qemu?, etc)

[varun-scifin]

Tested on the ubuntu-latest runner (Ubuntu 24, 6.8.0-1021-azure kernel, Go 1.23.3 and above, no QEMU): https://github.com/varun-scifin/go-71828/actions/runs/13411687068/job/37463124345

You can also clone the repo to repro.

[mohitaron]

The instructions don't seem sufficient to reproduce the issue, what's the environment it runs in? (os, distro, kernel, qemu?, etc)

As required by the ticket, I had posted the output of 'go env' already. Here's the output of 'uname -a' and 'lsb_release -a':

Ubuntu-iMac:sigchld>uname -a
Linux Ubuntu-iMac 6.8.0-52-generic #53-Ubuntu SMP PREEMPT_DYNAMIC Sat Jan 11 00:06:25 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

Ubuntu-iMac:sigchld>lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 24.04.2 LTS
Release: 24.04
Codename: noble

[mohitaron]

The problem is also reproducible on go version 1.24.0.

Ubuntu-iMac:sigchld>go version
go version go1.24.0 linux/amd64

Ubuntu-iMac:sigchld>go env
AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/aron/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/aron/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build4247764318=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/aron/workspace/main/go.mod'
GOMODCACHE='/home/aron/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/aron/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/snap/go/10853'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/aron/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/snap/go/10853/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.0'
GOWORK=''
PKG_CONFIG='pkg-config'

Ubuntu-iMac:sigchld>go run sigchld.go
I am a parent process
Received SIGCHLD 1
Received SIGCHLD 2
panic: Received more than one SIGCHLD

goroutine 8 [running]:
main.main.func1()
/home/aron/workspace/main/experimental/aron/sigchld/sigchld.go:64 +0xa5
created by main.main in goroutine 1
/home/aron/workspace/main/experimental/aron/sigchld/sigchld.go:57 +0x125
exit status 2

[ianlancetaylor]

The problem is that with the new pidfd code, we run a test fork to see if pidfd works. That test fork winds up sending a SIGCHLD signal. I think the fix is straightforward.

[ianlancetaylor]

@gopherbot Please open backport issues.

This bug causes a spurious SIGCHLD signal the first time a process is executed when running on Linux systems. This is a regression from past behavior and from behavior on non-Linux systems. The patch is small and safe.

[gopherbot]

Backport issue(s) opened: #71848 (for 1.23), #71849 (for 1.24).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/650835 mentions this issue: syscall: don't send child signal when testing pidfd

[gopherbot]

Change https://go.dev/cl/651035 mentions this issue: [release-branch.go1.24] syscall: don't send child signal when testing pidfd

[gopherbot]

Change https://go.dev/cl/651055 mentions this issue: [release-branch.go1.23] syscall: don't send child signal when testing pidfd

[gopherbot]

Change https://go.dev/cl/651415 mentions this issue: runtime: use WCLONE when waiting on pidfd test child

[gopherbot]

Change https://go.dev/cl/651495 mentions this issue: [release-branch.go1.23] runtime: use WCLONE when waiting on pidfd test child

[gopherbot]

Change https://go.dev/cl/651476 mentions this issue: [release-branch.go1.24] runtime: use WCLONE when waiting on pidfd test child