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

$ go version
go version go1.12.1 openbsd/amd64

Does this issue reproduce with the latest release?

Most likely, since the code that causes the issue is still there, but 1.12.1 is the latest version in ports for OpenBSD 6.5.

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="openbsd"
GOOS="openbsd"
GOPATH="/home/user/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/openbsd_amd64"
GCCGO="gccgo"
CC="cc"
CXX="c++"
CGO_ENABLED="1"
GOMOD=""
CGO_FLAGS="-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-build867684327=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Use the pledge(2) system call before something which reaches line 57 of socket() in src/net/sock_posix.go calling maxListenerBacklog().

package main

import (
	"io"
	"net/http"

	"golang.org/x/sys/unix"
)

func main() {
	unix.Pledge("stdio inet", "")
	h := func(w http.ResponseWriter, r *http.Request) {
		io.WriteString(w, "hi\n")
	}
	http.HandleFunc("/", h)
	http.ListenAndServe(":8080", nil)
}

What did you expect to see?

A functioning web server listening on port 8080.

What did you see instead?

signal: abort trap (core dumped)

Details

This is caused by @bradfitz's commit (related to #26775). The minimal example above would work fine prior to this commit.

In OpenBSD the pledge(2) system call is used to reduce the operating mode of the running process. It is common to reduce the permitted operations during the lifetime of the process. For example: allowing calls to read(2) and socket(2) then reducing to allow only socket(2) from then on out --- done through first pledging "rpath inet" then "inet" later. This greatly improves security during runtime and is used everywhere in OpenBSD.

When pledge(2) is first called it immediately restricts which sysctl(2) information can be queried (see here). This includes disallowing kern.somaxconn ie [CTL_KERN, KERN_SOMAXCONN].

Before the commit above, kern.somaxconn would be queried during initialization and thus would not interfere with any calls to pledge(2) during run time. After the commit, kern.somaxconn is queried in socket() causing pledge to shoot the disobeying process in the head.

This greatly restricts how pledge(2) can be used with certain aspects of the net package. For example, to bypass this issue one would need to do something like below (and still no calls to pledge(2) can happen before the call to http.ListenAndServe().

package main

import (
	"io"
	"net/http"
	"sync"
	"time"

	"golang.org/x/sys/unix"
)

func main() {
	h := func(w http.ResponseWriter, r *http.Request) {
		io.WriteString(w, "hi\n")
	}
	http.HandleFunc("/", h)

	var w sync.WaitGroup
	w.Add(1)

	go func() {
		defer w.Done()
		http.ListenAndServe(":8080", nil)
	}()

	time.Sleep(time.Millisecond)

	unix.Pledge("stdio inet", "")

	w.Wait()
}

I think we should opt to move the call to maxListenerBacklog() back into initialization. Perhaps it is possible to only have this done on OpenBSD systems where it's an issue. But I am open to other suggestions which are cleaner than my workaround above.