runtime: unix: sane exit in dieFromSignal for pid 1

A curious bug was reported to kubernetes[1] and runc[2] recently:
sometimes runc init reports exit status of 2.

Turns out, Go runtime assumes that on any UNIX system signals such as
SIGTERM (or any other that has _sigKill flag set in sigtable) with no
signal handler set up, will result in kernel terminating the program.
This is true, except for PID 1 which gets a custom treatment from the
kernel.

As a result, when a Go program that runs as PID 1 (which is easy to
achieve in Linux by using a new PID namespace) receives such a signal,
Go runtime calls dieFromSignal which falls through all the way to
exit(2), which is very confusing to a user.

This issue can be worked around by the program by adding custom handlers
for SIGTERM/SIGINT/SIGHUP, but that requires a goroutine to handle those
signals, which, in case of runc, unnecessarily raises its NPROC/pid.max
requirement (see discussion at [2]).

Since practically exit(2) in dieFromSignal can only happen when the
process is running as PID 1, replace it with exit(128+sig) to mimic
the shell convention when a child is terminated by a signal.

Add a test case which demonstrates the issue and validates the fix
(albeit only on Linux).

[An earlier version of this patch used to do nothing in dieFromSignal
for PID 1 case, but such behavior might be a breaking change for a Go
program running in a Linux container as PID 1.]

Fixes #78442

[1]: https://github.com/kubernetes/kubernetes/issues/135713
[2]: https://github.com/opencontainers/runc/pull/5189

Change-Id: I196e09e4b5ce84ce2c747a0c2d1fc6e9cf3a6131
Reviewed-on: https://go-review.googlesource.com/c/go/+/759040
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprog/signal_pid1.go b/src/runtime/testdata/testprog/signal_pid1.go
new file mode 100644
index 0000000000..7446056e2d
--- /dev/null
+++ b/src/runtime/testdata/testprog/signal_pid1.go
@@ -0,0 +1,26 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+func init() {
+	register("SignalPid1", SignalPid1)
+}
+
+// SignalPid1 is a helper for TestSignalPid1.
+func SignalPid1() {
+	if os.Getpid() != 1 {
+		fmt.Fprintln(os.Stderr, "I am not PID 1")
+		return
+	}
+	fmt.Println("ready")
+
+	time.Sleep(time.Hour)
+}