os: ignore SIGSYS in checkPidfd

In Android version 11 and earlier, pidfd-related system calls
are not allowed by the seccomp policy, which causes crashes due
to SIGSYS signals.

Fixes #69065

Change-Id: Ib29631639a5cf221ac11b4d82390cb79436b8657
GitHub-Last-Rev: aad6b3b32c
GitHub-Pull-Request: golang/go#69543
Reviewed-on: https://go-review.googlesource.com/c/go/+/614277
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/signal_unix.go

@@ -605,6 +605,19 @@ var crashing atomic.Int32
 var testSigtrap func(info *siginfo, ctxt *sigctxt, gp *g) bool
 var testSigusr1 func(gp *g) bool
 
+// sigsysIgnored is non-zero if we are currently ignoring SIGSYS. See issue #69065.
+var sigsysIgnored uint32
+
+//go:linkname ignoreSIGSYS os.ignoreSIGSYS
+func ignoreSIGSYS() {
+	atomic.Store(&sigsysIgnored, 1)
+}
+
+//go:linkname restoreSIGSYS os.restoreSIGSYS
+func restoreSIGSYS() {
+	atomic.Store(&sigsysIgnored, 0)
+}
+
 // sighandler is invoked when a signal occurs. The global g will be
 // set to a gsignal goroutine and we will be running on the alternate
 // signal stack. The parameter gp will be the value of the global g
@@ -715,6 +728,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		return
 	}
 
+	if sig == _SIGSYS && c.sigFromSeccomp() && atomic.Load(&sigsysIgnored) != 0 {
+		return
+	}
+
 	if flags&_SigKill != 0 {
 		dieFromSignal(sig)
 	}