runtime: always install new signal stack on NetBSD and DragonFly

On NetBSD and DragonFly a newly created thread inherits the signal stack
of the creating thread.  That means that in a cgo program a C thread
created using pthread_create will get the signal stack of the creating
thread, most likely a Go thread.  This will then lead to chaos if two
signals occur simultaneously.

We can't fix the general case.  But we can fix the case of a C thread
that calls a Go function, by installing a new signal stack and then
dropping it when we return to C.  That will break the case of a C thread
that calls sigaltstack and then calls Go, because we will drop the C
thread's alternate signal stack as we return from Go.  Still, this is
the 1.5 behavior.  And what else can we do?

Fixes #14051.
Fixes #14052.
Fixes #14067.

Change-Id: Iee286ca50b50ec712a4d929c7121c35e2383a7b9
Reviewed-on: https://go-review.googlesource.com/18835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/os1_dragonfly.go

@@ -140,22 +140,17 @@ func minit() {
 	// m.procid is a uint64, but lwp_start writes an int32. Fix it up.
 	_g_.m.procid = uint64(*(*int32)(unsafe.Pointer(&_g_.m.procid)))
 
-	// Initialize signal handling
-	var st sigaltstackt
-	sigaltstack(nil, &st)
-	if st.ss_flags&_SS_DISABLE != 0 {
+	// Initialize signal handling.
+
+	// On DragonFly a thread created by pthread_create inherits
+	// the signal stack of the creating thread.  We always create
+	// a new signal stack here, to avoid having two Go threads
+	// using the same signal stack.  This breaks the case of a
+	// thread created in C that calls sigaltstack and then calls a
+	// Go function, because we will lose track of the C code's
+	// sigaltstack, but it's the best we can do.
 	signalstack(&_g_.m.gsignal.stack)
 	_g_.m.newSigstack = true
-	} else {
-		// Use existing signal stack.
-		stsp := uintptr(unsafe.Pointer(st.ss_sp))
-		_g_.m.gsignal.stack.lo = stsp
-		_g_.m.gsignal.stack.hi = stsp + st.ss_size
-		_g_.m.gsignal.stackguard0 = stsp + _StackGuard
-		_g_.m.gsignal.stackguard1 = stsp + _StackGuard
-		_g_.m.gsignal.stackAlloc = st.ss_size
-		_g_.m.newSigstack = false
-	}
 
 	// restore signal mask from m.sigmask and unblock essential signals
 	nmask := _g_.m.sigmask

src/runtime/os1_netbsd.go

@@ -172,22 +172,17 @@ func minit() {
 	_g_ := getg()
 	_g_.m.procid = uint64(lwp_self())
 
-	// Initialize signal handling
-	var st sigaltstackt
-	sigaltstack(nil, &st)
-	if st.ss_flags&_SS_DISABLE != 0 {
+	// Initialize signal handling.
+
+	// On NetBSD a thread created by pthread_create inherits the
+	// signal stack of the creating thread.  We always create a
+	// new signal stack here, to avoid having two Go threads using
+	// the same signal stack.  This breaks the case of a thread
+	// created in C that calls sigaltstack and then calls a Go
+	// function, because we will lose track of the C code's
+	// sigaltstack, but it's the best we can do.
 	signalstack(&_g_.m.gsignal.stack)
 	_g_.m.newSigstack = true
-	} else {
-		// Use existing signal stack.
-		stsp := uintptr(unsafe.Pointer(st.ss_sp))
-		_g_.m.gsignal.stack.lo = stsp
-		_g_.m.gsignal.stack.hi = stsp + st.ss_size
-		_g_.m.gsignal.stackguard0 = stsp + _StackGuard
-		_g_.m.gsignal.stackguard1 = stsp + _StackGuard
-		_g_.m.gsignal.stackAlloc = st.ss_size
-		_g_.m.newSigstack = false
-	}
 
 	// restore signal mask from m.sigmask and unblock essential signals
 	nmask := _g_.m.sigmask