runtime: signal handling support for plan9_arm

Plan 9 trap/signal handling differs on ARM from other architectures
because ARM has a link register.  Also trap message syntax varies
between different architectures (historical accident?).
Revised 7 March to clarify a comment.

Change-Id: Ib6485f82857a2f9a0d6b2c375cf0aaa230b83656
Reviewed-on: https://go-review.googlesource.com/18969
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/defs_plan9_386.go

@@ -30,9 +30,13 @@ type sigctxt struct {
 
 func (c *sigctxt) pc() uintptr { return uintptr(c.u.pc) }
 func (c *sigctxt) sp() uintptr { return uintptr(c.u.sp) }
+func (c *sigctxt) lr() uintptr { return uintptr(0) }
 
 func (c *sigctxt) setpc(x uintptr) { c.u.pc = uint32(x) }
 func (c *sigctxt) setsp(x uintptr) { c.u.sp = uint32(x) }
+func (c *sigctxt) setlr(x uintptr) {}
+
+func (c *sigctxt) savelr(x uintptr) {}
 
 func dumpregs(u *ureg) {
 	print("ax    ", hex(u.ax), "\n")
@@ -49,3 +53,5 @@ func dumpregs(u *ureg) {
 	print("fs    ", hex(u.fs), "\n")
 	print("gs    ", hex(u.gs), "\n")
 }
+
+func sigpanictramp() {}

src/runtime/defs_plan9_amd64.go

@@ -39,9 +39,13 @@ type sigctxt struct {
 
 func (c *sigctxt) pc() uintptr { return uintptr(c.u.ip) }
 func (c *sigctxt) sp() uintptr { return uintptr(c.u.sp) }
+func (c *sigctxt) lr() uintptr { return uintptr(0) }
 
 func (c *sigctxt) setpc(x uintptr) { c.u.ip = uint64(x) }
 func (c *sigctxt) setsp(x uintptr) { c.u.sp = uint64(x) }
+func (c *sigctxt) setlr(x uintptr) {}
+
+func (c *sigctxt) savelr(x uintptr) {}
 
 func dumpregs(u *ureg) {
 	print("ax    ", hex(u.ax), "\n")
@@ -66,3 +70,5 @@ func dumpregs(u *ureg) {
 	print("fs    ", hex(u.fs), "\n")
 	print("gs    ", hex(u.gs), "\n")
 }
+
+func sigpanictramp() {}

src/runtime/os3_plan9.go

@@ -66,22 +66,37 @@ func sighandler(_ureg *ureg, note *byte, gp *g) int {
 			pc = 0
 		}
 
-		// Only push sigpanic if PC != 0.
+		// IF LR exists, sigpanictramp must save it to the stack
-		//
+		// before entry to sigpanic so that panics in leaf
+		// functions are correctly handled. This will smash
+		// the stack frame but we're not going back there
+		// anyway.
+		if usesLR {
+			c.savelr(c.lr())
+		}
+
 		// If PC == 0, probably panicked because of a call to a nil func.
-		// Not pushing that onto SP will make the trace look like a call
+		// Not faking that as the return address will make the trace look like a call
 		// to sigpanic instead. (Otherwise the trace will end at
 		// sigpanic and we won't get to see who faulted).
 		if pc != 0 {
-			if sys.RegSize > sys.PtrSize {
+			if usesLR {
+				c.setlr(pc)
+			} else {
+				if sys.RegSize > sys.PtrSize {
+					sp -= sys.PtrSize
+					*(*uintptr)(unsafe.Pointer(sp)) = 0
+				}
 				sp -= sys.PtrSize
-				*(*uintptr)(unsafe.Pointer(sp)) = 0
+				*(*uintptr)(unsafe.Pointer(sp)) = pc
+				c.setsp(sp)
 			}
-			sp -= sys.PtrSize
-			*(*uintptr)(unsafe.Pointer(sp)) = pc
-			c.setsp(sp)
 		}
-		c.setpc(funcPC(sigpanic))
+		if usesLR {
+			c.setpc(funcPC(sigpanictramp))
+		} else {
+			c.setpc(funcPC(sigpanic))
+		}
 		return _NCONT
 	}
 	if flags&_SigNotify != 0 {
@@ -105,7 +120,7 @@ Throw:
 	level, _, docrash = gotraceback()
 	if level > 0 {
 		goroutineheader(gp)
-		tracebacktrap(c.pc(), c.sp(), 0, gp)
+		tracebacktrap(c.pc(), c.sp(), c.lr(), gp)
 		tracebackothers(gp)
 		print("\n")
 		dumpregs(_ureg)

src/runtime/os_plan9.go

@@ -78,7 +78,15 @@ func sigpanic() {
 	note := gostringnocopy((*byte)(unsafe.Pointer(g.m.notesig)))
 	switch g.sig {
 	case _SIGRFAULT, _SIGWFAULT:
-		addr := note[index(note, "addr=")+5:]
+		i := index(note, "addr=")
+		if i >= 0 {
+			i += 5
+		} else if i = index(note, "va="); i >= 0 {
+			i += 3
+		} else {
+			panicmem()
+		}
+		addr := note[i:]
 		g.sigcode1 = uintptr(atolwhex(addr))
 		if g.sigcode1 < 0x1000 || g.paniconfault {
 			panicmem()

src/runtime/signal_plan9.go

@@ -22,8 +22,8 @@ var sigtable = [...]sigTabT{
 	{_SigThrow, "sys: trap: invalid opcode"},
 
 	// We can recover from some memory errors in runtime·sigpanic.
-	{_SigPanic, "sys: trap: fault read addr"},  // SIGRFAULT
+	{_SigPanic, "sys: trap: fault read"},  // SIGRFAULT
-	{_SigPanic, "sys: trap: fault write addr"}, // SIGWFAULT
+	{_SigPanic, "sys: trap: fault write"}, // SIGWFAULT
 
 	// We can also recover from math errors.
 	{_SigPanic, "sys: trap: divide error"}, // SIGINTDIV