runtime: fix up OS X kernel bug sending user-generated SIGTRAP

OS X unconditionally sets si_code = TRAP_BRKPT when sending SIGTRAP,
even if it was generated by kill -TRAP and not a breakpoint.
Correct the si_code by looking to see if the PC is after a breakpoint.

For #12906.

Change-Id: I998c2499f7f12b338e607282a325b045f1f4f690
Reviewed-on: https://go-review.googlesource.com/18347Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/signal_darwin.go

@@ -85,6 +85,8 @@ func sigtrampgo(fn uintptr, infostyle, sig uint32, info *siginfo, ctx unsafe.Poi
 	}
 
 	setg(g.m.gsignal)
+	c := &sigctxt{info, ctx}
+	c.fixsigcode(sig)
 	sighandler(sig, info, ctx, g)
 	setg(g)
 	sigreturn(ctx, infostyle)

src/runtime/signal_darwin_386.go

@@ -32,3 +32,25 @@ func (c *sigctxt) set_eip(x uint32)     { c.regs().eip = x }
 func (c *sigctxt) set_esp(x uint32)     { c.regs().esp = x }
 func (c *sigctxt) set_sigcode(x uint32) { c.info.si_code = int32(x) }
 func (c *sigctxt) set_sigaddr(x uint32) { c.info.si_addr = x }
+
+func (c *sigctxt) fixsigcode(sig uint32) {
+	switch sig {
+	case _SIGTRAP:
+		// OS X sets c.sigcode() == TRAP_BRKPT unconditionally for all SIGTRAPs,
+		// leaving no way to distinguish a breakpoint-induced SIGTRAP
+		// from an asynchronous signal SIGTRAP.
+		// They all look breakpoint-induced by default.
+		// Try looking at the code to see if it's a breakpoint.
+		// The assumption is that we're very unlikely to get an
+		// asynchronous SIGTRAP at just the moment that the
+		// PC started to point at unmapped memory.
+		pc := uintptr(c.eip())
+		// OS X will leave the pc just after the INT 3 instruction.
+		// INT 3 is usually 1 byte, but there is a 2-byte form.
+		code := (*[2]byte)(unsafe.Pointer(pc - 2))
+		if code[1] != 0xCC && (code[0] != 0xCD || code[1] != 3) {
+			// SIGTRAP on something other than INT 3.
+			c.set_sigcode(_SI_USER)
+		}
+	}
+}

src/runtime/signal_darwin_amd64.go

@@ -40,3 +40,25 @@ func (c *sigctxt) set_rip(x uint64)     { c.regs().rip = x }
 func (c *sigctxt) set_rsp(x uint64)     { c.regs().rsp = x }
 func (c *sigctxt) set_sigcode(x uint64) { c.info.si_code = int32(x) }
 func (c *sigctxt) set_sigaddr(x uint64) { c.info.si_addr = x }
+
+func (c *sigctxt) fixsigcode(sig uint32) {
+	switch sig {
+	case _SIGTRAP:
+		// OS X sets c.sigcode() == TRAP_BRKPT unconditionally for all SIGTRAPs,
+		// leaving no way to distinguish a breakpoint-induced SIGTRAP
+		// from an asynchronous signal SIGTRAP.
+		// They all look breakpoint-induced by default.
+		// Try looking at the code to see if it's a breakpoint.
+		// The assumption is that we're very unlikely to get an
+		// asynchronous SIGTRAP at just the moment that the
+		// PC started to point at unmapped memory.
+		pc := uintptr(c.rip())
+		// OS X will leave the pc just after the INT 3 instruction.
+		// INT 3 is usually 1 byte, but there is a 2-byte form.
+		code := (*[2]byte)(unsafe.Pointer(pc - 2))
+		if code[1] != 0xCC && (code[0] != 0xCD || code[1] != 3) {
+			// SIGTRAP on something other than INT 3.
+			c.set_sigcode(_SI_USER)
+		}
+	}
+}

src/runtime/signal_darwin_arm.go

@@ -42,3 +42,24 @@ func (c *sigctxt) set_r10(x uint32) { c.regs().r[10] = x }
 
 func (c *sigctxt) set_sigcode(x uint32) { c.info.si_code = int32(x) }
 func (c *sigctxt) set_sigaddr(x uint32) { c.info.si_addr = x }
+
+func (c *sigctxt) fixsigcode(sig uint32) {
+	switch sig {
+	case _SIGTRAP:
+		// OS X sets c.sigcode() == TRAP_BRKPT unconditionally for all SIGTRAPs,
+		// leaving no way to distinguish a breakpoint-induced SIGTRAP
+		// from an asynchronous signal SIGTRAP.
+		// They all look breakpoint-induced by default.
+		// Try looking at the code to see if it's a breakpoint.
+		// The assumption is that we're very unlikely to get an
+		// asynchronous SIGTRAP at just the moment that the
+		// PC started to point at unmapped memory.
+		pc := uintptr(c.pc())
+		// OS X will leave the pc just after the instruction.
+		code := (*uint32)(unsafe.Pointer(pc - 4))
+		if *code != 0xe7f001f0 {
+			// SIGTRAP on something other than breakpoint.
+			c.set_sigcode(_SI_USER)
+		}
+	}
+}

src/runtime/signal_darwin_arm64.go

@@ -58,3 +58,24 @@ func (c *sigctxt) set_r28(x uint64) { c.regs().x[28] = x }
 func (c *sigctxt) set_sigaddr(x uint64) {
 	c.info.si_addr = (*byte)(unsafe.Pointer(uintptr(x)))
 }
+
+func (c *sigctxt) fixsigcode(sig uint32) {
+	switch sig {
+	case _SIGTRAP:
+		// OS X sets c.sigcode() == TRAP_BRKPT unconditionally for all SIGTRAPs,
+		// leaving no way to distinguish a breakpoint-induced SIGTRAP
+		// from an asynchronous signal SIGTRAP.
+		// They all look breakpoint-induced by default.
+		// Try looking at the code to see if it's a breakpoint.
+		// The assumption is that we're very unlikely to get an
+		// asynchronous SIGTRAP at just the moment that the
+		// PC started to point at unmapped memory.
+		pc := uintptr(c.pc())
+		// OS X will leave the pc just after the instruction.
+		code := (*uint32)(unsafe.Pointer(pc - 4))
+		if *code != 0xd4200000 {
+			// SIGTRAP on something other than breakpoint.
+			c.set_sigcode(_SI_USER)
+		}
+	}
+}