runtime: factor out windows sigtramp

This CL factors out part of the Windows sigtramp implementation, which
was duplicated in all four architectures. The new common code is
implemented in Go rather than in assembly, which will make Windows
error handling easier to reason and maintain.

While here, implement the control flow guard workaround on
windows/386, which almost comes for free.

Change-Id: I0bf38c28c54793225126e161bd95527a62de05e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/458135
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 27 insertions, 56 deletions

diff --git a/src/runtime/sys_windows_386.s b/src/runtime/sys_windows_386.s
index 8713f7d0d9..0983cc7b1f 100644
--- a/src/runtime/sys_windows_386.s
+++ b/src/runtime/sys_windows_386.s
@@ -72,13 +72,20 @@ TEXT runtime·getlasterror(SB),NOSPLIT,$0
 	MOVL	AX, ret+0(FP)
 	RET
 
+TEXT runtime·sigFetchGSafe<ABIInternal>(SB),NOSPLIT|NOFRAME,$0
+	get_tls(AX)
+	CMPL	AX, $0
+	JE	2(PC)
+	MOVL	g(AX), AX
+	MOVL	AX, ret+0(FP)
+	RET
+
 // Called by Windows as a Vectored Exception Handler (VEH).
-// First argument is pointer to struct containing
+// AX is pointer to struct containing
 // exception record and context pointers.
-// Handler function is stored in AX.
-// Return 0 for 'not handled', -1 for handled.
+// CX is the kind of sigtramp function.
+// Return value of sigtrampgo is stored in AX.
 TEXT sigtramp<>(SB),NOSPLIT,$0-0
-	MOVL	ptrs+0(FP), CX
 	SUBL	$40, SP
 
 	// save callee-saved registers
@@ -87,58 +94,11 @@ TEXT sigtramp<>(SB),NOSPLIT,$0-0
 	MOVL	SI, 20(SP)
 	MOVL	DI, 24(SP)
 
-	MOVL	AX, SI	// save handler address
-
-	// find g
-	get_tls(DX)
-	CMPL	DX, $0
-	JNE	3(PC)
-	MOVL	$0, AX // continue
-	JMP	done
-	MOVL	g(DX), DX
-	CMPL	DX, $0
-	JNE	2(PC)
-	CALL	runtime·badsignal2(SB)
-
-	// save g in case of stack switch
-	MOVL	DX, 32(SP)	// g
-	MOVL	SP, 36(SP)
-
-	// do we need to switch to the g0 stack?
-	MOVL	g_m(DX), BX
-	MOVL	m_g0(BX), BX
-	CMPL	DX, BX
-	JEQ	g0
-
-	// switch to the g0 stack
-	get_tls(BP)
-	MOVL	BX, g(BP)
-	MOVL	(g_sched+gobuf_sp)(BX), DI
-	// make room for sighandler arguments
-	// and re-save old SP for restoring later.
-	// (note that the 36(DI) here must match the 36(SP) above.)
-	SUBL	$40, DI
-	MOVL	SP, 36(DI)
-	MOVL	DI, SP
-
-g0:
-	MOVL	0(CX), BX // ExceptionRecord*
-	MOVL	4(CX), CX // Context*
-	MOVL	BX, 0(SP)
+	MOVL	AX, 0(SP)
 	MOVL	CX, 4(SP)
-	MOVL	DX, 8(SP)
-	CALL	SI	// call handler
-	// AX is set to report result back to Windows
-	MOVL	12(SP), AX
+	CALL	runtime·sigtrampgo(SB)
+	MOVL	8(SP), AX
 
-	// switch back to original stack and g
-	// no-op if we never left.
-	MOVL	36(SP), SP
-	MOVL	32(SP), DX	// note: different SP
-	get_tls(BP)
-	MOVL	DX, g(BP)
-
-done:
 	// restore callee-saved registers
 	MOVL	24(SP), DI
 	MOVL	20(SP), SI
@@ -150,8 +110,18 @@ done:
 	BYTE $0xC2; WORD $4
 	RET // unreached; make assembler happy
 
+// Trampoline to resume execution from exception handler.
+// This is part of the control flow guard workaround.
+// It switches stacks and jumps to the continuation address.
+// DX and CX are set above at the end of sigtrampgo
+// in the context that starts executing at sigresume.
+TEXT runtime·sigresume(SB),NOSPLIT|NOFRAME,$0
+	MOVL	DX, SP
+	JMP	CX
+
 TEXT runtime·exceptiontramp(SB),NOSPLIT,$0
-	MOVL	$runtime·exceptionhandler(SB), AX
+	MOVL	argframe+0(FP), AX
+	MOVL	$const_callbackVEH, CX
 	JMP	sigtramp<>(SB)
 
 TEXT runtime·firstcontinuetramp(SB),NOSPLIT,$0-0
@@ -159,7 +129,8 @@ TEXT runtime·firstcontinuetramp(SB),NOSPLIT,$0-0
 	INT	$3
 
 TEXT runtime·lastcontinuetramp(SB),NOSPLIT,$0-0
-	MOVL	$runtime·lastcontinuehandler(SB), AX
+	MOVL	argframe+0(FP), AX
+	MOVL	$const_callbackLastVCH, CX
 	JMP	sigtramp<>(SB)
 
 GLOBL runtime·cbctxts(SB), NOPTR, $4