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, 20 insertions, 125 deletions

diff --git a/src/runtime/sys_windows_arm64.s b/src/runtime/sys_windows_arm64.s
index 4702a4d7d2..67fe21b2d0 100644
--- a/src/runtime/sys_windows_arm64.s
+++ b/src/runtime/sys_windows_arm64.s
@@ -131,143 +131,38 @@ TEXT runtime·getlasterror(SB),NOSPLIT|NOFRAME,$0
 	RET
 
 // Called by Windows as a Vectored Exception Handler (VEH).
-// First argument is pointer to struct containing
+// R0 is pointer to struct containing
 // exception record and context pointers.
-// Handler function is stored in R1
-// Return 0 for 'not handled', -1 for handled.
-// int32_t sigtramp(
-//     PEXCEPTION_POINTERS ExceptionInfo,
-//     func *GoExceptionHandler);
-TEXT sigtramp<>(SB),NOSPLIT|NOFRAME,$0
-	// Save R0, R1 (args) as well as LR, R27, R28 (callee-save).
+// R1 is the kind of sigtramp function.
+// Return value of sigtrampgo is stored in R0.
+TEXT sigtramp<>(SB),NOSPLIT,$176
+	// Switch from the host ABI to the Go ABI, safe args and lr.
 	MOVD	R0, R5
 	MOVD	R1, R6
 	MOVD	LR, R7
-	MOVD	R27, R16		// saved R27 (callee-save)
-	MOVD	g, R17 			// saved R28 (callee-save from Windows, not really g)
+	SAVE_R19_TO_R28(8*4)
+	SAVE_F8_TO_F15(8*14)
 
-	BL      runtime·load_g(SB)	// smashes R0, R27, R28 (g)
-	CMP	$0,	g		// is there a current g?
-	BNE	g_ok
-	MOVD	R7, LR
-	MOVD	R16, R27	// restore R27
-	MOVD	R17, g		// restore R28
-	MOVD	$0, R0		// continue 
-	RET
-
-g_ok:
-	// Do we need to switch to the g0 stack?
-	MOVD	g, R3			// R3 = oldg (for sigtramp_g0)
-	MOVD	g_m(g), R2		// R2 = m
-	MOVD	m_g0(R2), R2		// R2 = g0
-	CMP	g, R2			// if curg == g0
-	BNE	switch
+	BL	runtime·load_g(SB)	// Clobers R0, R27, R28 (g)
 
-	// No: on g0 stack already, tail call to sigtramp_g0.
-	// Restore all the callee-saves so sigtramp_g0 can return to our caller.
-	// We also pass R2 = g0, R3 = oldg, both set above.
 	MOVD	R5, R0
 	MOVD	R6, R1
-	MOVD	R7, LR
-	MOVD	R16, R27		// restore R27
-	MOVD	R17, g 			// restore R28
-	B	sigtramp_g0<>(SB)
-
-switch:
-	// switch to g0 stack (but do not update g - that's sigtramp_g0's job)
-	MOVD	RSP, R8
-	MOVD	(g_sched+gobuf_sp)(R2), R4	// R4 = g->gobuf.sp
-	SUB	$(6*8), R4			// alloc space for saves - 2 words below SP for frame pointer, 3 for us to use, 1 for alignment
-	MOVD	R4, RSP				// switch to g0 stack
+	// Calling ABIInternal because TLS might be nil.
+	BL	runtime·sigtrampgo<ABIInternal>(SB)
+	// Return value is already stored in R0.
 
-	MOVD	$0, (0*8)(RSP)	// fake saved LR
-	MOVD	R7, (1*8)(RSP)	// saved LR
-	MOVD	R8, (2*8)(RSP)	// saved SP
-
-	MOVD	R5, R0		// original args
-	MOVD	R6, R1		// original args
-	MOVD	R16, R27
-	MOVD	R17, g 		// R28
-	BL	sigtramp_g0<>(SB)
-
-	// switch back to original stack; g already updated
-	MOVD	(1*8)(RSP), R7	// saved LR
-	MOVD	(2*8)(RSP), R8	// saved SP
+	// Restore callee-save registers.
+	RESTORE_R19_TO_R28(8*4)
+	RESTORE_F8_TO_F15(8*14)
 	MOVD	R7, LR
-	MOVD	R8, RSP
-	RET
-
-// sigtramp_g0 is running on the g0 stack, with R2 = g0, R3 = oldg.
-// But g itself is not set - that's R28, a callee-save register,
-// and it still holds the value from the Windows DLL caller.
-TEXT sigtramp_g0<>(SB),NOSPLIT,$128
-	NO_LOCAL_POINTERS
-
-	// Push C callee-save registers R19-R28. LR, FP already saved.
-	// These registers will occupy the upper 10 words of the frame.
-	SAVE_R19_TO_R28(8*7)
-
-	MOVD	0(R0), R5	// R5 = ExceptionPointers->ExceptionRecord
-	MOVD	8(R0), R6	// R6 = ExceptionPointers->ContextRecord
-	MOVD	R6, context-(11*8)(SP)
-
-	MOVD	R2, g 			// g0
-	BL      runtime·save_g(SB)	// smashes R0
-
-	MOVD	R5, (1*8)(RSP)	// arg0 (ExceptionRecord)
-	MOVD	R6, (2*8)(RSP)	// arg1 (ContextRecord)
-	MOVD	R3, (3*8)(RSP)	// arg2 (original g)
-	MOVD	R3, oldg-(12*8)(SP)
-	BL	(R1)
-	MOVD	oldg-(12*8)(SP), g
-	BL      runtime·save_g(SB)	// smashes R0
-	MOVW	(4*8)(RSP), R0	// return value (0 or -1)
-
-	// if return value is CONTINUE_SEARCH, do not set up control
-	// flow guard workaround
-	CMP	$0, R0
-	BEQ	return
-
-	// Check if we need to set up the control flow guard workaround.
-	// On Windows, the stack pointer in the context must lie within
-	// system stack limits when we resume from exception.
-	// Store the resume SP and PC in alternate registers
-	// and return to sigresume on the g0 stack.
-	// sigresume makes no use of the stack at all,
-	// loading SP from R0 and jumping to R1.
-	// Note that smashing R0 and R1 is only safe because we know sigpanic
-	// will not actually return to the original frame, so the registers
-	// are effectively dead. But this does mean we can't use the
-	// same mechanism for async preemption.
-	MOVD	context-(11*8)(SP), R6
-	MOVD	context_pc(R6), R2		// load PC from context record
-	MOVD	$sigresume<>(SB), R1
-
-	CMP	R1, R2
-	BEQ	return				// do not clobber saved SP/PC
-
-	// Save resume SP and PC into R0, R1.
-	MOVD	context_xsp(R6), R2
-	MOVD	R2, (context_x+0*8)(R6)
-	MOVD	context_pc(R6), R2
-	MOVD	R2, (context_x+1*8)(R6)
-
-	// Set up context record to return to sigresume on g0 stack
-	MOVD	RSP, R2
-	MOVD	R2, context_xsp(R6)
-	MOVD	$sigresume<>(SB), R2
-	MOVD	R2, context_pc(R6)
-
-return:
-	RESTORE_R19_TO_R28(8*7)		// smashes g
 	RET
 
 // 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.
-// R0 and R1 are set above at the end of sigtramp<>
-// in the context that starts executing at sigresume<>.
-TEXT sigresume<>(SB),NOSPLIT|NOFRAME,$0
+// R0 and R1 are set above at the end of sigtrampgo
+// in the context that starts executing at sigresume.
+TEXT runtime·sigresume(SB),NOSPLIT|NOFRAME,$0
 	// Important: do not smash LR,
 	// which is set to a live value when handling
 	// a signal by pushing a call to sigpanic onto the stack.
@@ -275,15 +170,15 @@ TEXT sigresume<>(SB),NOSPLIT|NOFRAME,$0
 	B	(R1)
 
 TEXT runtime·exceptiontramp(SB),NOSPLIT|NOFRAME,$0
-	MOVD	$runtime·exceptionhandler(SB), R1
+	MOVD	$const_callbackVEH, R1
 	B	sigtramp<>(SB)
 
 TEXT runtime·firstcontinuetramp(SB),NOSPLIT|NOFRAME,$0
-	MOVD	$runtime·firstcontinuehandler(SB), R1
+	MOVD	$const_callbackFirstVCH, R1
 	B	sigtramp<>(SB)
 
 TEXT runtime·lastcontinuetramp(SB),NOSPLIT|NOFRAME,$0
-	MOVD	$runtime·lastcontinuehandler(SB), R1
+	MOVD	$const_callbackLastVCH, R1
 	B	sigtramp<>(SB)
 
 GLOBL runtime·cbctxts(SB), NOPTR, $4