feat(verify): add runtime ABI checks with sentinel registers and red-zone canary

Assisted-by: Qwen 3.8 Max Preview

verify/abi_amd64.s

@@ -0,0 +1,61 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+#include "textflag.h"
+
+// ABI-checking trampoline.  Sets sentinel values in the callee-saved
+// registers (BP, R14) before entering the JIT function and checks whether
+// they survived on return.
+//
+// The return trampoline (leaveJITCheckedRaw) is a raw TEXT symbol with no
+// Go function declaration, so the toolchain does NOT interpose an
+// ABIInternal wrapper — the JIT function RETs directly into the check code,
+// which sees the registers exactly as the function left them.
+//
+// Go ABI0 on amd64 guarantees:
+//   - BP is callee-saved (NOSPLIT frame=0 functions must not touch it).
+//   - R14 holds the goroutine pointer and must survive across any call.
+
+// Sentinel values chosen to be unlikely in normal execution.
+#define SENTINEL_BP  0xDEADBEEFCAFEF00D
+#define SENTINEL_R14 0x0BADF00DDEADBEEF
+
+// GLOBL holding the raw address of the leave trampoline, read by Go.
+GLOBL ·leaveCheckedPtr(SB), NOPTR, $8
+DATA  ·leaveCheckedPtr(SB)/8, $·leaveJITCheckedRaw(SB)
+
+// func enterJITChecked(fn uintptr, stack uintptr)
+// Sets sentinels in BP and R14, switches to the prepared stack and jumps
+// to fn.  The prepared stack's return address must be leaveJITCheckedRaw
+// (read from leaveCheckedPtr).
+TEXT ·enterJITChecked(SB), NOSPLIT, $0-16
+	MOVQ fn+0(FP), AX         // target (before SP switch)
+	MOVQ SP, ·savedSP(SB)     // preserve Go stack
+	MOVQ BP, ·savedBP(SB)     // preserve frame pointer (vet requires save before clobber)
+	MOVQ $SENTINEL_BP, BP     // sentinel in BP
+	MOVQ $SENTINEL_R14, R14   // sentinel in R14
+	MOVQ stack+8(FP), SP      // switch to prepared stack
+	JMP  AX
+
+// leaveJITCheckedRaw is the raw return trampoline.  It has NO Go function
+// declaration, so no ABIInternal wrapper is generated — the JIT function's
+// RET lands here directly, seeing BP and R14 exactly as the function left
+// them.  It checks the sentinels, records violations in abiResult, then
+// restores the Go stack and returns.
+TEXT ·leaveJITCheckedRaw(SB), NOSPLIT, $0-0
+	// Check BP against the sentinel.
+	MOVQ $SENTINEL_BP, CX
+	CMPQ BP, CX
+	JEQ  bp_ok
+	ORQ  $1, ·abiResult(SB)
+
+bp_ok:
+	// Check R14 against the sentinel.
+	MOVQ $SENTINEL_R14, CX
+	CMPQ R14, CX
+	JEQ  r14_ok
+	ORQ  $2, ·abiResult(SB)
+
+r14_ok:
+	MOVQ ·savedSP(SB), SP
+	RET