gasm-sdk/verify/call_arm64.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

//go:build arm64

package verify

import (
	"encoding/binary"
	"fmt"
	"syscall"
	"unsafe"
)

// enterJIT switches to the prepared stack and branches to fn.
//
//go:nosplit
func enterJIT(fn uintptr, stack uintptr)

// leaveJIT restores the Go stack after a JIT function returns.
//
//go:nosplit
func leaveJIT()

// leaveJITAddr is the raw ABI0 address of leaveJIT, handed over by the
// GLOBL/DATA in trampoline_arm64.s (reflect would return the interposed
// ABIInternal wrapper instead).
var leaveRawAddr uintptr

var leaveJITAddr = leaveRawAddr

const stackPad = 64

// Call invokes the assembled function at fnAddr with the given ABI0 argument
// block.  On arm64, the ABI0 convention places arguments on the stack starting
// at [SP+8] (same as amd64 ABI0 in this project).
//
// Not safe for concurrent use: only one JIT call may be in flight at a
// time, the trampolines keep the saved registers in package globals.
func Call(fnAddr uintptr, args []byte) ([]byte, error) {
	stackSize := stackPad + 8 + len(args) + 64
	stackMem, err := syscall.Mmap(-1, 0, stackSize,
		syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_PRIVATE|syscall.MAP_ANON)
	if err != nil {
		return nil, fmt.Errorf("verify: stack mmap: %w", err)
	}
	defer syscall.Munmap(stackMem)

	retOff := stackPad
	binary.LittleEndian.PutUint64(stackMem[retOff:retOff+8], uint64(leaveJITAddr))
	copy(stackMem[retOff+8:], args)

	stackBase := uintptr(unsafe.Pointer(&stackMem[retOff]))
	enterJIT(fnAddr, stackBase)

	out := make([]byte, len(args))
	copy(out, stackMem[retOff+8:retOff+8+len(args)])
	return out, nil
}