78 lines
2.6 KiB
Go
78 lines
2.6 KiB
Go
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// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
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// SPDX-License-Identifier: BSD-3-Clause
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//go:build amd64
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package verify
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import (
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"encoding/binary"
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"fmt"
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"reflect"
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"syscall"
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"unsafe"
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)
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// savedSP holds the Go stack pointer while a JIT call is in flight.
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// Referenced by the assembly trampoline (trampoline_amd64.s).
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var savedSP uintptr
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// enterJIT switches to the prepared stack and jumps to fn.
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// It does not return normally; the JIT function's RET transfers control
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// to leaveJIT, which restores the Go stack.
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//
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//go:nosplit
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func enterJIT(fn uintptr, stack uintptr)
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// leaveJIT restores the Go stack after a JIT function returns.
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// Its address is placed as the return address on the prepared stack.
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//
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//go:nosplit
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func leaveJIT()
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// leaveJITAddr is the machine address of leaveJIT, resolved once at init.
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var leaveJITAddr uintptr
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func init() {
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leaveJITAddr = reflect.ValueOf(leaveJIT).Pointer()
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}
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// stackPad is padding below the return address on the prepared stack.
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// The ABIInternal wrapper that leaveJIT's address resolves to executes
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// PUSHQ BP and CALL before reaching the raw assembly, writing up to 16
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// bytes below the return-address slot. 64 bytes of headroom is ample.
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const stackPad = 64
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// Call invokes the assembled function at fnAddr with the given ABI0 argument
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// block (the raw bytes that would appear at FP+0). It returns the argument
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// block after the call, which contains any results the function wrote back
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// (the ABI0 convention shares the argument area for inputs and outputs).
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//
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// The function must be NOSPLIT (no stack growth) and must not reference
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// external symbols — the image is self-contained.
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func Call(fnAddr uintptr, args []byte) ([]byte, error) {
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// Prepare the stack: [padding][leaveJIT addr][args...]
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stackSize := stackPad + 8 + len(args) + 64 // padding + ret + args + safety
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stackMem, err := syscall.Mmap(-1, 0, stackSize,
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syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_PRIVATE|syscall.MAP_ANON)
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if err != nil {
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return nil, fmt.Errorf("verify: stack mmap: %w", err)
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}
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defer syscall.Munmap(stackMem)
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// The return address sits after the padding; the function's SP will
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// point here, leaving stackPad bytes below for the wrapper's pushes.
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retOff := stackPad
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binary.LittleEndian.PutUint64(stackMem[retOff:retOff+8], uint64(leaveJITAddr))
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// The ABI0 argument area follows the return address.
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copy(stackMem[retOff+8:], args)
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stackBase := uintptr(unsafe.Pointer(&stackMem[retOff]))
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enterJIT(fnAddr, stackBase)
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// Copy out the (possibly modified) argument area.
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out := make([]byte, len(args))
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copy(out, stackMem[retOff+8:retOff+8+len(args)])
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return out, nil
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}
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