sdk/include/setjmp.h

// Copyright Microsoft and CHERIoT Contributors.
// SPDX-License-Identifier: MIT
#pragma once

/**
 * This is a minimal implementation of setjmp/longjmp.
 *
 * CHERIoT cannot store a `jmp_buf` anywhere other than the stack without
 * clearing tags (which will then cause `longjmp` to fail).
 */

#include <stddef.h>
#include <stdint.h>

/**
 * Jump buffer for setjmp/longjmp.
 */
struct __jmp_buf
{
	uintptr_t __cs0;
	uintptr_t __cs1;
	uintptr_t __csp;
	uintptr_t __cra;
};

/**
 * C requires that `setjmp` and `longjmp` take a `jmp_buf` by reference and so
 * this ends up being defined as an array of one element, which allows it to
 * both be allocated and passed by reference.
 */
typedef struct __jmp_buf jmp_buf[1];

/**
 * C `setjmp` function.  Returns (up to) twice.  First returns 0, returns a
 * value passed to `longjmp` on the second return.
 */
__attribute__((returns_twice)) extern "C" int setjmp(jmp_buf env);
__asm__(".section .text.setjmp,\"awG\",@progbits,setjmp,comdat\n"
        ".globl setjmp\n"
        ".p2align 2\n"
        ".type setjmp,@function\n"
        "setjmp:\n"
        "	csc	cs0, 0(ca0)\n"
        "	csc	cs1, 8(ca0)\n"
        "	csc	csp, 16(ca0)\n"
        "	csc	cra, 24(ca0)\n"
        "	li	a0, 0\n"
        "	cret\n");

/**
 * C `longjmp` function.  Does not return, jumps back to the `setjmp` call.
 */
extern "C" void longjmp(jmp_buf env, int val);
__asm__(".section .text.longjmp,\"awG\",@progbits,longjmp,comdat\n"
        ".globl longjmp\n"
        ".p2align 2\n"
        ".type longjmp,@function\n"
        "longjmp:\n"
        "	clc	cs0, 0(ca0)\n"
        "	clc	cs1, 8(ca0)\n"
        "	clc	csp, 16(ca0)\n"
        "	clc	cra, 24(ca0)\n"
        "	mv	a0, a1\n"
        "	cjr	cra\n");