libucontext/arch/x86/makecontext.c

/*
 * Copyright (c) 2018 Ariadne Conill <ariadne@dereferenced.org>
 * Copyright (c) 2019 A. Wilcox <awilfox@adelielinux.org>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * This software is provided 'as is' and without any warranty, express or
 * implied.  In no event shall the authors be liable for any damages arising
 * from the use of this software.
 */

#define _GNU_SOURCE
#include <stddef.h>
#include <stdarg.h>
#include <string.h>
#include <stdint.h>


extern void libucontext_trampoline(void);


void
libucontext_makecontext(ucontext_t *ucp, void (*func)(void), int argc, ...)
{
	libucontext_greg_t *sp, *argp;
	va_list va;
	int i;
	unsigned int uc_link;

	uc_link = (argc > 6 ? argc - 6 : 0) + 1;

	sp = (libucontext_greg_t *) ((uintptr_t) ucp->uc_stack.ss_sp + ucp->uc_stack.ss_size);
	sp -= uc_link;
	sp = (libucontext_greg_t *) (((uintptr_t) sp & -16L) - 8);

	ucp->uc_mcontext.gregs[REG_EIP] = (uintptr_t) func;
	ucp->uc_mcontext.gregs[REG_EBX] = (uintptr_t) argc;
	ucp->uc_mcontext.gregs[REG_ESP] = (uintptr_t) sp;

	argp = sp;
	*argp++ = (uintptr_t) &libucontext_trampoline;

	va_start(va, argc);

	for (i = 0; i < argc; i++)
		*argp++ = va_arg (va, libucontext_greg_t);

	va_end(va);

	*argp++ = (uintptr_t) ucp->uc_link;
}


extern __typeof(libucontext_makecontext) makecontext __attribute__((weak, __alias__("libucontext_makecontext")));