src/basic/raw-clone.h

/* SPDX-License-Identifier: LGPL-2.1-or-later */
#pragma once

/***
  Copyright © 2016 Michael Karcher
***/

#include <errno.h>
#include <sched.h>
#include <sys/syscall.h>

#include "log.h"
#include "macro.h"

/**
 * raw_clone() - uses clone to create a new process with clone flags
 * @flags: Flags to pass to the clone system call
 *
 * Uses the clone system call to create a new process with the cloning flags and termination signal passed in the flags
 * parameter. Opposed to glibc's clone function, using this function does not set up a separate stack for the child, but
 * relies on copy-on-write semantics on the one stack at a common virtual address, just as fork does.
 *
 * To obtain copy-on-write semantics, flags must not contain CLONE_VM, and thus CLONE_THREAD and CLONE_SIGHAND (which
 * require CLONE_VM) are not usable.
 *
 * Additionally, as this function does not pass the ptid, newtls and ctid parameters to the kernel, flags must not
 * contain CLONE_PARENT_SETTID, CLONE_CHILD_SETTID, CLONE_CHILD_CLEARTID or CLONE_SETTLS.
 *
 * Returns: 0 in the child process and the child process id in the parent.
 */
static inline pid_t raw_clone(unsigned long flags) {
        pid_t ret;

        assert((flags & (CLONE_VM|CLONE_PARENT_SETTID|CLONE_CHILD_SETTID|
                         CLONE_CHILD_CLEARTID|CLONE_SETTLS)) == 0);
#if defined(__s390x__) || defined(__s390__) || defined(__CRIS__)
        /* On s390/s390x and cris the order of the first and second arguments
         * of the raw clone() system call is reversed. */
        ret = (pid_t) syscall(__NR_clone, NULL, flags);
#elif defined(__sparc__)
        {
                /**
                 * sparc always returns the other process id in %o0, and
                 * a boolean flag whether this is the child or the parent in
                 * %o1. Inline assembly is needed to get the flag returned
                 * in %o1.
                 */
                int in_child, child_pid, error;

                asm volatile("mov %3, %%g1\n\t"
                             "mov %4, %%o0\n\t"
                             "mov 0 , %%o1\n\t"
#if defined(__arch64__)
                             "t 0x6d\n\t"
#else
                             "t 0x10\n\t"
#endif
                             "addx %%g0, 0, %2\n\t"
                             "mov %%o1, %0\n\t"
                             "mov %%o0, %1" :
                             "=r"(in_child), "=r"(child_pid), "=r"(error) :
                             "i"(__NR_clone), "r"(flags) :
                             "%o1", "%o0", "%g1", "cc" );

                if (error) {
                        errno = child_pid;
                        ret = -1;
                } else
                        ret = in_child ? 0 : child_pid;
        }
#else
        ret = (pid_t) syscall(__NR_clone, flags, NULL);
#endif

        if (ret == 0)
                reset_cached_pid();

        return ret;
}