Re: [PATCH v3 2/2] arch: wire-up clone3() syscall

[Christian Brauner <christian@brauner.io>]

On Tue, Jun 04, 2019 at 08:40:01PM +0200, Arnd Bergmann wrote:
> On Tue, Jun 4, 2019 at 6:09 PM Christian Brauner <christian@brauner.io> wrote:
> >
> > Wire up the clone3() call on all arches that don't require hand-rolled
> > assembly.
> >
> > Some of the arches look like they need special assembly massaging and it is
> > probably smarter if the appropriate arch maintainers would do the actual
> > wiring. Arches that are wired-up are:
> > - x86{_32,64}
> > - arm{64}
> > - xtensa
> 
> The ones you did look good to me. I would hope that we can do all other
> architectures the same way, even if they have special assembly wrappers
> for the old clone(). The most interesting cases appear to be ia64, alpha,
> m68k and sparc, so it would be good if their maintainers could take a
> look.

Yes, agreed. They can sort this out even after this lands.

> 
> What do you use for testing? Would it be possible to override the
> internal clone() function in glibc with an LD_PRELOAD library
> to quickly test one of the other architectures for regressions?

I have a test program that is rather horrendously ugly and I compiled
kernels for x86 and the arms and tested in qemu. The program basically
looks like [1].

Christian

[1]:
#define _GNU_SOURCE
#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/sched.h>
#include <linux/types.h>
#include <sched.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mount.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/sysmacros.h>
#include <sys/types.h>
#include <sys/un.h>
#include <sys/wait.h>
#include <unistd.h>

static pid_t raw_clone(struct clone_args *args)
{
	return syscall(__NR_clone3, args, sizeof(struct clone_args));
}

static pid_t raw_clone_legacy(int *pidfd, unsigned int flags)
{
	return syscall(__NR_clone, flags, 0, pidfd, 0, 0);
}

static int wait_for_pid(pid_t pid)
{
	int status, ret;

again:
	ret = waitpid(pid, &status, 0);
	if (ret == -1) {
		if (errno == EINTR)
			goto again;

		return -1;
	}

	if (ret != pid)
		goto again;

	if (!WIFEXITED(status) || WEXITSTATUS(status) != 0)
		return -1;

	return 0;
}

#define ptr_to_u64(ptr) ((__u64)((uintptr_t)(ptr)))
#define u64_to_ptr(n) ((uintptr_t)((__u64)(n)))

int main(int argc, char *argv[])
{
	int pidfd = -1;
	pid_t parent_tid = -1, pid = -1;
	struct clone_args args = {0};
	args.parent_tid = ptr_to_u64(&parent_tid);
	args.pidfd = ptr_to_u64(&pidfd);
	args.flags = CLONE_PIDFD | CLONE_PARENT_SETTID;
	args.exit_signal = SIGCHLD;

	pid = raw_clone(&args);
	if (pid < 0) {
		fprintf(stderr, "%s - Failed to create new process\n",
			strerror(errno));
		exit(EXIT_FAILURE);
	}

	if (pid == 0) {
		printf("I am the child with pid %d\n", getpid());
		exit(EXIT_SUCCESS);
	}

	printf("raw_clone: I am the parent. My child's pid is   %d\n", pid);
	printf("raw_clone: I am the parent. My child's pidfd is %d\n",
	       *(int *)args.pidfd);
	printf("raw_clone: I am the parent. My child's paren_tid value is %d\n",
	       *(pid_t *)args.parent_tid);

	if (wait_for_pid(pid))
		exit(EXIT_FAILURE);

	if (pid != *(pid_t *)args.parent_tid)
		exit(EXIT_FAILURE);

	close(pidfd);

	printf("\n\n");
	pidfd = -1;
	pid = raw_clone_legacy(&pidfd, CLONE_PIDFD | SIGCHLD);
	if (pid < 0) {
		fprintf(stderr, "%s - Failed to create new process\n",
			strerror(errno));
		exit(EXIT_FAILURE);
	}

	if (pid == 0) {
		printf("I am the child with pid %d\n", getpid());
		exit(EXIT_SUCCESS);
	}

	printf("raw_clone_legacy: I am the parent. My child's pid is   %d\n",
	       pid);
	printf("raw_clone_legacy: I am the parent. My child's pidfd is %d\n",
	       pidfd);

	if (wait_for_pid(pid))
		exit(EXIT_FAILURE);

	if (pid != *(pid_t *)args.parent_tid)
		exit(EXIT_FAILURE);

	return 0;
}