pddupfd(2)

[kib]

Summary

This is inspired by Linux' pidfd_getfd(), and should put our procdesc facility mostly on par with Linux.

[kib]

The test program

/* $Id: pddupfd.c,v 1.2 2026/05/22 03:59:31 kostik Exp kostik $ */

#include <sys/types.h>
#include <sys/fcntl.h>
#include <sys/mman.h>
#include <sys/procdesc.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <err.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define MY_SYS_pdopenpid	603
#define MY_SYS_pddupfd		604

static int
my_pdopenpid(pid_t pid, int flags)
{
	return (syscall(MY_SYS_pdopenpid, pid, flags));
}

static int
my_pddupfd(pid_t pid, int remote_fd, int flags)
{
	return (syscall(MY_SYS_pddupfd, pid, remote_fd, flags));
}

static void
check(int fd, int child, int *comm)
{
	struct __wrusage wu;
	struct __siginfo si;
	int error, f, pp, status, workfd;
	char cmd[128];

	error = pdgetpid(fd, &pp);
	if (error == -1)
		err(1, "pgdetpid");
	printf("child pid %d %d\n", child, pp);

	while (*comm == -1)
		;
	workfd = my_pddupfd(fd, *comm, 0);
	if (workfd == -1)
		err(1, "pddupfd");

	f = fcntl(workfd, F_GETFD);
	if (f == -1)
		err(1, "F_GETFD %d", workfd);
	f &= ~FD_CLOEXEC;
	error = fcntl(workfd, F_SETFD, f);
	if (error == -1)
		err(1, "F_SETFD %d", workfd);

	snprintf(cmd, sizeof(cmd), "cat 0<&%d", workfd);
	system(cmd);
	close(workfd);

	*comm = 0;
	
	error = pdwait(fd, &status, WEXITED, &wu, &si);
	if (error == -1)
		err(1, "pdwait");
	printf("exited pid %d status %#x si.si_status %#x si.si_code %#x\n",
	    pp, status, si.si_status, si.si_code);
}

int
main(void)
{
	int *comm;
	pid_t child;
	int fd, workfd;

	comm = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED |
	    MAP_ANONYMOUS, -1, 0);
	if (comm == MAP_FAILED)
		err(1, "mmap");
	*comm = -1;

	child = fork();
	if (child == -1)
		err(1, "fork");
	if (child == 0) {
		workfd = open("/etc/passwd", O_RDONLY);
		if (workfd == -1)
			err(1, "open");
		*comm = workfd;
		while (*comm != 0)
			sleep(1);
		_exit(0x123);
	}

	fd = my_pdopenpid(child, 0);
	if (fd == -1)
		err(1, "pdopenpid");

	*comm = 1;
	check(fd, child, comm);
	close(fd);
}

[kib]

Document pddupfd.

[kib]

Preseve capabilities and RESOLVE_BENEATH of the source fd.

[markj]

I cannot apply the patch, it fails in pdfork.2. (I already applied the pdopenpid() patch.) Could you please rebase? Or, if your branch is already public, could you please include a pointer to it in the review description so that it's easier for me to fetch? That makes it easier for me to e.g., write syzkaller descriptions for new syscalls.

[kib]

In D57163#1311399, @markj wrote:

I cannot apply the patch, it fails in pdfork.2. (I already applied the pdopenpid() patch.) Could you please rebase? Or, if your branch is already public, could you please include a pointer to it in the review description so that it's easier for me to fetch? That makes it easier for me to e.g., write syzkaller descriptions for new syscalls.

I had to apply ACL to my gitweb server due to 'AI'. If you provide me with the list of IP addresses to enable, I will open for them.

I pushed the code to github.com/kostikbel/freebsd-src branch pdrfork.