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/* Check if kernel supports PID file descriptors.
Copyright (C) 2023 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <atomic.h>
#include <sys/wait.h>
#include <sysdep.h>
/* The PID file descriptors was added during multiple releases:
- Linux 5.2 added CLONE_PIDFD support for clone and __clone_pidfd_supported
syscall.
- Linux 5.3 added support for poll and CLONE_PIDFD for clone3.
- Linux 5.4 added P_PIDFD support on waitid.
For internal usage on spawn and fork, it only make sense to return a file
descriptor if caller can actually waitid on it. */
static int __waitid_pidfd_supported = 0;
bool
__clone_pidfd_supported (void)
{
int state = atomic_load_relaxed (&__waitid_pidfd_supported);
if (state == 0)
{
/* Linux define the maximum allocated file descriptor value as
0x7fffffc0 (from fs/file.c):
#define __const_min(x, y) ((x) < (y) ? (x) : (y))
unsigned int sysctl_nr_open_max =
__const_min(INT_MAX, ~(size_t)0/sizeof(void *)) & -BITS_PER_LONG;
So we can detect whether kernel supports all pidfd interfaces by
using a valid but never allocated file descriptor: if is not
supported waitid will return EINVAL, otherwise EBADF.
Also the waitid is a cancellation entrypoint, so issue the syscall
directly. */
int r = INTERNAL_SYSCALL_CALL (waitid, P_PIDFD, INT_MAX, NULL,
WEXITED | WNOHANG);
state = r == -EBADF ? 1 : -1;
atomic_store_relaxed (&__waitid_pidfd_supported, state);
}
return state > 0;
}
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