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61 lines
2.2 KiB
C
61 lines
2.2 KiB
C
/* Check if kernel supports PID file descriptors.
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Copyright (C) 2023-2025 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <atomic.h>
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#include <sys/wait.h>
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#include <sysdep.h>
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/* The PID file descriptors was added during multiple releases:
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- Linux 5.2 added CLONE_PIDFD support for clone and __clone_pidfd_supported
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syscall.
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- Linux 5.3 added support for poll and CLONE_PIDFD for clone3.
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- Linux 5.4 added P_PIDFD support on waitid.
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For internal usage on spawn and fork, it only make sense to return a file
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descriptor if caller can actually waitid on it. */
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static int __waitid_pidfd_supported = 0;
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bool
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__clone_pidfd_supported (void)
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{
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int state = atomic_load_relaxed (&__waitid_pidfd_supported);
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if (state == 0)
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{
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/* Linux define the maximum allocated file descriptor value as
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0x7fffffc0 (from fs/file.c):
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#define __const_min(x, y) ((x) < (y) ? (x) : (y))
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unsigned int sysctl_nr_open_max =
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__const_min(INT_MAX, ~(size_t)0/sizeof(void *)) & -BITS_PER_LONG;
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So we can detect whether kernel supports all pidfd interfaces by
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using a valid but never allocated file descriptor: if is not
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supported waitid will return EINVAL, otherwise EBADF.
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Also the waitid is a cancellation entrypoint, so issue the syscall
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directly. */
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int r = INTERNAL_SYSCALL_CALL (waitid, P_PIDFD, INT_MAX, NULL,
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WEXITED | WNOHANG);
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state = r == -EBADF ? 1 : -1;
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atomic_store_relaxed (&__waitid_pidfd_supported, state);
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}
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return state > 0;
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}
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