| 1 | /* Check if kernel supports PID file descriptors. |
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| 2 | Copyright (C) 2023-2024 Free Software Foundation, Inc. |
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| 3 | This file is part of the GNU C Library. |
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| 4 | |
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| 5 | The GNU C Library is free software; you can redistribute it and/or |
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| 6 | modify it under the terms of the GNU Lesser General Public |
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| 7 | License as published by the Free Software Foundation; either |
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| 8 | version 2.1 of the License, or (at your option) any later version. |
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| 9 | |
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| 10 | The GNU C Library is distributed in the hope that it will be useful, |
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| 11 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
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| 12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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| 13 | Lesser General Public License for more details. |
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| 14 | |
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| 15 | You should have received a copy of the GNU Lesser General Public |
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| 16 | License along with the GNU C Library; if not, see |
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| 17 | <https://www.gnu.org/licenses/>. */ |
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| 18 | |
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| 19 | #include <atomic.h> |
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| 20 | #include <sys/wait.h> |
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| 21 | #include <sysdep.h> |
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| 22 | |
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| 23 | /* The PID file descriptors was added during multiple releases: |
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| 24 | - Linux 5.2 added CLONE_PIDFD support for clone and __clone_pidfd_supported |
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| 25 | syscall. |
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| 26 | - Linux 5.3 added support for poll and CLONE_PIDFD for clone3. |
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| 27 | - Linux 5.4 added P_PIDFD support on waitid. |
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| 28 | |
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| 29 | For internal usage on spawn and fork, it only make sense to return a file |
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| 30 | descriptor if caller can actually waitid on it. */ |
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| 31 | |
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| 32 | static int __waitid_pidfd_supported = 0; |
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| 33 | |
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| 34 | bool |
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| 35 | __clone_pidfd_supported (void) |
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| 36 | { |
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| 37 | int state = atomic_load_relaxed (&__waitid_pidfd_supported); |
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| 38 | if (state == 0) |
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| 39 | { |
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| 40 | /* Linux define the maximum allocated file descriptor value as |
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| 41 | 0x7fffffc0 (from fs/file.c): |
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| 42 | |
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| 43 | #define __const_min(x, y) ((x) < (y) ? (x) : (y)) |
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| 44 | unsigned int sysctl_nr_open_max = |
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| 45 | __const_min(INT_MAX, ~(size_t)0/sizeof(void *)) & -BITS_PER_LONG; |
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| 46 | |
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| 47 | So we can detect whether kernel supports all pidfd interfaces by |
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| 48 | using a valid but never allocated file descriptor: if is not |
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| 49 | supported waitid will return EINVAL, otherwise EBADF. |
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| 50 | |
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| 51 | Also the waitid is a cancellation entrypoint, so issue the syscall |
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| 52 | directly. */ |
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| 53 | int r = INTERNAL_SYSCALL_CALL (waitid, P_PIDFD, INT_MAX, NULL, |
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| 54 | WEXITED | WNOHANG); |
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| 55 | state = r == -EBADF ? 1 : -1; |
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| 56 | atomic_store_relaxed (&__waitid_pidfd_supported, state); |
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| 57 | } |
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| 58 | |
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| 59 | return state > 0; |
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| 60 | } |
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| 61 | |
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