linux: Add pidfd_getpid

This interface allows to obtain the associated process ID from the
process file descriptor.  It is done by parsing the procps fdinfo
information.  Its prototype is:

   pid_t pidfd_getpid (int fd)

It returns the associated pid or -1 in case of an error and sets the
errno accordingly.  The possible errno values are those from open, read,
and close (used on procps parsing), along with:

   - EBADF if the FD is negative, does not have a PID associated, or if
     the fdinfo fields contain a value larger than pid_t.

   - EREMOTE if the PID is in a separate namespace.

   - ESRCH if the process is already terminated.

Checked on x86_64-linux-gnu on Linux 4.15 (no CLONE_PIDFD or waitid
support), Linux 5.4 (full support), and Linux 6.2.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

NEWS

@@ -27,6 +27,10 @@ Major new features:
   The pidfd functionality avoids the issue of PID reuse with the traditional
   posix_spawn interface.
 
+* On Linux, the pidfd_getpid function has been added.  It allows retrieving
+  the process ID associated with the process file descriptor created by
+  pid_spawn, fork_np, or pidfd_open.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
   [Add deprecations, removals and changes affecting compatibility here]

manual/process.texi

@@ -33,6 +33,7 @@ primitive functions to do each step individually instead.
 * Process Creation Concepts::   An overview of the hard way to do it.
 * Process Identification::      How to get the process ID of a process.
 * Creating a Process::          How to fork a child process.
+* Querying a Process::          How to query a child process.
 * Executing a File::            How to make a process execute another program.
 * Process Completion::          How to tell when a child process has completed.
 * Process Completion Status::   How to interpret the status value
@@ -362,6 +363,43 @@ the proper precautions for using @code{vfork}, your program will still
 work even if the system uses @code{fork} instead.
 @end deftypefun
 
+@node Querying a Process
+@section Querying a Process
+
+The file descriptor returned by the @code{pidfd_fork} function can be used to
+query process extra information.
+
+@deftypefun pid_t pidfd_getpid (int @var{fd})
+@standards{GNU, sys/pidfd.h}
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
+
+The @code{pidfd_getpid} function retrieves the process ID associated with process
+file descriptor created with @code{pid_spawn}, @code{pidfd_fork}, or
+@code{pidfd_open}.
+
+If the operation fails, @code{pidfd_getpid} return @code{-1} and the following
+@code{errno} error conditionas are defined:
+
+@table @code
+@item EBADF
+The input file descriptor is invalid, does not have a pidfd associated, or an
+error has occurred parsing the kernel data.
+@item EREMOTE
+There is no process ID to denote the process in the current namespace.
+@item ESRCH
+The process for which the file descriptor refers to is terminated.
+@item ENOENT
+The procfs is not mounted.
+@item ENFILE.
+Too many open files in system (@code{pidfd_open} tries to open a procfs file and
+read its contents).
+@item ENOMEM
+Insufficient kernel memory was available.
+@end table
+
+This function is specific to Linux.
+@end deftypefun
+
 @node Executing a File
 @section Executing a File
 @cindex executing a file

sysdeps/unix/sysv/linux/Makefile

@@ -213,6 +213,7 @@ tests += \
   tst-ofdlocks \
   tst-personality \
   tst-pidfd \
+  tst-pidfd_getpid \
   tst-pkey \
   tst-ppoll \
   tst-prctl \
@@ -493,8 +494,10 @@ sysdep_headers += \
 sysdep_routines += \
   getcpu \
   oldglob \
+  pidfd_getpid \
   pidfd_spawn \
   pidfd_spawnp \
+  procutils \
   sched_getcpu \
   spawnattr_getcgroup_np \
   spawnattr_setcgroup_np \

sysdeps/unix/sysv/linux/Versions

@@ -322,6 +322,7 @@ libc {
 %endif
   }
   GLIBC_2.39 {
+    pidfd_getpid;
     pidfd_spawn;
     pidfd_spawnp;
     posix_spawnattr_getcgroup_np;

sysdeps/unix/sysv/linux/aarch64/libc.abilist

@@ -2673,6 +2673,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/alpha/libc.abilist

@@ -2782,6 +2782,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/arc/libc.abilist

@@ -2434,6 +2434,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/arm/be/libc.abilist

@@ -554,6 +554,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/arm/le/libc.abilist

@@ -551,6 +551,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/csky/libc.abilist

@@ -2710,6 +2710,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/hppa/libc.abilist

@@ -2659,6 +2659,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/i386/libc.abilist

@@ -2843,6 +2843,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/ia64/libc.abilist

@@ -2608,6 +2608,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/loongarch/lp64/libc.abilist

@@ -2194,6 +2194,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist

@@ -555,6 +555,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist

@@ -2786,6 +2786,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/microblaze/be/libc.abilist

@@ -2759,6 +2759,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/microblaze/le/libc.abilist

@@ -2756,6 +2756,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist

@@ -2751,6 +2751,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist

@@ -2749,6 +2749,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist

@@ -2757,6 +2757,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist

@@ -2659,6 +2659,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/nios2/libc.abilist

@@ -2798,6 +2798,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/or1k/libc.abilist

@@ -2180,6 +2180,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/pidfd_getpid.c

@@ -0,0 +1,126 @@
+/* pidfd_getpid - Get the associated pid from the pid file descriptor.
+   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 <_itoa.h>
+#include <errno.h>
+#include <intprops.h>
+#include <procutils.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sysdep.h>
+#include <unistd.h>
+
+#define FDINFO_TO_FILENAME_PREFIX "/proc/self/fdinfo/"
+
+#define FDINFO_FILENAME_LEN \
+  (sizeof (FDINFO_TO_FILENAME_PREFIX) + INT_STRLEN_BOUND (int))
+
+struct parse_fdinfo_t
+{
+  bool found;
+  pid_t pid;
+};
+
+/* Parse the PID field in the fdinfo entry, if existent.  Avoid strtol or
+   similar to not be locale dependent.  */
+static int
+parse_fdinfo (const char *l, void *arg)
+{
+  enum { fieldlen = sizeof ("Pid:") - 1 };
+  if (strncmp (l, "Pid:", fieldlen) != 0)
+    return 0;
+
+  l += fieldlen;
+
+  /* Skip leading spaces.  */
+  while (*l == ' ' || (unsigned int) (*l) -'\t' < 5)
+    l++;
+
+  bool neg = false;
+  switch (*l)
+    {
+    case '-':
+      neg = true;
+      l++;
+      break;
+    case '+':
+      return -1;
+    }
+
+  if (*l == '\0')
+    return 0;
+
+  int n = 0;
+  while (*l != '\0')
+    {
+      /* Check if '*l' is a digit.  */
+      if ('0' > *l || *l > '9')
+        return -1;
+
+      /* Ignore invalid large values.  */
+      if (INT_MULTIPLY_WRAPV (10, n, &n)
+          || INT_ADD_WRAPV (n, *l++ - '0', &n))
+        return -1;
+    }
+
+  /* -1 indicates that the process is terminated.  */
+  if (neg && n != 1)
+    return -1;
+
+  struct parse_fdinfo_t *fdinfo = arg;
+  fdinfo->pid = neg ? -n : n;
+  fdinfo->found = true;
+
+  return 1;
+}
+
+pid_t
+pidfd_getpid (int fd)
+{
+  if (__glibc_unlikely (fd < 0))
+    {
+      __set_errno (EBADF);
+      return -1;
+    }
+
+  char fdinfoname[FDINFO_FILENAME_LEN];
+
+  char *p = mempcpy (fdinfoname, FDINFO_TO_FILENAME_PREFIX,
+		     strlen (FDINFO_TO_FILENAME_PREFIX));
+  *_fitoa_word (fd, p, 10, 0) = '\0';
+
+  struct parse_fdinfo_t fdinfo = { .found = false, .pid = -1 };
+  if (!procutils_read_file (fdinfoname, parse_fdinfo, &fdinfo))
+    /* The fdinfo contains an invalid 'Pid:' value.  */
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (EBADF);
+
+  /* The FD does not have a 'Pid:' entry associated.  */
+  if (!fdinfo.found)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (EBADF);
+
+  /* The pidfd cannot be resolved because it is in a separate pid
+     namespace.  */
+  if (fdinfo.pid == 0)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (EREMOTE);
+
+  /* A negative value means the process is terminated.  */
+  if (fdinfo.pid < 0)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (ESRCH);
+
+  return fdinfo.pid;
+}

sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist

@@ -2825,6 +2825,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist

@@ -2858,6 +2858,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist

@@ -2579,6 +2579,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist

@@ -2893,6 +2893,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/procutils.c

@@ -0,0 +1,97 @@
+/* Utilities functions to read/parse Linux procfs and sysfs.
+   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 <assert.h>
+#include <not-cancel.h>
+#include <procutils.h>
+#include <string.h>
+
+static int
+next_line (char **r, int fd, char *const buffer, char **cp, char **re,
+           char *const buffer_end)
+{
+  char *res = *cp;
+  char *nl = memchr (*cp, '\n', *re - *cp);
+  if (nl == NULL)
+    {
+      if (*cp != buffer)
+        {
+          if (*re == buffer_end)
+            {
+              memmove (buffer, *cp, *re - *cp);
+              *re = buffer + (*re - *cp);
+              *cp = buffer;
+
+              ssize_t n = TEMP_FAILURE_RETRY (
+		__read_nocancel (fd, *re, buffer_end - *re));
+              if (n < 0)
+                return -1;
+
+              *re += n;
+
+              nl = memchr (*cp, '\n', *re - *cp);
+	      if (nl == NULL)
+	        /* Line too long.  */
+		return 0;
+            }
+          else
+            nl = memchr (*cp, '\n', *re - *cp);
+
+          res = *cp;
+        }
+
+      if (nl == NULL)
+        nl = *re - 1;
+    }
+
+  *nl = '\0';
+  *cp = nl + 1;
+  assert (*cp <= *re);
+
+  if (res == *re)
+    return 0;
+
+  *r = res;
+  return 1;
+}
+
+bool
+procutils_read_file (const char *filename, procutils_closure_t closure,
+		     void *arg)
+{
+  enum { buffer_size = PROCUTILS_MAX_LINE_LEN };
+  char buffer[buffer_size];
+  char *buffer_end = buffer + buffer_size;
+  char *cp = buffer_end;
+  char *re = buffer_end;
+
+  int fd = TEMP_FAILURE_RETRY (
+    __open64_nocancel (filename, O_RDONLY | O_CLOEXEC));
+  if (fd == -1)
+    return false;
+
+  char *l;
+  int r;
+  while ((r = next_line (&l, fd, buffer, &cp, &re, buffer_end)) > 0)
+    if (closure (l, arg) != 0)
+      break;
+
+  __close_nocancel_nostatus (fd);
+
+  return r == 1;
+}

sysdeps/unix/sysv/linux/procutils.h

@@ -0,0 +1,43 @@
+/* Utilities functions to read/parse Linux procfs and sysfs.
+   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/>.  */
+
+#ifndef _PROCUTILS_H
+#define _PROCUTILS_H
+
+#include <stdbool.h>
+
+typedef int (*procutils_closure_t) (const char *line, void *arg);
+
+#define PROCUTILS_MAX_LINE_LEN 256
+
+/* Open and read the path FILENAME, line per line, and call CLOSURE with
+   argument ARG on each line.  The read is done with a static buffer,
+   with non-cancellable calls, and the line is null terminated.
+
+   The CLOSURE should return 0 if the read should continue, otherwise the
+   the function should stop and return early.
+
+   The '\n' is not included in the CLOSURE input argument and lines longer
+   than PROCUTILS_MAX_LINE_LEN characteres are ignored.
+
+   It returns true in case the file is fully read or false if CLOSURE
+   returns a value diferent than 0.  */
+bool procutils_read_file (const char *filename, procutils_closure_t closure,
+			  void *arg) attribute_hidden;
+
+#endif

sysdeps/unix/sysv/linux/riscv/rv32/libc.abilist

@@ -2436,6 +2436,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist

@@ -2636,6 +2636,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist

@@ -2823,6 +2823,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist

@@ -2616,6 +2616,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/sh/be/libc.abilist

@@ -2666,6 +2666,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/sh/le/libc.abilist

@@ -2663,6 +2663,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist

@@ -2818,6 +2818,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist

@@ -2631,6 +2631,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/sys/pidfd.h

@@ -46,4 +46,8 @@ extern int pidfd_getfd (int __pidfd, int __targetfd,
 extern int pidfd_send_signal (int __pidfd, int __sig, siginfo_t *__info,
 			      unsigned int __flags) __THROW;
 
+/* Query the process ID (PID) from process descriptor FD.  Return the PID
+   or -1 in case of an error.  */
+extern pid_t pidfd_getpid (int __fd) __THROW;
+
 #endif /* _PIDFD_H  */

sysdeps/unix/sysv/linux/tst-pidfd.c

@@ -18,6 +18,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <support/capture_subprocess.h>
 #include <support/check.h>
 #include <support/process_state.h>
@@ -27,6 +28,8 @@
 #include <support/xsocket.h>
 #include <sys/pidfd.h>
 #include <sys/wait.h>
+#include <stdlib.h>
+#include <unistd.h>
 
 #define REMOTE_PATH "/dev/null"
 
@@ -102,6 +105,44 @@ do_test (void)
   ppid = getpid ();
   puid = getuid ();
 
+  /* Sanity check for invalid inputs.  */
+  TEST_COMPARE (pidfd_getpid (-1), -1);
+  TEST_COMPARE (errno, EBADF);
+
+  {
+    pid_t pid = pidfd_getpid (STDOUT_FILENO);
+    TEST_COMPARE (pid, -1);
+    TEST_COMPARE (errno, EBADF);
+  }
+
+  /* Check if pidfd_getpid returns ESRCH for exited subprocess.  */
+  {
+    pid_t pidfork = xfork ();
+    if (pidfork == 0)
+      _exit (EXIT_SUCCESS);
+    int pidfork_pidfd = pidfd_open (pidfork, 0);
+
+    /* The process might be still running or already in zombie state, in
+       either case the PID is still allocated to the process.  */
+    pid_t pid = pidfd_getpid (pidfork_pidfd);
+    TEST_COMPARE (pidfork, pid);
+    if (pid > 0)
+      support_process_state_wait (pid, support_process_state_zombie);
+
+    siginfo_t info;
+    TEST_COMPARE (waitid (P_PIDFD, pidfork_pidfd, &info, WEXITED), 0);
+    TEST_COMPARE (info.si_pid, pidfork);
+    TEST_COMPARE (info.si_status, 0);
+    TEST_COMPARE (info.si_code, CLD_EXITED);
+
+    /* Once the process is reaped the associated PID is not available.  */
+    pid = pidfd_getpid (pidfork_pidfd);
+    TEST_COMPARE (pid, -1);
+    TEST_COMPARE (errno, ESRCH);
+
+    xclose (pidfork_pidfd);
+  }
+
   TEST_COMPARE (socketpair (AF_UNIX, SOCK_STREAM, 0, sockets), 0);
 
   pid_t pid = xfork ();
@@ -118,6 +159,12 @@ do_test (void)
   int pidfd = pidfd_open (pid, 0);
   TEST_VERIFY (pidfd != -1);
 
+  TEST_COMPARE (pidfd_getpid (INT_MAX), -1);
+  {
+    pid_t querypid = pidfd_getpid (pidfd);
+    TEST_COMPARE (querypid, pid);
+  }
+
   /* Wait for first sigtimedwait.  */
   support_process_state_wait (pid, support_process_state_sleeping);
   TEST_COMPARE (pidfd_send_signal (pidfd, SIGUSR1, NULL, 0), 0);

sysdeps/unix/sysv/linux/tst-pidfd_getpid.c

@@ -0,0 +1,123 @@
+/* Specific tests for Linux pidfd_getpid.
+   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 <errno.h>
+#include <sched.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/xunistd.h>
+#include <support/test-driver.h>
+#include <sys/pidfd.h>
+#include <sys/wait.h>
+#include <sys/mount.h>
+
+static int
+do_test (void)
+{
+  {
+    /* The pidfd_getfd syscall was the last in the set of pidfd related
+       syscalls added to the kernel.  Use pidfd_getfd to decide if this
+       kernel has pidfd support that we can test.  */
+    int r = pidfd_getfd (0, 0, 1);
+    TEST_VERIFY_EXIT (r == -1);
+    if (errno == ENOSYS)
+      FAIL_UNSUPPORTED ("kernel does not support pidfd_getfd, skipping test");
+    if (errno == EPERM)
+      FAIL_UNSUPPORTED ("kernel does not allow pidfd_getfd, skipping test");
+  }
+
+  /* Check if pidfd_getpid returns EREMOTE for process not in current
+     namespace.  */
+  {
+    pid_t child0 = xfork ();
+    TEST_VERIFY_EXIT (child0 >= 0);
+    if (child0 == 0)
+      {
+	/* Create another unrelated descriptor, so child2 will inherit the
+	   file descriptor.  */
+	pid_t child1 = xfork ();
+	TEST_VERIFY_EXIT (child1 >= 0);
+        if (child1 == 0)
+	  _exit (0);
+	int child1_pidfd = pidfd_open (child1, 0);
+	TEST_VERIFY_EXIT (child1_pidfd != -1);
+
+	if (unshare (CLONE_NEWNS | CLONE_NEWUSER | CLONE_NEWPID) < 0)
+	  {
+	    /* Older kernels may not support all the options, or security
+	       policy may block this call.  */
+	    if (errno == EINVAL || errno == EPERM || errno == ENOSPC)
+	      exit (EXIT_UNSUPPORTED);
+	    FAIL_EXIT1 ("unshare user/fs/pid failed: %m");
+	  }
+
+	if (mount (NULL, "/", NULL, MS_REC | MS_PRIVATE, 0) != 0)
+	  {
+	    /* This happens if we're trying to create a nested container,
+	       like if the build is running under podman, and we lack
+	       priviledges.  */
+	    if (errno  == EPERM)
+	      _exit (EXIT_UNSUPPORTED);
+	    else
+	      _exit (EXIT_FAILURE);
+	  }
+
+	pid_t child2 = xfork ();
+	if (child2 > 0)
+	  {
+	    int status;
+	    xwaitpid (child2, &status, 0);
+	    TEST_VERIFY (WIFEXITED (status));
+	    xwaitpid (child1, &status, 0);
+	    TEST_VERIFY (WIFEXITED (status));
+
+	    _exit (WEXITSTATUS (status));
+	  }
+
+	/* Now that we're pid 1 (effectively "root") we can mount /proc  */
+	if (mount ("proc", "/proc", "proc", 0, NULL) != 0)
+	  {
+	    if (errno == EPERM)
+	      _exit (EXIT_UNSUPPORTED);
+	    else
+	      _exit (EXIT_FAILURE);
+	  }
+
+	TEST_COMPARE (pidfd_getpid (child1_pidfd), -1);
+	TEST_COMPARE (errno, EREMOTE);
+
+	_exit (EXIT_SUCCESS);
+      }
+      int child0_pidfd = pidfd_open (child0, 0);
+      TEST_VERIFY_EXIT (child0_pidfd != -1);
+
+      pid_t child0pid = pidfd_getpid (child0_pidfd);
+
+      siginfo_t info;
+      TEST_COMPARE (waitid (P_PIDFD, child0_pidfd, &info, WEXITED), 0);
+      if (info.si_status == EXIT_UNSUPPORTED)
+	FAIL_UNSUPPORTED ("unable to unshare user/fs/pid");
+      TEST_COMPARE (info.si_status, 0);
+      TEST_COMPARE (info.si_code, CLD_EXITED);
+      TEST_COMPARE (info.si_pid, child0pid);
+  }
+
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/unix/sysv/linux/x86_64/64/libc.abilist

@@ -2582,6 +2582,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F

sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist

@@ -2688,6 +2688,7 @@ GLIBC_2.38 strlcat F
 GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
+GLIBC_2.39 pidfd_getpid F
 GLIBC_2.39 pidfd_spawn F
 GLIBC_2.39 pidfd_spawnp F
 GLIBC_2.39 posix_spawnattr_getcgroup_np F