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>

1 files changed, 123 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/tst-pidfd_getpid.c b/sysdeps/unix/sysv/linux/tst-pidfd_getpid.c
new file mode 100644
index 0000000000..e48eed79ab
--- /dev/null
+++ b/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>