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, 97 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/procutils.c b/sysdeps/unix/sysv/linux/procutils.c
new file mode 100644
index 0000000..30fc3a0
--- /dev/null
+++ b/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;
+}