hurd: Implement SHM_ANON

This adds a special SHM_ANON value that can be passed into shm_open ()
in place of a name. When called in this way, shm_open () will create a
new anonymous shared memory file. The file will be created in the same
way that other shared memory files are created (i.e., under /dev/shm/),
except that it is not given a name and therefore cannot be reached from
the file system, nor by other calls to shm_open (). This is accomplished
by utilizing O_TMPFILE.

This is intended to be compatible with FreeBSD's API of the same name.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230130125216.6254-4-bugaevc@gmail.com>

posix/shm-directory.c

@@ -23,17 +23,34 @@
 #include <alloc_buffer.h>
 #include <shm-directory.h>
 #include <string.h>
+#include <sys/mman.h>
+#include <fcntl.h>
 
 int
 __shm_get_name (struct shmdir_name *result, const char *name, bool sem_prefix)
 {
+  struct alloc_buffer buffer;
+  size_t namelen;
+
+  buffer = alloc_buffer_create (result->name, sizeof (result->name));
+  alloc_buffer_copy_bytes (&buffer, SHMDIR, strlen (SHMDIR));
+
+#if defined (SHM_ANON) && defined (O_TMPFILE)
+  if (name == SHM_ANON)
+    {
+      /* For SHM_ANON, we want shm_open () to pass O_TMPFILE to open (),
+         with SHMDIR itself as the path.  So, leave it at that.  */
+      alloc_buffer_add_byte (&buffer, 0);
+      if (alloc_buffer_has_failed (&buffer))
+        return -1;
+      return 0;
+    }
+#endif
+
   while (name[0] == '/')
     ++name;
-  size_t namelen = strlen (name);
+  namelen = strlen (name);
 
-  struct alloc_buffer buffer
-    = alloc_buffer_create (result->name, sizeof (result->name));
-  alloc_buffer_copy_bytes (&buffer, SHMDIR, strlen (SHMDIR));
   if (sem_prefix)
     alloc_buffer_copy_bytes (&buffer, "sem.", strlen ("sem."));
   alloc_buffer_copy_bytes (&buffer, name, namelen + 1);

rt/shm_open.c

@@ -23,6 +23,7 @@
 #include <shlib-compat.h>
 #include <shm-directory.h>
 #include <unistd.h>
+#include <sys/mman.h>
 
 /* Open shared memory object.  */
 int
@@ -36,6 +37,10 @@ __shm_open (const char *name, int oflag, mode_t mode)
     }
 
   oflag |= O_NOFOLLOW | O_CLOEXEC;
+#if defined (SHM_ANON) && defined (O_TMPFILE)
+  if (name == SHM_ANON)
+    oflag |= O_TMPFILE;
+#endif
 
   int fd = __open64_nocancel (dirname.name, oflag, mode);
   if (fd == -1 && __glibc_unlikely (errno == EISDIR))

sysdeps/mach/hurd/bits/mman_ext.h

@@ -0,0 +1,25 @@
+/* System-specific extensions of <sys/mman.h>, Hurd version.
+   Copyright (C) 2022 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 _SYS_MMAN_H
+# error "Never include <bits/mman_ext.h> directly; use <sys/mman.h> instead."
+#endif
+
+#ifdef __USE_GNU
+# define SHM_ANON	((const char *) 1)
+#endif /* __USE_GNU  */