linux: Require /dev/shm as the shared memory file system

Previously, glibc would pick an arbitrary tmpfs file system from
/proc/mounts if /dev/shm was not available.  This could lead to
an unsuitable file system being picked for the backing storage for
shm_open, sem_open, and related functions.

This patch introduces a new function, __shm_get_name, which builds
the file name under the appropriate (now hard-coded) directory.  It is
called from the various shm_* and sem_* function.  Unlike the
SHM_GET_NAME macro it replaces, the callers handle the return values
and errno updates.  shm-directory.c is moved directly into the posix
subdirectory because it can be implemented directly using POSIX
functionality.  It resides in libc because it is needed by both
librt and nptl/htl.

In the sem_open implementation, tmpfname is initialized directly
from a string constant.  This happens to remove one alloca call.

Checked on x86_64-linux-gnu.

1 files changed, 0 insertions, 147 deletions

diff --git a/sysdeps/unix/sysv/linux/shm-directory.c b/sysdeps/unix/sysv/linux/shm-directory.c
deleted file mode 100644
index ea6d2a46c1..0000000000
--- a/sysdeps/unix/sysv/linux/shm-directory.c
+++ /dev/null
@@ -1,147 +0,0 @@
-/* Determine directory for shm/sem files.  Linux version.
-   Copyright (C) 2000-2021 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 "shm-directory.h"
-
-#include <errno.h>
-#include <mntent.h>
-#include <paths.h>
-#include <stdio.h>
-#include <string.h>
-#include <sys/statfs.h>
-#include <libc-lock.h>
-#include "linux_fsinfo.h"
-
-
-/* Mount point of the shared memory filesystem.  */
-static struct
-{
-  char *dir;
-  size_t dirlen;
-} mountpoint;
-
-/* This is the default directory.  */
-static const char defaultdir[] = "/dev/shm/";
-
-/* Protect the `mountpoint' variable above.  */
-__libc_once_define (static, once);
-
-
-/* Determine where the shmfs is mounted (if at all).  */
-static void
-where_is_shmfs (void)
-{
-  char buf[512];
-  struct statfs f;
-  struct mntent resmem;
-  struct mntent *mp;
-  FILE *fp;
-
-  /* The canonical place is /dev/shm.  This is at least what the
-     documentation tells everybody to do.  */
-  if (__statfs (defaultdir, &f) == 0 && (f.f_type == SHMFS_SUPER_MAGIC
-                                         || f.f_type == RAMFS_MAGIC))
-    {
-      /* It is in the normal place.  */
-      mountpoint.dir = (char *) defaultdir;
-      mountpoint.dirlen = sizeof (defaultdir) - 1;
-
-      return;
-    }
-
-  /* OK, do it the hard way.  Look through the /proc/mounts file and if
-     this does not exist through /etc/fstab to find the mount point.  */
-  fp = __setmntent ("/proc/mounts", "r");
-  if (__glibc_unlikely (fp == NULL))
-    {
-      fp = __setmntent (_PATH_MNTTAB, "r");
-      if (__glibc_unlikely (fp == NULL))
-        /* There is nothing we can do.  Blind guesses are not helpful.  */
-        return;
-    }
-
-  /* Now read the entries.  */
-  while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
-    /* The original name is "shm" but this got changed in early Linux
-       2.4.x to "tmpfs".  */
-    if (strcmp (mp->mnt_type, "tmpfs") == 0
-        || strcmp (mp->mnt_type, "shm") == 0)
-      {
-        /* Found it.  There might be more than one place where the
-           filesystem is mounted but one is enough for us.  */
-        size_t namelen;
-
-        /* First make sure this really is the correct entry.  At least
-           some versions of the kernel give wrong information because
-           of the implicit mount of the shmfs for SysV IPC.  */
-        if (__statfs (mp->mnt_dir, &f) != 0 || (f.f_type != SHMFS_SUPER_MAGIC
-                                                && f.f_type != RAMFS_MAGIC))
-          continue;
-
-        namelen = strlen (mp->mnt_dir);
-
-        if (namelen == 0)
-          /* Hum, maybe some crippled entry.  Keep on searching.  */
-          continue;
-
-        mountpoint.dir = (char *) malloc (namelen + 2);
-        if (mountpoint.dir != NULL)
-          {
-            char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
-            if (cp[-1] != '/')
-              *cp++ = '/';
-            *cp = '\0';
-            mountpoint.dirlen = cp - mountpoint.dir;
-          }
-
-        break;
-      }
-
-  /* Close the stream.  */
-  __endmntent (fp);
-}
-
-
-const char *
-__shm_directory (size_t *len)
-{
-  /* Determine where the shmfs is mounted.  */
-  __libc_once (once, where_is_shmfs);
-
-  /* If we don't know the mount points there is nothing we can do.  Ever.  */
-  if (__glibc_unlikely (mountpoint.dir == NULL))
-    {
-      __set_errno (ENOSYS);
-      return NULL;
-    }
-
-  *len = mountpoint.dirlen;
-  return mountpoint.dir;
-}
-#if IS_IN (libpthread)
-hidden_def (__shm_directory)
-
-/* Make sure the table is freed if we want to free everything before
-   exiting.  */
-void
-__shm_directory_freeres (void)
-{
-  if (mountpoint.dir != defaultdir)
-    free (mountpoint.dir);
-}
-#endif