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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.
This commit is contained in:
Florian Weimer
committed by
Adhemerval Zanella
Adhemerval Zanella
parent
7f61f785f1
commit
e9fed2438a
@@ -1,147 +0,0 @@
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/* Determine directory for shm/sem files. Linux version.
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Copyright (C) 2000-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include "shm-directory.h"
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#include <errno.h>
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#include <mntent.h>
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#include <paths.h>
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#include <stdio.h>
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#include <string.h>
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#include <sys/statfs.h>
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#include <libc-lock.h>
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#include "linux_fsinfo.h"
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/* Mount point of the shared memory filesystem. */
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static struct
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{
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char *dir;
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size_t dirlen;
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} mountpoint;
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/* This is the default directory. */
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static const char defaultdir[] = "/dev/shm/";
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/* Protect the `mountpoint' variable above. */
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__libc_once_define (static, once);
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/* Determine where the shmfs is mounted (if at all). */
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static void
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where_is_shmfs (void)
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{
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char buf[512];
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struct statfs f;
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struct mntent resmem;
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struct mntent *mp;
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FILE *fp;
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/* The canonical place is /dev/shm. This is at least what the
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documentation tells everybody to do. */
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if (__statfs (defaultdir, &f) == 0 && (f.f_type == SHMFS_SUPER_MAGIC
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|| f.f_type == RAMFS_MAGIC))
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{
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/* It is in the normal place. */
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mountpoint.dir = (char *) defaultdir;
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mountpoint.dirlen = sizeof (defaultdir) - 1;
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return;
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}
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/* OK, do it the hard way. Look through the /proc/mounts file and if
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this does not exist through /etc/fstab to find the mount point. */
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fp = __setmntent ("/proc/mounts", "r");
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if (__glibc_unlikely (fp == NULL))
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{
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fp = __setmntent (_PATH_MNTTAB, "r");
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if (__glibc_unlikely (fp == NULL))
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/* There is nothing we can do. Blind guesses are not helpful. */
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return;
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}
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/* Now read the entries. */
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while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
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/* The original name is "shm" but this got changed in early Linux
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2.4.x to "tmpfs". */
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if (strcmp (mp->mnt_type, "tmpfs") == 0
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|| strcmp (mp->mnt_type, "shm") == 0)
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{
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/* Found it. There might be more than one place where the
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filesystem is mounted but one is enough for us. */
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size_t namelen;
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/* First make sure this really is the correct entry. At least
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some versions of the kernel give wrong information because
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of the implicit mount of the shmfs for SysV IPC. */
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if (__statfs (mp->mnt_dir, &f) != 0 || (f.f_type != SHMFS_SUPER_MAGIC
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&& f.f_type != RAMFS_MAGIC))
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continue;
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namelen = strlen (mp->mnt_dir);
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if (namelen == 0)
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/* Hum, maybe some crippled entry. Keep on searching. */
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continue;
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mountpoint.dir = (char *) malloc (namelen + 2);
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if (mountpoint.dir != NULL)
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{
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char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
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if (cp[-1] != '/')
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*cp++ = '/';
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*cp = '\0';
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mountpoint.dirlen = cp - mountpoint.dir;
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}
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break;
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}
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/* Close the stream. */
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__endmntent (fp);
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}
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const char *
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__shm_directory (size_t *len)
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{
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/* Determine where the shmfs is mounted. */
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__libc_once (once, where_is_shmfs);
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/* If we don't know the mount points there is nothing we can do. Ever. */
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if (__glibc_unlikely (mountpoint.dir == NULL))
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{
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__set_errno (ENOSYS);
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return NULL;
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}
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*len = mountpoint.dirlen;
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return mountpoint.dir;
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}
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#if IS_IN (libpthread)
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hidden_def (__shm_directory)
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/* Make sure the table is freed if we want to free everything before
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exiting. */
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void
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__shm_directory_freeres (void)
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{
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if (mountpoint.dir != defaultdir)
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free (mountpoint.dir);
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}
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#endif
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