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, 41 insertions, 0 deletions

diff --git a/include/shm-directory.h b/include/shm-directory.h
new file mode 100644
index 0000000000..1d3ffb6137
--- /dev/null
+++ b/include/shm-directory.h
@@ -0,0 +1,41 @@
+/* Header for directory for shm/sem files.
+   Copyright (C) 2014-2021 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/>.  */
+
+#ifndef _SHM_DIRECTORY_H
+
+#include <limits.h>
+#include <paths.h>
+#include <stdbool.h>
+
+/* The directory that contains shared POSIX objects.  */
+#define SHMDIR _PATH_DEV "shm/"
+
+struct shmdir_name
+{
+  /* The combined prefix/name.  The sizeof includes the terminating
+     NUL byte.  4 bytes are needed for the optional "sem." prefix.  */
+  char name[sizeof (SHMDIR) + 4 + NAME_MAX];
+};
+
+/* Sets RESULT->name to the constructed name and returns 0 on success,
+   or -1 on failure.  Includes the "sem." prefix in the name if
+   SEM_PREFIX is true.  */
+int __shm_get_name (struct shmdir_name *result, const char *name,
+		    bool sem_prefix);
+
+#endif  /* shm-directory.h */