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, 9 insertions, 4 deletions

diff --git a/sysdeps/posix/shm_unlink.c b/sysdeps/posix/shm_unlink.c
index f3258a0..c90b854 100644
--- a/sysdeps/posix/shm_unlink.c
+++ b/sysdeps/posix/shm_unlink.c
@@ -25,16 +25,21 @@
 
 #include <errno.h>
 #include <string.h>
-#include "shm-directory.h"
+#include <shm-directory.h>
 
 
 /* Remove shared memory object.  */
 int
 shm_unlink (const char *name)
 {
-  SHM_GET_NAME (ENOENT, -1, "");
-
-  int result = unlink (shm_name);
+  struct shmdir_name dirname;
+  if (__shm_get_name (&dirname, name, false) != 0)
+    {
+      __set_errno (ENOENT);
+      return -1;
+    }
+
+  int result = unlink (dirname.name);
   if (result < 0 && errno == EPERM)
     __set_errno (EACCES);
   return result;