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, 7 insertions, 3 deletions

diff --git a/sysdeps/pthread/sem_unlink.c b/sysdeps/pthread/sem_unlink.c
index faafbe4e91..1f06a55b8e 100644
--- a/sysdeps/pthread/sem_unlink.c
+++ b/sysdeps/pthread/sem_unlink.c
@@ -27,11 +27,15 @@
 int
 sem_unlink (const char *name)
 {
-  /* Construct the filename.  */
-  SHM_GET_NAME (ENOENT, -1, SEM_SHM_PREFIX);
+  struct shmdir_name dirname;
+  if (__shm_get_name (&dirname, name, true) != 0)
+    {
+      __set_errno (ENOENT);
+      return -1;
+    }
 
   /* Now try removing it.  */
-  int ret = unlink (shm_name);
+  int ret = unlink (dirname.name);
   if (ret < 0 && errno == EPERM)
     __set_errno (EACCES);
   return ret;