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

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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
2021-02-03 08:55:13 -03:00
committed by Adhemerval Zanella
parent 7f61f785f1
commit e9fed2438a
19 changed files with 86 additions and 300 deletions
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34
sysdeps/pthread/sem_open.c
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@@ -62,8 +62,9 @@ int __sem_mappings_lock attribute_hidden = LLL_LOCK_INITIALIZER;
/* Search for existing mapping and if possible add the one provided. */
static sem_t *
check_add_mapping (const char *name, size_t namelen, int fd, sem_t *existing)
check_add_mapping (const char *name, int fd, sem_t *existing)
{
size_t namelen = strlen (name);
sem_t *result = SEM_FAILED;
/* Get the information about the file. */
@@ -150,8 +151,12 @@ sem_open (const char *name, int oflag, ...)
return SEM_FAILED;
}
/* Create the name of the final file in local variable SHM_NAME. */
SHM_GET_NAME (EINVAL, SEM_FAILED, SEM_SHM_PREFIX);
struct shmdir_name dirname;
if (__shm_get_name (&dirname, name, true) != 0)
{
__set_errno (EINVAL);
return SEM_FAILED;
}
/* Disable asynchronous cancellation. */
#ifdef __libc_ptf_call
@@ -164,7 +169,7 @@ sem_open (const char *name, int oflag, ...)
if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
{
try_again:
fd = __libc_open (shm_name,
fd = __libc_open (dirname.name,
(oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);
if (fd == -1)
@@ -178,13 +183,12 @@ sem_open (const char *name, int oflag, ...)
else
/* Check whether we already have this semaphore mapped and
create one if necessary. */
result = check_add_mapping (name, namelen, fd, SEM_FAILED);
result = check_add_mapping (name, fd, SEM_FAILED);
}
else
{
/* We have to open a temporary file first since it must have the
correct form before we can start using it. */
char *tmpfname;
mode_t mode;
unsigned int value;
va_list ap;
@@ -217,16 +221,11 @@ sem_open (const char *name, int oflag, ...)
memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',
sizeof (sem_t) - sizeof (struct new_sem));
tmpfname = __alloca (shm_dirlen + sizeof SEM_SHM_PREFIX + 6);
char *xxxxxx = __mempcpy (tmpfname, shm_dir, shm_dirlen);
char tmpfname[] = SHMDIR "sem.XXXXXX";
int retries = 0;
#define NRETRIES 50
while (1)
{
/* Add the suffix for mktemp. */
strcpy (xxxxxx, "XXXXXX");
/* We really want to use mktemp here. We cannot use mkstemp
since the file must be opened with a specific mode. The
mode cannot later be set since then we cannot apply the
@@ -244,7 +243,12 @@ sem_open (const char *name, int oflag, ...)
if (errno == EEXIST)
{
if (++retries < NRETRIES)
continue;
{
/* Restore the six placeholder bytes before the
null terminator before the next attempt. */
memcpy (tmpfname + sizeof (tmpfname) - 7, "XXXXXX", 6);
continue;
}
__set_errno (EAGAIN);
}
@@ -265,7 +269,7 @@ sem_open (const char *name, int oflag, ...)
fd, 0)) != MAP_FAILED)
{
/* Create the file. Don't overwrite an existing file. */
if (link (tmpfname, shm_name) != 0)
if (link (tmpfname, dirname.name) != 0)
{
/* Undo the mapping. */
(void) munmap (result, sizeof (sem_t));
@@ -290,7 +294,7 @@ sem_open (const char *name, int oflag, ...)
/* Insert the mapping into the search tree. This also
determines whether another thread sneaked by and already
added such a mapping despite the fact that we created it. */
result = check_add_mapping (name, namelen, fd, result);
result = check_add_mapping (name, fd, result);
}
/* Now remove the temporary name. This should never fail. If