如何删除另一个用户创建的共享内存文件？

Question

I am just running into a problem with the permissions of /dev/shm: I have an suite of software, which multiple users should be able to use. Permissions of shared resources are given by using the same group. But in case of shared memory I am running into this problem:

• a c-program run by user "a" should be able to remove a shared memory in /dev/shm, which was created by user "b"

• due to the sticky bit of /dev/shm it is forbidden to delete a shared memory of another user - even if both users and the shared memory are belonging to the same group

• creating a subdirectory in /dev/shm with appropriate permissions does not work since inner '/' are not allowed in the filename used by shm_open()

Well, I do not want to remove the sticky bit of /dev/shm since this is linux standard.

Eg it would be very fine if there was a possibility to place the shared memory files of my application suite in another directory than /dev/shm - ideally a subdirectory of /dev/shm. But I did not find any hint how to do this.

Does anyone has an idea, how to allow user "a" to delete a shared memory created by user "b"?

Answer 1

it would be very fine if there was a possibility to place the shared memory files of my application suite in another directory than /dev/shm

There's nothing special about files used by shm_open() other than they have to be in /dev/shm . You can mmap() any file with shared access exactly the same way.

Don't use shm_open() to open a file for use as shared memory. The only thing shm_open() really does is give you a file descriptor to a file located in /dev/shm . This is the entirety of glibc's shm_open() implementation :

/* Open shared memory object.  */
int
shm_open (const char *name, int oflag, mode_t mode)
{
  SHM_GET_NAME (EINVAL, -1, "");

  oflag |= O_NOFOLLOW | O_CLOEXEC;

  /* Disable asynchronous cancellation.  */
  int state;
  pthread_setcancelstate (PTHREAD_CANCEL_DISABLE, &state);

  int fd = open (shm_name, oflag, mode);
  if (fd == -1 && __glibc_unlikely (errno == EISDIR))
    /* It might be better to fold this error with EINVAL since
       directory names are just another example for unsuitable shared
       object names and the standard does not mention EISDIR.  */
    __set_errno (EINVAL);

  pthread_setcancelstate (state, NULL);

  return fd;
}

All that does is make sure the file name is for a file in /dev/shm .

Just replace shm_open() with a plain open() and put your shared memory file where you want.

Instead of

int fd = shm_open( "shared_memory_file", oflag, mode );

simply use

int fd = open( "/path/to/my/shared_memory_file", oflag, mode );