Delete POSIX shared memory owned by different user

Question

I have a program that creates four shared memory objects. The memory creation routine calls shm_unlink() before attempting to create them, and the program calls another routine to delete them with shm_unlink() at the end of the run.

Today I got "permission denied" on objects 2-4 (but not object 1) when attempting to re-create them. Linux fuser showed that they were all owned by the same process, so I couldn't kill that.

I went to /dev/shm and did ls -l, and found that 2-4 were owned by root, not by my sudo user. I rarely log on as root so I don't know how they became owned by root. To solve the problem, I logged on as root and deleted them manually.

My question is: is there a programmatic way to delete shared memory objects owned by a different user (e.g. root) without logging on as that user? Is there a way to call chown on the shm objects and change the owner?

Answer 1

Root can delete shared memory (or other IPC items) owned by any user. If you need a pragmatic way to do this, do this as root.

Otherwise, you will need to possibly alter the permissions on the created items, either as they are created or afterwards. All filesystem entries (including things that aren't files) use the POSIX permissions, So you will need to ensure that the item is writable by the user who is removing it (and the directory above it is writable by that same removing user).

The less desirable way to ensure this is to make the item (and its parent directory) world writable. This, of course, opens huge security holes and is not generally recommended.

If the process that runs is launched as root, or transitions from a regular user to root and back, then the created IPC items are likely to be owned by root. If the process that does this can be modified, then you may be able to get the process to create the items with the appropriate group permissions to permit removal by a user other than the launching user.