A Curious Permissions Problem with ALSA

While I’m waiting for kde4 to reach everyday usability (defined for my purposes as supporting panel auto-hiding), I periodically check out an updated subversion tree of kde4, compile it, and try it out.

There’s some pain with switching between kde3 and kde4 and back again, so I try kde4 with a different login, specially created for testing kde4.

There are still bugs in kde4, particularly when you compile subversion trees, and not specific tagged releases. In my latest foray into kde4, several applications did crash, including amarok, the audio media player.

After spending some time in kde4, I logged out and switched back to kde3 with my regular username. Once there, I found that there was no audio from ALSA applications. So, the usual course when this happens is to examine the permissions on the appropriate audio devices. In this case, however, all of the permissions looked fine.

So, what was preventing ALSA applications from running? When I ran strace on an ALSA application, I found that I was getting permissions problems (reported as EPERM on the return from a syscall) on semctl() syscalls. So, the next step is to run ipcs. This shows the SYSV IPC resources currently in use. There, I found two shared memory segments and two semaphores that were owned by the “kde4” username. Since all kde4-owned processes had exited, this indicated that some process had experienced an abnormal exit without releasing some SYSV IPC resources.

ALSA uses such resources when applications want to generate sound, and it is not possible for an unprivileged user to obtain or release the resources of another user. This produced the permission problems and prevented the applications from working correctly with ALSA.

The solution was to become the root user, and use the ipcrm command to release all resources owned by the kde4 user. Once that was done, ALSA applications run as my regular username could, once again, produce sound.

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