CODEWITHSUNDEEP
clinuxlockingfcntl
Maksim Skurydzin
Maksim Skurydzin

Reputation: 10541

Forcefully remove fcntl locks from a different process

Is there any way I can remove fcntl byte range locks on a file from a process that did not lock these ranges?

I have several processes that put byte range locks on files. What I basically need to come up with is an external tool that would help me remove byte range locks for files I specify.

Upvotes: 0

Views: 750

Answers (1)

Nominal Animal
Nominal Animal

Reputation: 39298

There are two options that immediately come to mind.

  1. Write a kernel module to do this. As far as I know, there is no kernel facility to do this as of right now. (You could add a new command to fcntl(), that given superuser privileges or same user as the owner of the lock, does the force-unlock or lock stealing.)

  2. Write a small library, that installs a realtime signal handler, say SIGRTMAX. When this signal is caught, sent by sigqueue(), and the int payload describes an open file descriptor, release all byte locks on that descriptor.

    Alternatively, you can have the signal handler open and read a file or pipe (say /tmp/PID.lock, where the file or pipe contains a data packet defining which file or file descriptor and byte range to unlock.

    As long as the library is loaded when the process starts (and possibly interposing all signal() and sigaction() calls to make sure your signal is kept in the call chain), this should work fine.

The second option requires that you preload the library (via LD_PRELOAD environment variable, or preloading it for all binaries using /etc/ld.so.conf).

The interposing library is not difficult at all to write. I have shown an example of using an interposing library to monitor fork() calls. In your case, you'd have to think of a good way to define the byte ranges to be unlocked (in file or pipe, triggered by a signal), and handle all that in the signal handler context; but there are enough async-signal-safe low-level unistd.h I/O functions to do this.

Upvotes: 1

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