What the Fork!?

Posted on June 25, 2021


What do you do when you want fork a program? What do you think is right?

File Descriptors

Do you close all file descriptors except ones that are passed in explicitly when you fork a process? You probably should. If you accidentally inherit a file descriptor and don’t close it, it, well.. it never closes (until you and all your children die). The OS can’t free the inode, and processes waiting for the handle to be closed will wait for you to die. Also, you are giving the child permission to read or write files you might not want it to. Ever used <(cmd) to get a secret into a program?

Ok. So you close all file descriptors. But.. how do you know which file descriptors to close? Maybe your caller set some environment variable to a file descriptor and your child will want to use it, or passed in as an argument (e.g. using bashes process substitution <(cmd)).

Solution: make sure you set CLOEXEC on all file descriptors that you don’t want to be inherited when they are created. Unfortunately, your libraries probably won’t do this. Double unfortunately, on some operating systems it is impossible to atomically create a file descriptor and set CLOEXEC (cough macOS cough). So if you use threads (or any of your libraries do), you are even more out of luck.

You might be tempted to scan your environment and args for things that might look like file descriptors, and then not close those ones. But you will probably run into false positives. File descriptors are usually lowish numbers which could commonly crop up for a variety of reasons.

The best approach is probably to not close file descriptors, and to open, and set CLOEXEC on any that you will need as early as possible, before there is any chance of spawning threads that might ruin your day. You may also want to avoid threads. Or at least, avoid creating file descriptors or forking from threads. And keep a keen eye on your dependencies. Good luck.

What to do after the fork?

Do you do anything after a fork? You probably should not. It would be wise to stick to the list of functions documented in man 7 signal-safety. Sure, this is not a signal handler, but we are actually under the same constraints. That is, we are in a state where any number of locks might be being held by threads that are no longer going to run. So, even calling malloc() may hang the child. Don’t use threads? You are probably OK. Just make sure none of your libraries do (or ever will)!

Want to read /proc/self/fd to know which file descriptors to close? Too bad, opendir is not in the list of safe functions to use.

What do you do with SIGINT?

I’ll just leave this here