## The impossible EFAULT I have written a program; suppose it's called `worker`. (While the program is written in Haskell, I don't think that's particularly relevant to this post.) (EDIT: Reproducer can be found [here](https://git.tomsmeding.com/snap-efault/tree/).) (EDIT 2: Diagnosis by `int-e` on irc [here](https://paste.tomsmeding.com/D22SvR2T).) When run, `worker` starts a bunch of copies of a script. Under normal circumstances this script sets up a container using Linux cgroups and Linux user namespaces, but none of that is relevant because the strange behaviour in question occurs just fine without all of that -- in fact, we'll let it start the following script, say `./sleep.sh`: ```bash #!/bin/bash sleep 10 ``` Clearly, there is no weird behaviour here, assuming that the system has `bash` under `/bin`, and mine does. The copies of `sleep.sh` are started by passing `./sleep.sh` to `posix_spawnp(3)`. (The Haskell `process` library does this for me.) The thing is, occasionally (once every 5 to 10 invocations of `./worker`, approximately), `posix_spawnp` returns `EFAULT` ("Bad Address"). The manpage for `posix_spawnp` says that: > **ERRORS** > > The posix_spawn() and posix_spawnp() functions fail only in the case where the underlying fork(2), vfork(2) or clone(2) call fails; in these cases, these functions return an error number, which will be one of the errors described for fork(2), vfork(2) or clone(2). > > In addition, these functions fail if: > > **ENOSYS** Function not supported on this system. Okay, so I should look for `EFAULT` in `fork(2)`, `vfork(2)` and `clone(2)` to figure out what goes wrong, right? Wrong. Or, in any case, none of those manpages mention `EFAULT`. I've looked through the source code of `posix_spawnp` in glibc and it at least doesn't throw `EFAULT` directly; presumably, one of the subroutines it calls does. glibc is large and I don't think looking through the entire call tree will be very productive, so I tried to diagnose the issue from the outside instead. And this is where the weirdness starts. Whenever my program encounters `EFAULT` from `posix_spawnp`, it prints `Oops EFAULT`; hence grepping for `EFAULT` gives output precisely if the error occurred in this run. I get the following observations: - `./worker 2>&1 | grep EFAULT`: errors occur. - `./worker 2>&1 | grep EFAULT | cat`: errors DO NOT occur. - `./worker 2>&1 | grep --line-buffered EFAULT | cat`: errors occur. - `./worker 2>&1 | grep --line-buffered EFAULT`: errors occur. ("errors occur" means that once every few executions I get output indicating that `EFAULT` occurred; in the negative case I've run it for >20x the number of invocations that are necessary to produce `EFAULT` in the other cases, without any `EFAULT`.) The only situation in which `posix_spawnp` seems to always succeed, is when `stdout` of the process that `worker`'s output is piped to, is block-buffered. But this makes no sense: there shouldn't even be a reasonable way in which `worker` _can_ even determine whether this is the case! Surely it can distinguish between `./worker | cat` and `./worker` (using `isatty(3)` -- this is precisely what `grep` does when not passed `--line-buffered`), but in all of the above cases the output is piped to another process anyway. This is already spooky, but it gets even spookier: if I replace the invocation of `./sleep.sh` by an invocation of `sleep` (i.e. removing the indirection of the shell script), errors occur in none of the above setups. Somehow, starting a script is different from starting a native process (and changing `bash` to `dash` in `sleep.sh` doesn't change anything). `posix_spawnp` shouldn't care what it is starting! That's the job of the loader, as far as I know. So what gives? ### The cause I'll try to reduce my own program to a minimal reproducer, and if I find anything I'll post an update to this post. In the meantime, spookiness. `snap-server` [modifies the environment](https://github.com/snapframework/snap-server/blob/8d89c10014d8d295bfbf5419bbb8551de32d7f85/src/Snap/Http/Server.hs#L161) to set the locale, and `setenv(3)` is not atomic. In particular, it breaks `execve(2)` when they race, and this is what happens. All possible solutions to this problem are hacks.