os: StartProcess ETXTBSY race on Unix systems #22315
Modern Unix systems appear to have a fundamental design flaw in the interaction between multithreaded programs, fork+exec, and the prohibition on executing a program if that program is open for writing.
Below is a simple multithreaded C program. It creates 20 threads all doing the same thing: write an exit 0 shell script to /var/tmp/fork-exec-N (for different N), and then fork and exec that script. Repeat ad infinitum. Note that the shell script fds are opened O_CLOEXEC, so that an fd being written by one thread does not leak into the fork+exec's shell script of a different thread.
On my Linux workstation, this program produces a never-ending stream of ETXTBSY errors. The problem is that O_CLOEXEC is not enough. The fd being written by one thread can leak into the forked child of a second thread, and it stays there until that child calls exec. If the first thread closes the fd and calls exec before the second thread's child does exec, then the first thread's exec will get ETXTBSY, because somewhere in the system (specifically, in the child of the second thread), there is an fd still open for writing the first thread's shell script, and according to modern Unix rules, one must not exec a program if there exists any fd anywhere open for writing that program.
Five years ago this bit us because cmd/go installed cmd/cgo (that is, copied the binary from a temporary location to somewhere/bin/cgo) and then executed it. To fix this we put a sleep+retry loop around the fork+exec of cgo when it gets ETXTBSY. Now (as of last week or so) we don't ever install cmd/cgo and execute it in the same cmd/go process, so that specific race is gone, although as I write this cmd/go still has the sleep+retry loop, which I intend to remove.
Last week this bit us again because cmd/go updated a build stamp in the binary, closed it, and executed it. The resulting flaky ETXTBSY failures were reported as #22220. A pending CL fixes this by not updating the build stamp in temporary binaries, which are the main ones we execute. There's still one case where we write+execute a program, which is
In general this race is going to happen every time anyone writes a program that both writes and executes a program. It's easy to imagine other build systems running into this, but also programs that do things like unzip a zip file and then run a program inside it - think a program supervisor or mini container runtime. As soon as there are multiple threads doing fork+exec at the same time, and one of them is doing fork+exec of a program that was previously open for write in the same process, you have a mysterious flaky problem.
It seems like maybe Go should take care of this, if possible. We've now hit it twice in cmd/go, five years apart, and at least this past time it took the better part of a day to figure out. (I don't remember how long it took five years ago, in part because I don't remember anything about discovering it five years ago. I also don't want to rediscover all this five years from now.)
There are a few hacks we could use:
None of these seem great. The ETXTBSY sleep, up to 1 second, might be the best option. It would certainly reduce the flake rate and in many cases would probably make it undetectable. It would not help exec of very slow-to-load programs, but that's not the common case.
I wondered how Java deals with this, and the answer seems to be that Java doesn't deal with this. https://bugs.openjdk.java.net/browse/JDK-8068370 was filed in 2014 and is still open.
referenced this issue
Oct 18, 2017
Userspace workarounds seem flawed or less than ideal. This is a kernel
I'm going to repeat a hack I described elsewhere that I believe would work for pure Go programs.
The effect of this should be that when
The disadvantages are that all forks are serialized, and that all forks waste time closing descriptors that will shortly be closed anyhow. Also, of course, forks temporarily block closes, but that is unlikely to be significant.
I'm happy to have a go, but I'm a nobody on that list. I was assuming folks here might have the ear of a Google kernel developer or two in that area that would vet the idea and suggest it to the list if worthy. :-)
Is there a place to file a feature request against the Linux kernel? I know nothing about the kernel development process. I hear it uses git.
As far as I can see it doesn't matter if
@ianlancetaylor thanks, yes, the explicit closes would solve the problem with slow execs, which would be nice. That might make this actually palatable. You also don't even need the extra pipe if you use vfork in this approach.
I agree with @RalphCorderoy that there's a race between the "maintain the max" and "fork", in that Open might create a new fd, then fork runs in a different thread before Open can update the max. But since fds are created lowest-available, it should suffice for the child to assume that max is, say, 10 larger than it is.
Also note that this need not be an RWMutex (and for that matter the current syscall.ForkMutex need not be an RWMutex either). It just needs to be an "either-or" mutex. An RWMutex allows N readers or 1 writer. The mutex we need would allow N of type A or N of type B, just never a mix. If we built that (not difficult, I don't think), then programs that never fork would not serialize any of their closes, and programs that fork a lot but don't close things would not serialize any of their forks.
O_CLOFORK would require having fcntl F_SETFL/F_GETFL support for that bit too, and it would complicate fork a little more than it already is. An alternative that would be equally fine for us would be a "close all fd's above" or "tell me the maximum fd of my process" syscall. I don't know if a new bit or a new syscall is more likely.
linux-kernel mailing-list archive of post: https://marc.info/?l=linux-kernel&m=150834137201488