Re: Difficulty emulating F_DUPFD_CLOEXEC

[Zvi Gilboa <zg7s@eservices.virginia.edu>]

On 03/23/2013 09:59 PM, Rich Felker wrote:
> Hi again,
>
> Old kernels lack fcntl F_DUPFD_CLOEXEC, which musl needs internally
> and wants to provide to applications. Thus, I'd like to emulate it
> like we do for pipe2, dup3, socket SOCK_CLOEXEC, etc.; the emulation
> has a race condition that leaks fds, but it's better than nothing.
>
> The problem I'm having is how to detect the case where the kernel
> lacks F_DUPFD_CLOEXEC. For other other atomic close-on-exec
> operations, we either have ENOSYS (newly added syscall) or an
> unambiguous EINVAL. But for fcntl, we could get EINVAL because
> F_DUPFD_CLOEXEC is not recognized by the kernel, or because the
> argument is larger than OPEN_MAX. So we need a test for the cause
> behind EINVAL.
>
> False positives are not an option, because if we wrongly detect that
> F_DUPFD_CLOEXEC was not supported, we would emulate it with the code
> that has race conditions and fd leaks, even on new kernels which
> should not have these problems.
>
> The best idea I have so far is:
>
> 1. Try F_DUPFD_CLOEXEC. If it succeeds or fails with any error other
>     than EINVAL, we're done.
>
> 2. If it fails with EINVAL, retry with an argument of 0. This will
>     eliminate the other cause of EINVAL, so now we should get EINVAL
>     only on old kernels that lack F_DUPFD_CLOEXEC. If this test
>     succeeds, we need to close the new duplicate fd we made (on the
>     wrong fd number) and report EINVAL back to the caller.
>
> 3. If the test in step 2 failed, F_DUPFD_CLOEXEC is unsupported, and
>     we have to use the fallback code with race conditions.
>
> This uglifies fcntl.c a bit more, but I think it works. Does the above
> reasoning make sense? Any other ideas?

In the hope that this matches the project's spirit... how about running 
these tests during the build, and have a script (or a simple test 
program) #define whether the target architecture supports 
F_DUPFD_CLOEXEC or not?  Potentially, this test could be added at the 
very end of alltypes.h.sh

>
> Rich