Re: [PATCH v2] Implement close-on-fork

Eric Dumazet
2020-05-15T15:31:02Z

On Fri, May 15, 2020 at 8:23 AM Nate Karstens <nate.karstens@garmin.com> wrote:
>
>
> Series of 4 patches to implement close-on-fork. Tests have been
> published to https://github.com/nkarstens/ltp/tree/close-on-fork
> and cover close-on-fork functionality in the following syscalls:
>
> * accept(4)
> * dup3(2)
> * fcntl(2)
> * open(2)
> * socket(2)
> * socketpair(2)
> * unshare(2)
>
> Addresses underlying issue in that there is no way to prevent
> a fork() from duplicating a file descriptor. The existing
> close-on-exec flag partially-addresses this by allowing the
> parent process to mark a file descriptor as exclusive to itself,
> but there is still a period of time the failure can occur
> because the auto-close only occurs during the exec().
>
> One manifestation of this is a race conditions in system(), which
> (depending on the implementation) is non-atomic in that it first
> calls a fork() and then an exec().
>
> This functionality was approved by the Austin Common Standards
> Revision Group for inclusion in the next revision of the POSIX
> standard (see issue 1318 in the Austin Group Defect Tracker).
>
> ---
>
> This is v2 of the change. See https://lkml.org/lkml/2020/4/20/113
> for the original work.
>
> Thanks to everyone who provided comments on the first series of
> patches. Here are replies to specific comments:
>
> > I suggest we group the two bits of a file (close_on_exec, close_on_fork)
> > together, so that we do not have to dirty two separate cache lines.
>
> I could be mistaken, but I don't think this would improve efficiency.
> The close-on-fork and close-on-exec flags are read at different
> times. If you assume separate syscalls for fork and exec then
> there are several switches between when the two flags are read.
> In addition, the close-on-fork flags in the new process must be
> cleared, which will be much harder if the flags are interleaved.

:/

Fast path in big and performance sensitive applications is not fork()
and/or exec().

This is open()/close() and others (socket(), accept(), ...)

We do not want them to access extra cache lines for this new feature.

Sorry, I will say no to these patches in their current form.