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

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Date:   Fri, 15 May 2020 08:30:47 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Nate Karstens <nate.karstens@...min.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-alpha@...r.kernel.org, linux-parisc@...r.kernel.org,
        sparclinux@...r.kernel.org, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Changli Gao <xiaosuo@...il.com>
Subject: Re: [PATCH v2] Implement close-on-fork

On Fri, May 15, 2020 at 8:23 AM Nate Karstens <nate.karstens@...min.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.