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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 2 Jun 2020 17:08:22 -0700
Message-ID: <CAHk-=wimp3tNuMcix2Z3uCF0sFfQt5GhVku=yhJAmSALucYGjg@mail.gmail.com>
Subject: Re: [PATCH v5 0/3] close_range()
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kyle Evans <self@kyle-evans.net>,
        Victor Stinner <victor.stinner@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Shuah Khan <shuah@kernel.org>,
        David Howells <dhowells@redhat.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>
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On Tue, Jun 2, 2020 at 4:33 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
> >
> > And maybe this _did_ get mentioned last time, and I just don't find
> > it. I also don't see anything like that in the patches, although the
> > flags argument is there.
>
> I spent some good time digging and I couldn't find this mentioned
> anywhere so maybe it just never got sent to the list?

It's entirely possible that it was just a private musing, and you
re-opening this issue just resurrected the thought.

I'm not sure how simple it would be to implement, but looking at it it
shouldn't be problematic to add a "max_fd" argument to unshare_fd()
and dup_fd().

Although the range for unsharing is obviously reversed, so I'd suggest
not trying to make "dup_fd()" take the exact range into account.

More like just making __close_range() do basically something like

        rcu_read_lock();
        cur_max = files_fdtable(files)->max_fds;
        rcu_read_unlock();

        if (flags & CLOSE_RANGE_UNSHARE) {
                unsigned int max_unshare_fd = ~0u;
                if (cur_max >= max_fd)
                        max_unshare_fd = fd;
                unshare_fd(max_unsgare_fd);
        }

        .. do the rest of __close_range() here ..

and all that "max_unsgare_fd" would do would be to limit the top end
of the file descriptor table unsharing: we'd still do the exact range
handling in __close_range() itself.

Because teaching unshare_fd() and dup_fd() about anything more complex
than the above doesn't sound worth it, but adding a way to just avoid
the unnecessary copy of any high file descriptors sounds simple
enough.

But I haven't thought deeply about this. I might have missed something.

            Linus