linux-kernel - Re: [PATCH v5 0/3] close

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Date:   Thu, 4 Jun 2020 03:15:09 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kyle Evans <self@...e-evans.net>,
        Victor Stinner <victor.stinner@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Shuah Khan <shuah@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Dmitry V. Levin" <ldv@...linux.org>
Subject: Re: [PATCH v5 0/3] close_range()

On Wed, Jun 03, 2020 at 05:13:36PM -0700, Linus Torvalds wrote:
> On Wed, Jun 3, 2020 at 4:24 PM Christian Brauner
> <christian.brauner@...ntu.com> wrote:
> >
> > Ok, here's what I have. Does the below look somewhat sane?
> 
> Probably. Needs lots of testing. But this one looks wrong:

Right, there's a patch for a test-suite for the new flag too using
CLONE_FILES to create a shared fdtable and the proceeds to close all
(or subsets of) fds:

https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/commit/?h=close_range&id=498e7e844fe6e3f3306b2cd1b5e926e1cd394b99

I've been running that in an endless loop for a while.

> 
> > +int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
> >  {
> > +               if ((max_fd + 1) >= cur_max)
> > +                       max_unshare_fds = fd;
> 
> A normal value for "close everything starting at X" would have a
> max_fd value of ~0.

Ugh, obvious braino from my side. This should just be:

if (max_fd >= cur_max)
	max_unshare_fds = fd;

> 
> So "max_fd+1" would overflow to 0, and then this would never trigger.
> 
> Other than that it looks what what I imagine my feverdreams were about.

Thanks!
Christian