linux-kernel - Re: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes

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Date:   Fri, 12 Jun 2020 11:38:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Sargun Dhillon <sargun@...gun.me>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Robert Sesek <rsesek@...gle.com>,
        Chris Palmer <palmer@...gle.com>, Jann Horn <jannh@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "containers@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Matt Denton <mpdenton@...gle.com>, Tejun Heo <tj@...nel.org>,
        David Laight <David.Laight@...LAB.COM>,
        Al Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to
 move fds across processes

On Fri, Jun 12, 2020 at 08:28:16PM +0200, Christian Brauner wrote:
> Al didn't want the PAGE_SIZE limit in there because there's nothing
> inherently wrong with copying insane amounts of memory.

Right, ok.

> (Another tangent. I've asked this on Twitter not too long ago: do we
> have stats how long copy_from_user()/copy_struct_from_user() takes with
> growing struct/memory size? I'd be really interested in this. I have a
> feeling that clone3()'s and - having had a chat with David Howells -
> openat2()'s structs will continue to grow for a while... and I'd really
> like to have some numbers on when copy_struct_from_user() becomes
> costly or how costly it becomes.)

How long it takes? It should be basically the same, the costs should be
mostly in switching memory protections, etc. I wouldn't imagine how many
bytes being copied would matter much here, given the sub-page sizes.

> > Ah yeah, I like this because of what you mention below: it's forward
> > compat too. (I'd just use the ioctl masks directly...)
> > 
> > 	switch (cmd & ~(_IOC_SIZEMASK | _IOC_DIRMASK))
> > 
> > > 		return seccomp_notify_addfd(filter, buf, _IOC_SIZE(cmd));
> > 
> > I really like that this ends up having the same construction as a
> > standard EA syscall: the size is part of the syscall arguments.
> 
> This is basically what I had proposed in my previous mail, right?

I guess I missed it! Well, then I think we're all in agreement? :)

-- 
Kees Cook