linux-kernel - Re: [RFC v2 1/2] kernel/sysctl: support setting sysctl parameters from kernel command line

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Date:   Mon, 30 Mar 2020 22:09:39 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-mm@...ck.org, Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Guilherme G . Piccoli" <gpiccoli@...onical.com>
Subject: Re: [RFC v2 1/2] kernel/sysctl: support setting sysctl parameters
 from kernel command line

On Thu, Mar 26, 2020 at 07:45:13AM -0500, Eric W. Biederman wrote:
> > On Wed 25-03-20 17:20:40, Eric W. Biederman wrote:
> plus I want to get very far away from the incorrect idea that you
> can have sysctls without compiling in proc support.  That is not how
> the code works, that is not how the code is tested.

Agreed.

> It is also worth pointing out that:
> 
> 	proc_mnt = kern_mount(proc_fs_type);
>         for_each_sysctl_cmdline() {
>         	...
> 		file = file_open_root(proc_mnt->mnt_root, proc_mnt, sysctl_path, O_WRONLY, 0);
> 		kernel_write(file, value, value_len);
>         }
>         kern_umount(proc_mnt);
> 
> Is not an unreasonable implementation.

This:

> There are problems with a persistent mount of proc in that it forces
> userspace not to use any proc mount options.  But a temporary mount of
> proc to deal with command line options is not at all unreasonable.
> Plus it looks like we can have kern_write do all of the kernel/user
> buffer silliness.

Is a bit of tribal knowledge worth documenting for the approach taken
forward. Vlastimil can you add a little comment mentioning some of this
logic?

  Luis