linux-kernel - Re: [PATCH v2 0/3] support setting sysctl parameters from kernel command line

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Date:   Thu, 16 Apr 2020 06:38:34 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Guilherme G . Piccoli" <gpiccoli@...onical.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH v2 0/3] support setting sysctl parameters from kernel
 command line

On Thu, Apr 16, 2020 at 03:02:06PM +0900, Masami Hiramatsu wrote:
> Hi Luis,
> 
> On Wed, 15 Apr 2020 06:30:41 +0000
> Luis Chamberlain <mcgrof@...nel.org> wrote:
> > Currently the maximum config size size is 32KB and the total key-words
> > (not     key-value entries) must be under 1024 nodes.  Note: this is not
> > the number of entries but nodes, an entry must consume more than 2 nodes
> > (a key-word and a value). So theoretically, it will be up to 512
> > key-value pairs. If keys contains 3 words in average, it can contain 256
> > key-value pairs. In most cases, the number of config items will be under
> > 100 entries and smaller than 8KB, so it would be enough.  If the node
> > number exceeds 1024, parser returns an error even if the file       size
> > is smaller than 32KB.  Anyway, since bootconfig command verifies it when
> > appending a boot config       to initrd image, user can notice it before
> > boot.  
> > ```
> > *recommending* bootconfig due to the limitation of cmdline seems
> > sensible, however if we advise that.. wouldn't the space for 512
> > theoretical entries full up rather fast?
> 
> Yeah, I think it is easier to hit the node number limitation rather
> than fill up the space. However, since the bootconfig supports comments,
> if user writes enough readable config file, I think it's probably the
> right balance :)
> If you think the 512 entries is too small, it is easy to expand it
> upto 32K (64K nodes). But it may consume 512KB memory only for the
> node (meta) data. Current 1024 nodes consumes 8KB (8bytes/node), so
> compared with the max data size (32KB), I think it is a better balance.

Yeah, and well at least x86 / x86_64 sets COMMAND_LINE_SIZE to 2048
right now (2 KB), that'd hit the limit of abuse of cmdline pretty fast
too. I see no way to scale this reasonably if people abuse syctls on
the command line but to use bootconfig and bite the bullet on size,
to keep sanity.

  Luis