linux-kernel - Re: [PATCH v3] module: add in-kernel support for decompressing

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Date:   Mon, 20 Dec 2021 08:52:30 -0800
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Martin Wilck <martin.wilck@...e.com>
Cc:     Jessica Yu <jeyu@...nel.org>, Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] module: add in-kernel support for decompressing

On Fri, Dec 10, 2021 at 05:09:23PM -0800, Dmitry Torokhov wrote:
> On Fri, Dec 10, 2021 at 04:11:21PM -0800, Luis Chamberlain wrote:
> > On Thu, Dec 09, 2021 at 10:09:17PM -0800, Dmitry Torokhov wrote:
> > > diff --git a/init/Kconfig b/init/Kconfig
> > > index cd23faa163d1..d90774ff7610 100644
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -2305,6 +2305,19 @@ config MODULE_COMPRESS_ZSTD
> > >  
> > >  endchoice
> > >  
> > > +config MODULE_DECOMPRESS
> > > +	bool "Support in-kernel module decompression"
> > > +	depends on MODULE_COMPRESS_GZIP || MODULE_COMPRESS_XZ
> > > +	select ZLIB_INFLATE if MODULE_COMPRESS_GZIP
> > > +	select XZ_DEC if MODULE_COMPRESS_XZ
> > 
> > What if MODULE_COMPRESS_GZIP and MODULE_COMPRESS_XZ are enabled?
> > These are not mutually exclusive.
> 
> They are mutually exclusive, the kernel uses the same (one) compression
> method for all kernel modules that it generates (i.e we do not compress
> drivers/usb/... with gzip while drivers/net/... with xz).

Ah yes I failed to see the choice/prompt for it.

> The idea here is to allow the kernel consume the same format that was
> used when generating modules. Supporting multiple formats at once is
> overkill IMO.

Indeed.

> > > +	help
> > > +
> > > +	  Support for decompressing kernel modules by the kernel itself
> > > +	  instead of relying on userspace to perform this task. Useful when
> > > +	  load pinning security policy is enabled.
> > 
> > Shouldn't kernel decompression be faster too? If so, what's the
> > point of doing it in userspace?
> 
> Make the kernel smaller?

Yes this I buy.

> Have more flexibility with exotic compression
> formats?

I just have a hunch that doing module decompression in the kernel will
speed things quite a bit... any chance you can provide some before and
after systemd-analyze ?

  Luis