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

[Luis Chamberlain <mcgrof@kernel.org>]

On Mon, Dec 20, 2021 at 07:17:57PM -0800, Dmitry Torokhov wrote:
> On Mon, Dec 20, 2021 at 08:52:30AM -0800, Luis Chamberlain wrote:
> > 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 ?
> 
> If you insist I can try running it, 

If you can run the test, yes it would be appreciated.

> but it should be slower unless your
> memory controller is so slow that reading file from disk and dealing
> with page by page decompression is quicker than copying already
> decompressed data from userspace. 

With userspace decompression I'd imagine we also have more context switches.

> We still reading and uncompressing
> file in kmod (to make sure the format is valid)

I don't understand, that seems wasteful.

> and we can uncompress
> using large buffers (we are not concerned with using unswappable kernel
> memory).
> 
> Maybe in the future when we have streaming and accelerated in-kernel
> decompression API we could optimize for that in kmod and see some
> savings on very large modules.

That would be very nice.

  Luis