Subject: Re: [PATCH v2 0/5] MODVERSIONS + RUST Redux

From: Greg KH <[email-masked]>
Date: Thu, 23 Nov 2023 12:12:56 +0000

On Thu, Nov 23, 2023 at 08:38:45PM +0900, Masahiro Yamada wrote:
> On Thu, Nov 23, 2023 at 6:05 PM Greg KH <[email-masked]> wrote:
> >
> > On Wed, Nov 22, 2023 at 01:04:09PM -0800, Matthew Maurer wrote:
> > > > So, even if you enable CONFIG_MODVERSIONS,
> > > > nothing is checked for Rust.
> > > > Genksyms computes a CRC from "int foo", and
> > > > the module subsystem confirms it is a "int"
> > > > variable.
> > > >
> > > > We know this check always succeeds.
> > > >
> > > > Why is this useful?
> > > The reason this is immediately useful is that it allows us to have Rust
> > > in use with a kernel where C modules are able to benefit from MODVERSIONS
> > > checking. The check would effectively be a no-op for now, as you have correctly
> > > determined, but we could refine it to make it more restrictive later.
> > > Since the
> > > existing C approach errs on the side of "it could work" rather than "it will
> > > work", I thought being more permissive was the correct initial solution.
> >
> > But it's just providing "fake" information to the CRC checker, which
> > means that the guarantee of a ABI check is not true at all.
> >
> > So the ask for the user of "ensure that the ABI checking is correct" is
> > being circumvented here, and any change in the rust side can not be
> > detected at all.
> >
> > The kernel is a "whole", either an option works for it, or it doesn't,
> > and you are splitting that guarantee here by saying "modversions will
> > only work for a portion of the kernel, not the whole thing" which is
> > going to cause problems for when people expect it to actually work
> > properly.
> >
> > So, I'd strongly recommend fixing this for the rust code if you wish to
> > allow modversions to be enabled at all.
> >
> > > With regards to future directions that likely won't work for loosening it:
> > > Unfortunately, the .rmeta format itself is not stable, so I wouldn't want to
> > > teach genksyms to open it up and split out the pieces for specific functions.
> > > Extending genksyms to parse Rust would also not solve the situation -
> > > layouts are allowed to differ across compiler versions or even (in rare
> > > cases) seemingly unrelated code changes.
> >
> > What do you mean by "layout" here?  Yes, the crcs can be different
> > across compiler versions and seemingly unrelated code changes (genksyms
> > is VERY fragile) but that's ok, that's not what you are checking here.
> > You want to know if the rust function signature changes or not from the
> > last time you built the code, with the same compiler and options, that's
> > all you are verifying.
> >
> > > Future directions that might work for loosening it:
> > > * Generating crcs from debuginfo + compiler + flags
> > > * Adding a feature to the rust compiler to dump this information. This
> > > is likely to
> > >   get pushback because Rust's current stance is that there is no ability to load
> > >   object code built against a different library.
> >
> > Why not parse the function signature like we do for C?
> >
> > > Would setting up Rust symbols so that they have a crc built out of .rmeta be
> > > sufficient for you to consider this useful? If not, can you help me understand
> > > what level of precision would be required?
> >
> > What exactly does .rmeta have to do with the function signature?  That's
> > all you care about here.
> 
> 
> 
> 
> rmeta is generated per crate.
> 
> CRC is computed per symbol.
> 
> They have different granularity.
> It is weird to refuse a module for incompatibility
> of a symbol that it is not using at all.

I agree, this should be on a per-symbol basis, so the Rust
infrastructure in the kernel needs to be fixed up to support this
properly, not just ignored like this patchset does.

thanks,

greg k-h