On Thu, Nov 23, 2023 at 6:05 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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. > thanks, > > greg k-h -- Best Regards Masahiro Yamada
