Messages in this thread | | | From | Florian Weimer <> | Subject | Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute | Date | Mon, 07 Aug 2023 14:36:53 +0200 |
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* Marco Elver:
> Good idea. I had already created > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899, and we need > better spec to proceed for GCC anyway.
Thanks for the reference.
>> Doesn't this change impact the kernel module ABI? >> >> I would really expect a check here >> >> > +#if __has_attribute(__preserve_most__) >> > +# define __preserve_most notrace __attribute__((__preserve_most__)) >> > +#else >> > +# define __preserve_most >> > +#endif >> >> that this is not a compilation for a module. Otherwise modules built >> with a compiler with __preserve_most__ attribute support are >> incompatible with kernels built with a compiler without that attribute. > > That's true, but is it a real problem? Isn't it known that trying to > make kernel modules built for a kernel with a different config (incl. > compiler) is not guaranteed to work? See IBT, CFI schemes, kernel > sanitizers, etc? > > If we were to start trying to introduce some kind of minimal kernel to > module ABI so that modules and kernels built with different toolchains > keep working together, we'd need a mechanism to guarantee this minimal > ABI or prohibit incompatible modules and kernels somehow. Is there a > precedence for this somewhere?
I think the GCC vs Clang thing is expected to work today, isn't it? Using the Clang-based BPF tools with a GCC-compiled kernel requires a matching ABI.
The other things you listed result in fairly obvious breakage, sometimes even module loading failures. Unconditional crashes are possible as well. With __preserve_most__, the issues are much more subtle and may only appear for some kernel/module compielr combinations and optimization settings. The impact of incorrectly clobbered registers tends to be like that.
Thanks, Florian
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