Messages in this thread | | | Date | Sat, 16 Oct 2021 14:16:43 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C |
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On Sat, Oct 16, 2021 at 12:17:40AM +0200, Thomas Gleixner wrote: > For actually callable functions, by some definition of callable, > e.g. the clear_page_*() variants a proper attribute would be definitely > preferred.
See my last email, clear_page_*() has nothing to do with CFI in the first place.
> That attribute should tell the compiler that the function is using the > register arguments correctly but is not suitable for direct invocation > because it clobbers registers. > > So the compiler can just refuse to call such a function if used directly > without an inline asm wrapper which describes the clobbers, right? > > But thinking more about clobbers. The only "annotation" of clobbers we > have today are the clobbers in the inline asm, which is fragile too. > > Something like > > __attribute__ ((clobbers ("rcx", "rax"))) > > might be useful by itself because it allows validation of the clobbers > in the inline asm wrappers and also allows a analysis tool to look at > the ASM code and check whether the above list is correct. > > Hmm?
Functions are allowed to clobber rcx and rax anyway.
The clear_page_*() functions follow the C ABI, like (almost) every other asm function in the kernel. I think there's a misunderstanding here, as most of this doesn't have anything to do with CFI anyway.
-- Josh
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