Re: [PATCH] static_call,x86: Robustify trampoline patching

Andy Lutomirski
2021-11-03T19:32:22Z

On 11/3/21 01:35, Peter Zijlstra wrote:
> On Tue, Nov 02, 2021 at 05:20:05PM -0700, Andy Lutomirski wrote:
>> I think that's a big mistake -- any sane ENDBR-using scheme would
>> really prefer that ENDBR to be right next to the actual function body,
>> and really any scheme would benefit due to better cache locality.
>
> Agreed, IBT/BTI want the landing pad in front of the actual function.
>
>> But, more importantly, IMO any sane ENDBR-using scheme wants to
>> generate the indirect stub as part of code gen for the actual
>> function.
>
> Sorta, I really want to be able to not have a landing pad for functions
> whose address is never taken. At that point it doesn't matter if it gets
> generated along with the function and then stripped/poisoned later, or
> generated later.

Stripping is conceptually straightforward even without LTO.

foo.indirect:
ENDBR
foo:
...

and the linker learns (using magic function sections?) that, if
foo.indirect is not referenced, then it should not be generated. Or a
very straightforward walk over all the relocations in an object to
poison the unused .indirect entries could be done. Making this work
with DSOs, EXPORT_SYMBOL, etc will be somewhat nontrivial, but not
impossible.

--Andy