Messages in this thread | | | Date | Wed, 3 Nov 2021 12:32:20 -0700 | Subject | Re: [PATCH] static_call,x86: Robustify trampoline patching | From | Andy Lutomirski <> |
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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
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