Messages in this thread | | | From | Ard Biesheuvel <> | Date | Sat, 30 Oct 2021 20:55:20 +0200 | Subject | Re: [PATCH] static_call,x86: Robustify trampoline patching |
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On Sat, 30 Oct 2021 at 20:03, Peter Zijlstra <peterz@infradead.org> wrote: > > On Sat, Oct 30, 2021 at 07:19:53PM +0200, Ard Biesheuvel wrote: > > I just realized that arm64 has the exact same problem, which is not > > being addressed by my v5 of the static call support patch. > > Yeah, it would. > > > As it turns out, the v11 Clang that I have been testing with is broken > > wrt BTI landing pads, and omits them from the jump table entries. > > Clang 12+ adds them properly, which means that both the jump table > > entry and the static call trampoline may start with BTI C + direct > > branch, and we also need additional checks to disambiguate. > > I'm not sure, why would the static_call trampoline need a BTI C ? The > whole point of static_call() is to be a direct call, we should never > have an indirect call to the trampoline, that would defeat the whole > purpose.
This might happen when the distance between the caller and the trampoline is more than 128 MB, in which case we emit a veneer that uses an indirect call as well. So we definitely need the landing pad in the trampoline.
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