Messages in this thread |  | | Date | Sun, 31 Oct 2021 17:39:20 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] static_call,x86: Robustify trampoline patching |
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On Sun, Oct 31, 2021 at 05:24:13PM +0100, Ard Biesheuvel wrote: > On Sat, 30 Oct 2021 at 20:55, Ard Biesheuvel <ardb@kernel.org> wrote: > > > > 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. > > Something like the below seems to work to prevent getting the wrong > trampoline address into arch_static_call_transform:
Is is also a terriblly gross hack. I really want the clang-cfi stuff to improve, not add layers of hacks on top of it.
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