Re: [PATCH] static_call: use CFI-compliant return0 stubs

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 12, 2026 at 08:40:11AM +0100, Ard Biesheuvel wrote:
> So far, we have managed to avoid the blessings of objtool on arm64,
> and the complexity associated with the inline patching is not really
> justified, given that on arm64, there is not really a need to avoid
> indirect calls (and as Peter says, we might end up with them anyway)
> 
> A while ago, I had a stab at implementing the out-of-line variety [0],
> but nobody cared enough to even respond. It is rather concise, and
> localised to arm64, so it is something we might consider for
> CONFIG_CFI builds. It is essentially the same sequence that arm64 uses
> for trampolines between modules and the kernel if they are out of
> direct branching range, with some .rodata patching to change the
> target. (arm64 basically only permits code patching without stopping
> the machine when it involves patching branch opcodes into NOPS or vice
> versa).
> 
> Doing so for only CONFIG_CFI makes sense because it removes the CFI
> overhead for all static calls, although it adds back some overhead for
> the trampoline. But there is currently no need to do this
> unconditionally.

Right, so your v3 is very simple and straight forward, and should work
as an end run around the CFI issue, by effectively doing that indirect
tail call in the trampoline outside of the compiler generated software
cfi things.

And I think I like your thing better because it handles all possible
cases, not just the ret0 oddity and isn't in fact much larger.