Re: [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Aug 14, 2025 at 2:17 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Hi!
>
> A while ago FineIBT started using the instruction 0xEA to generate #UD.
> All existing parts will generate #UD in 64bit mode on that instruction.
>
> However; Intel/AMD have not blessed using this instruction, it is on
> their 'reserved' list for future use.
>
> Peter Anvin worked the committees and got use of 0xD6 blessed, and it
> will be called UDB (per the next SDM or so).
>
> Reworking the FineIBT code to use UDB wasn't entirely trivial, and I've
> had to switch the hash register to EAX in order to free up some bytes.
>
> Per the x86_64 ABI, EAX is used to pass the number of vector registers
> for varargs -- something that should not happen in the kernel. More so,
> we build with -mskip-rax-setup, which should leave EAX completely unused
> in the calling convention.

rax is used to pass tail_call count.
See diagram in commit log:
https://lore.kernel.org/all/20240714123902.32305-2-hffilwlqm@gmail.com/
Before that commit rax was used differently.
Bottom line rax was used for a long time to support bpf_tail_calls.
I'm traveling atm. So cc-ing folks for follow ups.