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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 15, 2025 at 08:42:39AM +0300, Alexei Starovoitov wrote:
> 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.

IIRC the bpf2bpf tailcall doesn't use CFI at the moment. But let me
double check.

So emit_cfi() is called at the very start of emit_prologue() and
__arch_prepare_bpf_trampoline() in the BPF_TRAMP_F_INDIRECT case.

Now, emit_prologue() starts with the CFI bits, but the tailcall lands at
X86_TAIL_CALL_OFFSET, at which spot we only have EMIT_ENDBR(), nothing
else. So RAX should be unaffected at that point.

So, AFAICT, we're good on that point. It is just the C level indirect
function call ABI that is affected, BPF internal conventions are
unaffected.