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

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Message-ID: <CAADnVQLyahEsFereM_-Y-MUdWm2mLHNKfffwNKX5Fvy+EaH-Nw@mail.gmail.com>
Date: Fri, 15 Aug 2025 08:42:39 +0300
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>, Leon Hwang <hffilwlqm@...il.com>, bpf <bpf@...r.kernel.org>, 
	Andrii Nakryiko <andrii@...nel.org>
Cc: X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Kees Cook <kees@...nel.org>, 
	alyssa.milburn@...el.com, scott.d.constable@...el.com, 
	Joao Moreira <joao@...rdrivepizza.com>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Nathan Chancellor <nathan@...nel.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, ojeda@...nel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA

On Thu, Aug 14, 2025 at 2:17 PM Peter Zijlstra <peterz@...radead.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.