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

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Message-ID: <2c121572-8fde-4288-80ca-ab79f2b22cce@zytor.com>
Date: Thu, 14 Aug 2025 18:27:44 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Cc: kees@...nel.org, alyssa.milburn@...el.com, scott.d.constable@...el.com,
        joao@...rdrivepizza.com, andrew.cooper3@...rix.com,
        samitolvanen@...gle.com, nathan@...nel.org,
        alexei.starovoitov@...il.com, mhiramat@...nel.org, ojeda@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA

On 2025-08-14 04:17, Peter Zijlstra 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.
> 
> The code boots and passes the LKDTM CFI_FORWARD_PROTO test for various
> combinations (non exhaustive so far).
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Looks good to me (and using %eax will save one byte per call site as
well), but as per our IRC discussion, *my understanding* is that the
best possible performance (least branch predictor impact across
implementations) is to use a forward branch with a 2E prefix (jcc,pn in
GAS syntax) rather than a reverse branch, if space allows.

	-hpa