Re: [PATCH] x86: add hintable NOPs emulation

[David Laight]

On Wed, 20 Aug 2025 11:07:33 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Wed, Aug 20, 2025 at 03:34:46AM +0200, Marcos Del Sol Vives wrote:
> > Hintable NOPs are a series of instructions introduced by Intel with the
> > Pentium Pro (i686), and described in US patent US5701442A.
> > 
> > These instructions were reserved to allow backwards-compatible changes
> > in the instruction set possible, by having old processors treat them as
> > variable-length NOPs, while having other semantics in modern processors.
> > 
> > Some modern uses are:
> >  - Multi-byte/long NOPs
> >  - Indirect Branch Tracking (ENDBR32)
> >  - Shadow Stack (part of CET)
> > 
> > Some processors advertising i686 compatibility lack full support for
> > them, which may cause #UD to be incorrectly triggered, crashing software
> > that uses then with an unexpected SIGILL.
> > 
> > One such software is sudo in Debian bookworm, which is compiled with
> > GCC -fcf-protection=branch and contains ENDBR32 instructions. It crashes
> > on my Vortex86DX3 processor and VIA C3 Nehalem processors [1].
> > 
> > This patch is a much simplified version of my previous patch for x86
> > instruction emulation [2], that only emulates hintable NOPs.
> > 
> > When #UD is raised, it checks if the opcode corresponds to a hintable NOP
> > in user space. If true, it warns the user via the dmesg and advances the
> > instruction pointer, thus emulating its expected NOP behaviour.
> > 
> > [1]: https://lists.debian.org/debian-devel/2023/10/msg00118.html
> > [2]: https://lore.kernel.org/all/20210626130313.1283485-1-marcos@xxxxxxxx/
> > 
> > Signed-off-by: Marcos Del Sol Vives <marcos@xxxxxxxx>  
> 
> This is going to be terribly slow if there's a significant number of
> traps (like with endbr32), but yeah, this ought to work.

Could you patch the memory resident page to contain a supported nop?
(without marking it 'dirty')
Then the same function wouldn't trap until the code page was reloaded
from the source file.

	David