Re: [PATCH] x86/kcfi: Optimize call sequence

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 16, 2026 at 09:47:22PM +0100, David Laight wrote:

> > --- a/arch/x86/kernel/alternative.c
> > +++ b/arch/x86/kernel/alternative.c
> > @@ -1356,6 +1356,10 @@ early_param("cfi", cfi_parse_cmdline);
> >   *  "Make conditional jumps most often not taken: The efficiency and throughput
> >   *   for not-taken branches is better than for taken branches on most
> >   *   processors. Therefore, it is good to place the most frequent branch first"
> > + *
> > + * NOTE: Update the kCFI caller sequence to make use of this observation.
> > + * Replace the "je 1f; ud2" sequence with "jne +1; test $0xd6, %al". This
> > + * clobbers flags, but those are clobbered by the hash test anyway.
> 
> I think it would be better to give the byte sequences for both pairs of
> instructions - it takes a bit of sleuthing to check they are the same size.

You mean, expand the comment like a few lines above, where we have the
kCFI/FineIBT contrast? Sure, I suppose I can make this comment longer
still.

> I think it would also be better it the code doing the patching checked
> what it was overwriting.

Ye of little faith :-)

> Also, what actually generates the list of cfi locations in the first place?
> If it is objtool, then maybe it could do the rewrite instead.

The list with UD2 locations is compiler generated. Also, objtool
typically avoids actually modifying code and generally prefers to just
ship additional sections such that the kernel can modify itself. There
is an exception to this, but there was definite grumbling about that.