Re: [PATCH] x86/boot/compressed: Disable jump tables for clang

[David Laight <david.laight.linux@gmail.com>]

On Wed, 24 Jun 2026 15:17:39 -0700
Nathan Chancellor <nathan@kernel.org> wrote:

> On Wed, Jun 24, 2026 at 11:55:10AM +0200, Ingo Molnar wrote:
> > 
> > * Ingo Molnar <mingo@kernel.org> wrote:
> > 
> >   
> > > > I'm sitting on a patch to unconditionally disable jump-tables for
> > > > x86_64:
> > > > 
> > > >   https://web.git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=x86/syscall  
> > > 
> > > In particular:
> > > 
> > >   https://web.git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=x86/syscall&id=76612388fe7aa41a8eb88f890d451bc17255eda0  
> > 
> > Side note: since arch/x86/boot/compressed/Makefile constructs
> > its own KBUILD_CFLAGS, so a change to that Makefile will still
> > be required to universally apply -fno-jump-tables and work
> > around this Clang optimization in the decompression code.  
> 
> Right. I had intentionally kept my change scoped to clang to be less
> controversial but in the face of Peter's series, it makes sense to do it
> for all compilers like Ingo suggested. I have no preference for how we
> proceed here. I don't mind sending a v2 with something like

Isn't this solving a different problem?
Jump tables are disabled for the kernel build to avoid speculation of
mispredicted indirect jumps.
Here they are needed to stop the compiler output containing 'things' the
restricted environment can't support.

Someone building a kernel for a local machine may want to disable all of
the mitigations to avoid their associated costs and also enable jump
tables to avoid the cost of all the mispredicted branches in the comparison
tree.

	David