Re: [PATCH 02/11] riscv: update asm call site in `call_on_irq_stack` to setup correct label

[Deepak Gupta]

On Fri, Jul 25, 2025 at 08:23:44AM +0200, Heinrich Schuchardt wrote:
> On 25.07.25 01:36, Deepak Gupta wrote:
> > Call sites written in asm performing indirect call, they need to setup
> > label register (t2/x7) with correct label.
> >
> > Currently first kernel was compiled with `-save-temps` option and
> > normalized function signature string is captured and then placed at the
> > asm callsite.
> >
> > TODO: to write a macro wrapper with toolchain support.
> >
> > Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> > ---
> >  arch/riscv/kernel/entry.S | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > index 2660faf52232..598e17e800ae 100644
> > --- a/arch/riscv/kernel/entry.S
> > +++ b/arch/riscv/kernel/entry.S
> > @@ -389,6 +389,7 @@ SYM_FUNC_START(call_on_irq_stack)
> >  	load_per_cpu t0, irq_stack_ptr, t1
> >  	li	t1, IRQ_STACK_SIZE
> >  	add	sp, t0, t1
> > +	lui t2, %lpad_hash("FvP7pt_regsE")
>
> In patch 1 you use lpad 0 due to missing tool support for signature hashing.
>
> Wouldn't it be preferable to have a first patch series introducing 
> landing pad support with lpad 0 and once tool support for signature 
> hashing has landed create a second patch series using tags?
>
> Such a first patch series would not have to be an RFC but might be 
> merged soon.

It's mostly about security guarantees. Coarser grained cfi (only landing pad)
has been proved many times not that effective. Kernel is a monolithic piece of
code. If there is a good chance of adoption anywhere for labeled landing pads,
its kernel. If it becomes a long pole, it's a possible direction to go back to
unlabeled landing pad.

> Best regards
>
> Heinrich
>
> >  	jalr	a1
> >  	/* Switch back to the thread shadow call stack */