Re: [RFC] rust: kernel: Add KUnit tests for ARCH_WARN_ASM bug table emission

[Mukesh Kumar Chaurasiya]

On Thu, Sep 24, 2026 at 02:05:57PM +0100, Mark Rutland wrote:
> On Thu, Sep 24, 2026 at 02:52:53PM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 24, 2026 at 01:46:59PM +0100, Mark Rutland wrote:
> > > What problem is this trying to solve?
> > > 
> > > Mukesh's original patch seems to be trying to check whether we compiled
> > > things correctly (such that the bug_table entry points at the expected
> > > insturction), and I don't think that warrants a runtime check, but (as
> > > per my reply to him) I don't understand why that's a thing to check in
> > > the first place.
> > > 
> > > On arm64 we only call is_valid_bugaddr() under do_el1_brk64() ->
> > > call_el1_break_hook() -> bug_brk_handler(). We only call
> > > bug_brk_handler() when HW has taken a BRK exception with the BUG_BRK_IMM
> > > immediate, and that can only be triggered by executing the right
> > > instruction (BRK #BUG_BRK_IMM), which we ONLY use for BUG(). and WARN().
> > > 
> > > So I don't see why any change is necessary here.
> > > 
> > > If the HW has somehow taken a BRK exception from any other instruction
> > > (which would presumably be a HW bug), there won't be a bug_table entry,
> > > and so __report_bug() will fail to find an entry, and will return
> > > BUG_TRAP_TYPE_NONE, same as for the !is_valid_bugaddr() case.
> > 
> > Right, so I wasn't at all sure what he was doing, Rust is still mostly
> > unreadable garbage to me.
> > 
> > But I figured I'd much rather see him use is_valid_bugaddr() to do
> > runtime checks than whatever dodgy games he was playing with that label.
> > 
> > So *if* doing runtime checks is what is wanted, then is_valid_bugaddr()
> > needs to actually work, because then it grows a user outside of the BRK
> > handler.
> 
> Thanks; I think we're aligned. :)
> 
> Before we delve into implementation details, I'd like to understand the
> problem Mukesh is trying to solve.
> 
> Mark.

Hey Peter and Mark,

It's not as a problem, it's something i faced while implementing the
ARCH_WARN_ASM, the generated assembly was not correct so i wrote a test
for it, I sent it out along with the same patch[1] but then the
discussion led to it being for all archs so I sent it out as an RFC to
know others opinion.

After Peter's last comment i found is_valid_bugaddr can use a bit more
work(atleast on ppc) so i think i am going to drop this and go ahead
with improving is_valid_bugaddr.

Regards,
Mukesh

[1]https://lore.kernel.org/all/20260915090453.1227034-1-mkchauras@xxxxxxxxx/