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

[Mark Rutland <mark.rutland@arm.com>]

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.