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

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:

quoted

On Thu, Sep 24, 2026 at 01:46:59PM +0100, Mark Rutland wrote:

quoted

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@gmail.com/ (local)