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

[Peter Zijlstra <peterz@infradead.org>]

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

> > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > index 914282016069..3d8a969eab5f 100644
> > --- a/arch/arm64/kernel/traps.c
> > +++ b/arch/arm64/kernel/traps.c
> > @@ -987,14 +987,13 @@ void do_serror(struct pt_regs *regs, unsigned long esr)
> >  #ifdef CONFIG_GENERIC_BUG
> >  int is_valid_bugaddr(unsigned long addr)
> >  {
> > -	/*
> > -	 * bug_brk_handler() only called for BRK #BUG_BRK_IMM.
> > -	 * So the answer is trivial -- any spurious instances with no
> > -	 * bug table entry will be rejected by report_bug() and passed
> > -	 * back to the debug-monitors code and handled as a fatal
> > -	 * unexpected debug exception.
> > -	 */
> > -	return 1;
> > +	u32 insn;
> > +
> > +	if (aarch64_insn_read((u32 *)addr, &insn))
> > +		return 0;
> > +
> > +	/* Match ASM_BUG_FLAGS() / BUG() instructions. */
> > +	return insn == (0xd4200000 | (BUG_BRK_IMM << 5));
> >  }
> >  #endif
> 
> 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.

Lots of IFs etc, but since I am having a head-ache and am mostly
useless, I figured I'd distract myself doing whatever :-)