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 01:15:57PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 23, 2026 at 11:58:13AM +0530, Mukesh Kumar Chaurasiya wrote:
> > On Tue, Sep 22, 2026 at 08:47:35AM +0200, Peter Zijlstra wrote:
> > > On Tue, Sep 22, 2026 at 11:20:00AM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:
> > > > Verify that the __bug_table entry emitted by ARCH_WARN_ASM has a correct
> > > > bug_addr displacement — i.e. the arch's trap label reference resolves to
> > > > the trap instruction — by calling find_bug() with the exact virtual address
> > > > of the trap, mirroring what the real trap handler does.
> > > > 
> > > > To support all architectures, each arch that implements ARCH_WARN_ASM now
> > > > defines ARCH_WARN_ASM_TRAP_LABEL, a string constant naming the local label
> > > > at which the trap instruction is placed:
> > > > 
> > > >   x86       "1"     (ud2 at label 1:)
> > > >   powerpc   "1"     (twi at label 1:)
> > > >   riscv     "1"     (ebreak at label 1:)
> > > >   arm64     "14471" (brk placed at 14471: by __BUG_ENTRY_END)
> > > >   s390      "0"     (mc at label 0:)
> > > > 
> > > > The label is used consistently: in ARCH_WARN_ASM itself, in the
> > > > bug_addr back-reference inside __BUG_ENTRY / _EMIT_BUG_ENTRY, 

> > > Not really a fan of that. And I can't really tell what you're doing with
> > > it either. The kunit is in Rust and thus unreadable :-(
> > > 
> > > I would rather you fix up is_valid_bugaddr(), some architectures seem to
> > > have an always true stub because of the callchains always being from the
> > > break instruction.
> > 
> > Hey Peter,
> > 
> > thanks for the suggestion. I will fix up the is_valid_bugaddr for
> > powerpc. I am not very good with other archs asm but i can do it for
> > ppc32 and ppc64.
> 
> Untested thingies for arm64 and s390x.
> 
> 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.

Mark.