Re: [PATCH bpf-next v1 04/14] bpf: Drop dead spill diagnostic condition

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2026-08-16 at 03:57 +0200, Kumar Kartikeya Dwivedi wrote:
> diag_record_mod() appears to allow identical spill events to be suppressed,
> but diag_mod_keeps_lineage() only accepts register write events. The spill
> producer also supplies a stack target and an explicit origin, so a spill can
> never satisfy this conjunction.
> 
> Restrict the condition to writes to reflect the filter's actual contract and
> avoid suggesting that it applies to spills.
> 
> Link: https://lore.kernel.org/bpf/48e6f021b89562f68850fe21ef8c78719819b04cf9c4e4f50bc791937d37ace8@mail.kernel.org/
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  kernel/bpf/diagnostics.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/diagnostics.c b/kernel/bpf/diagnostics.c
> index 33b7d9e8e2c3..37ed22a764e0 100644
> --- a/kernel/bpf/diagnostics.c
> +++ b/kernel/bpf/diagnostics.c
> @@ -1531,8 +1531,7 @@ static void diag_record_mod(struct bpf_verifier_env *env, u32 insn_idx,
>  	} else if (diag_mod_insn_origin(env, insn_idx, &target, &event.mod.origin)) {
>  		event.mod.origin_valid = true;
>  	}
> -	if (old_reg && new_reg &&
> -	    (reason == BPF_DIAG_MOD_WRITE || reason == BPF_DIAG_MOD_SPILL) &&
> +	if (old_reg && new_reg && reason == BPF_DIAG_MOD_WRITE &&
>  	    !memcmp(&event.mod.old, &event.mod.new, sizeof(event.mod.old)) &&
>  	    !event.mod.origin_valid &&
>  	    diag_mod_keeps_lineage(env, &event))

Q: does this filter ever fires?
   The condition appears to be very narrow.