[PATCH bpf-next v3 3/7] bpf: Drop dead spill diagnostic condition

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

diag_record_mod() suppresses lineage-preserving register writes when the
diagnostic snapshot is unchanged. This is reachable for ALU operations that
do not change the represented type or bounds, avoiding a redundant history
event while retaining the register's earlier lineage.

The condition appears to extend this suppression to spill events, but
diag_mod_keeps_lineage() only accepts register writes. The spill producer also
supplies a stack target and an explicit origin, so a spill can never satisfy
the 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 5ecfa86ed49f..496d24064532 100644
--- a/kernel/bpf/diagnostics.c
+++ b/kernel/bpf/diagnostics.c
@@ -1567,8 +1567,7 @@ 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))
-- 
2.53.0