[PATCH net-next] llc: avoid sparse cast-truncates warning in counter clamps

[PATCH net-next] llc: avoid sparse cast-truncates warning in counter clamps

[Avinash Duduskar]

llc_conn_ac_inc_npta_value() and llc_conn_ac_inc_tx_win_size()
clamp their counters to the maximum valid 7-bit value via
(u8) ~LLC_2_SEQ_NBR_MODULO. LLC_2_SEQ_NBR_MODULO is defined as
((u8) 128) in include/net/llc_pdu.h, but the (u8) cast does not
prevent integer promotion of the operand of ~: ~128 is computed
as int (0xffffff7f), and the surrounding (u8) cast truncates
back to 0x7f. The result is correct (127), but the implicit
truncation is flagged by sparse:

  net/llc/llc_c_ac.c:1008:38: warning: cast truncates bits from
      constant value (ffffff7f becomes 7f)
  (and three more at lines 1009, 1099, 1100)

Replace the (u8) ~LLC_2_SEQ_NBR_MODULO expression with
LLC_2_SEQ_NBR_MODULO - 1, which evaluates to 127 directly and
silences sparse.

The same ~LLC_2_SEQ_NBR_MODULO pattern also appears in
include/net/llc_pdu.h:148 as part of PDU_GET_NEXT_Vr, but there
the result is immediately &-masked, so the int promotion is
harmless and sparse does not flag it; it is left alone.

This patch is the minimum diff to silence the warning. The
counter-clamp idiom itself could be modernized to
min_t(u8, ..., LLC_2_SEQ_NBR_MODULO - 1), but that is a
separate cleanup left for another patch.

No functional change.

Signed-off-by: Avinash Duduskar <avinash.duduskar@gmail.com>
---
Built with CONFIG_LLC2=m; full kernel build clean, sparse no
longer warns on the touched lines.

 net/llc/llc_c_ac.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/llc/llc_c_ac.c b/net/llc/llc_c_ac.c
index 0779daa8aa8f..ab86c720b3ec 100644
--- a/net/llc/llc_c_ac.c
+++ b/net/llc/llc_c_ac.c
@@ -1005,8 +1005,8 @@ static int llc_conn_ac_inc_npta_value(struct sock *sk, struct sk_buff *skb)
 		llc->dec_step = 0;
 		llc->dec_cntr = llc->inc_cntr = 2;
 		++llc->npta;
-		if (llc->npta > (u8) ~LLC_2_SEQ_NBR_MODULO)
-			llc->npta = (u8) ~LLC_2_SEQ_NBR_MODULO;
+		if (llc->npta > LLC_2_SEQ_NBR_MODULO - 1)
+			llc->npta = LLC_2_SEQ_NBR_MODULO - 1;
 	} else
 		--llc->inc_cntr;
 	return 0;
@@ -1096,8 +1096,8 @@ int llc_conn_ac_inc_tx_win_size(struct sock *sk, struct sk_buff *skb)
 	struct llc_sock *llc = llc_sk(sk);
 
 	llc->k += 1;
-	if (llc->k > (u8) ~LLC_2_SEQ_NBR_MODULO)
-		llc->k = (u8) ~LLC_2_SEQ_NBR_MODULO;
+	if (llc->k > LLC_2_SEQ_NBR_MODULO - 1)
+		llc->k = LLC_2_SEQ_NBR_MODULO - 1;
 	return 0;
 }
 

base-commit: 8ebd24a7822cbae25beeafba49b2159d6a68a5f2
-- 
2.54.0