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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48febe5821fsm33738735e9.14.2026.05.16.03.23.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 16 May 2026 03:23:48 -0700 (PDT) Date: Sat, 16 May 2026 11:23:47 +0100 From: David Laight To: Avinash Duduskar Cc: netdev@vger.kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net-next] llc: avoid sparse cast-truncates warning in counter clamps Message-ID: <20260516112347.68aba670@pumpkin> In-Reply-To: <20260513092253.3035961-1-avinash.duduskar@gmail.com> References: <20260513092253.3035961-1-avinash.duduskar@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 13 May 2026 14:52:53 +0530 Avinash Duduskar wrote: > 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, That (u8) cast is entirely pointless. Whenever the value is used it is subject to integer promotion and becomes a signed 128. > 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. Using min_t() (especially with u8 or u16) is a very bad idea. Consider what would would happen if the limit were 255. > > No functional change. > > Signed-off-by: Avinash Duduskar > --- > 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; If you rewrite it, do the check before the increment. The compiler doesn't know that llc->npta isn't 255 - when the result of the above is 0. That can generate rather more code. Probably best to do: if (llc->npta < LLC_2_SEQ_NBR_MODULO - 1) llc->npta++; -- David > } 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