netdev - [PATCH 5/9] ipv4: tcp_output: avoid warning about NET_ADD

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Date: Thu, 28 Mar 2024 15:30:43 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: linux-kernel@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Nathan Chancellor <nathan@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Dmitry Safonov <0x7f454c46@...il.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	"mfreemon@...udflare.com" <mfreemon@...udflare.com>,
	Yan Zhai <yan@...udflare.com>,
	netdev@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: [PATCH 5/9] ipv4: tcp_output: avoid warning about NET_ADD_STATS

From: Arnd Bergmann <arnd@...db.de>

Clang warns about a range check in percpu_add_op() being impossible
to hit for an u8 variable:

net/ipv4/tcp_output.c:188:3: error: result of comparison of constant -1 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/net/ip.h:291:41: note: expanded from macro 'NET_ADD_STATS'
 #define NET_ADD_STATS(net, field, adnd) SNMP_ADD_STATS((net)->mib.net_statistics, field, adnd)
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/net/snmp.h:143:4: note: expanded from macro 'SNMP_ADD_STATS'
                        this_cpu_add(mib->mibs[field], addend)
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/percpu-defs.h:509:33: note: expanded from macro 'this_cpu_add'
 #define this_cpu_add(pcp, val)          __pcpu_size_call(this_cpu_add_, pcp, val)
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
<scratch space>:187:1: note: expanded from here
this_cpu_add_8
^
arch/x86/include/asm/percpu.h:326:35: note: expanded from macro 'this_cpu_add_8'
 #define this_cpu_add_8(pcp, val)                percpu_add_op(8, volatile, (pcp), val)
                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/include/asm/percpu.h:127:31: note: expanded from macro 'percpu_add_op'
                              ((val) == 1 || (val) == -1)) ?            \
                                             ~~~~~ ^  ~~

Avoid this warning with a cast to a signed 'int'.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 net/ipv4/tcp_output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e3167ad96567..dbe54fceee08 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -183,7 +183,7 @@ static inline void tcp_event_ack_sent(struct sock *sk, u32 rcv_nxt)
 
 	if (unlikely(tp->compressed_ack)) {
 		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
-			      tp->compressed_ack);
+			      (int)tp->compressed_ack);
 		tp->compressed_ack = 0;
 		if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
 			__sock_put(sk);
-- 
2.39.2