[PATCH v1 1/1] x86/percpu: Cast -1 to argument type when comparing in percpu_add_op()

Andy Shevchenko
2024-09-05T17:04:06Z

When percpu_add_op() is used with unsigned argument, it prevents kernel builds
with clang, `make W=1` and CONFIG_WERROR=y:

net/ipv4/tcp_output.c:187:3: 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]
187 | NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
188 | tp->compressed_ack);
| ~~~~~~~~~~~~~~~~~~~
...
arch/x86/include/asm/percpu.h:238:31: note: expanded from macro 'percpu_add_op'
238 | ((val) == 1 || (val) == -1)) ? \
| ~~~~~ ^ ~~

Fix this by casting -1 to the type of the parameter and then compare.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
arch/x86/include/asm/percpu.h | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index c55a79d5feae..e525cd85f999 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -234,9 +234,10 @@ do { \
*/
#define percpu_add_op(size, qual, var, val) \
do { \
- const int pao_ID__ = (__builtin_constant_p(val) && \
- ((val) == 1 || (val) == -1)) ? \
- (int)(val) : 0; \
+ const int pao_ID__ = \
+ (__builtin_constant_p(val) && \
+ ((val) == 1 || \
+ (val) == (typeof(val))-1)) ? (int)(val) : 0; \
\
if (0) { \
typeof(var) pao_tmp__; \
--
2.43.0.rc1.1336.g36b5255a03ac