linux-kernel - [PATCH v2 1/4] s390/bitops: Limit return value range of _

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Message-ID: <20250916134803.874580-2-hca@linux.ibm.com>
Date: Tue, 16 Sep 2025 15:48:00 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Nathan Chancellor <nathan@...nel.org>, Miguel Ojeda <ojeda@...nel.org>,
        Kees Cook <kees@...nel.org>,
        Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Juergen Christ <jchrist@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Sven Schnelle <svens@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: [PATCH v2 1/4] s390/bitops: Limit return value range of __flogr()

With the recent ffs() and ffs64() optimization a logical AND operation was
removed, which allowed the compiler to tell the return value range of both
functions. This may lead to compile warnings as reported by the kernel test
robot:

   drivers/infiniband/hw/mlx5/mr.c: In function 'mlx5r_cache_create_ent_locked':
>> drivers/infiniband/hw/mlx5/mr.c:840:31: warning: 'sprintf' may write a terminating nul past the end of the destination [-Wformat-overflow=]
     840 |         sprintf(ent->name, "%d", order);
         |                               ^
   In function 'mlx5_mkey_cache_debugfs_add_ent',
       inlined from 'mlx5r_cache_create_ent_locked' at drivers/infiniband/hw/mlx5/mr.c:930:3:
   drivers/infiniband/hw/mlx5/mr.c:840:9: note: 'sprintf' output between 2 and 5 bytes into a destination of size 4
     840 |         sprintf(ent->name, "%d", order);
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add the AND operation again to address the warning.

>From a correctness point of view the AND operation is not necessary,
however there is no other way to tell the compiler that the returned
value of the flogr inline assembly is in the range of 0..64.

This increases the kernel image size by 566 bytes (defconfig, gcc 15.2.0).

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508211859.UoYsJbLN-lkp@intel.com/
Fixes: de88e74889a3 ("s390/bitops: Slightly optimize ffs() and fls64()")
Suggested-by: Juergen Christ <jchrist@...ux.ibm.com>
Reviewed-by: Juergen Christ <jchrist@...ux.ibm.com>
Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>
---
 arch/s390/include/asm/bitops.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/bitops.h b/arch/s390/include/asm/bitops.h
index 56376b6ff809..a1dd72b16f54 100644
--- a/arch/s390/include/asm/bitops.h
+++ b/arch/s390/include/asm/bitops.h
@@ -169,7 +169,7 @@ static __always_inline unsigned char __flogr(unsigned long word)
 		asm volatile(
 			"       flogr   %[rp],%[rp]\n"
 			: [rp] "+d" (rp.pair) : : "cc");
-		return rp.even;
+		return rp.even & 127;
 	}
 }
 
-- 
2.48.1