From: Yury Norov <ynorov@nvidia.com>
Date: Thu, 12 Mar 2026 23:08:16 +0000 (-0400)
Subject: lib: count_zeros: fix 32/64-bit inconsistency in count_trailing_zeros()
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4e64c91b813f666dffc3962a815a8a50b7d6f468;p=thirdparty%2Fkernel%2Flinux.git

lib: count_zeros: fix 32/64-bit inconsistency in count_trailing_zeros()

Based on 'sizeof(x) == 4' condition, in 32-bit case the function is wired
to ffs(), while in 64-bit case to __ffs(). The difference is substantial:
ffs(x) == __ffs(x) + 1. Also, ffs(0) == 0, while __ffs(0) is undefined.

The 32-bit behaviour is inconsistent with the function description, so it
needs to get fixed.

There are 9 individual users for the function in 6 different subsystems.
Some arches and drivers are 64-bit only:
 - arch/loongarch/kvm/intc/eiointc.c;
 - drivers/hv/mshv_vtl_main.c;
 - kernel/liveupdate/kexec_handover.c;

The others are:
 - ib_umem_find_best_pgsz(): as per comment, __ffs() should be correct;
 - rzv2m_csi_reg_write_bit(): ARCH_RENESAS only, unclear;
 - lz77_match_len(): CIFS_COMPRESSION only, unclear, experimental;

IB and CIFS are explicitly OK with the change.

The attached patch gets rid of 32-bit explicit support, so that both
32- and 64-bit versions rely on __ffs().

CC: K. Y. Srinivasan <kys@microsoft.com>
CC: Haiyang Zhang <haiyangz@microsoft.com>
CC: Mark Brown <broonie@kernel.org>
CC: Steve French <sfrench@samba.org>
CC: Alexander Graf <graf@amazon.com>
CC: Mike Rapoport <rppt@kernel.org>
CC: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Yury Norov <ynorov@nvidia.com>
---

diff --git a/include/linux/count_zeros.h b/include/linux/count_zeros.h
index 4e5680327ece8..5034a30b5c7c7 100644
--- a/include/linux/count_zeros.h
+++ b/include/linux/count_zeros.h
@@ -10,6 +10,8 @@
 
 #include <asm/bitops.h>
 
+#define COUNT_TRAILING_ZEROS_0 (-1)
+
 /**
  * count_leading_zeros - Count the number of zeros from the MSB back
  * @x: The value
@@ -40,12 +42,7 @@ static inline int count_leading_zeros(unsigned long x)
  */
 static inline int count_trailing_zeros(unsigned long x)
 {
-#define COUNT_TRAILING_ZEROS_0 (-1)
-
-	if (sizeof(x) == 4)
-		return ffs(x);
-	else
-		return (x != 0) ? __ffs(x) : COUNT_TRAILING_ZEROS_0;
+	return (x != 0) ? __ffs(x) : COUNT_TRAILING_ZEROS_0;
 }
 
 #endif /* _LINUX_BITOPS_COUNT_ZEROS_H_ */