linux-kernel - [PATCH v4 next 1/9] lib: mul_u64_u64_div

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20251029173828.3682-2-david.laight.linux@gmail.com>
Date: Wed, 29 Oct 2025 17:38:20 +0000
From: David Laight <david.laight.linux@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Cc: David Laight <david.laight.linux@...il.com>,
	u.kleine-koenig@...libre.com,
	Nicolas Pitre <npitre@...libre.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Biju Das <biju.das.jz@...renesas.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Li RongQing <lirongqing@...du.com>,
	Yu Kuai <yukuai3@...wei.com>,
	Khazhismel Kumykov <khazhy@...omium.org>,
	Jens Axboe <axboe@...nel.dk>,
	x86@...nel.org
Subject: [PATCH v4 next 1/9] lib: mul_u64_u64_div_u64() rename parameter 'c' to 'd'

Change to prototype from mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
to mul_u64_u64_div_u64(u64 a, u64 b, u64 d).
Using 'd' for 'divisor' makes more sense.

An upcoming change adds a 'c' parameter to calculate (a * b + c)/d.

Signed-off-by: David Laight <david.laight.linux@...il.com>
Reviewed-by: Nicolas Pitre <npitre@...libre.com>
---
 lib/math/div64.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/lib/math/div64.c b/lib/math/div64.c
index bf77b9843175..0ebff850fd4d 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -184,10 +184,10 @@ u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
 EXPORT_SYMBOL(iter_div_u64_rem);
 
 #ifndef mul_u64_u64_div_u64
-u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
+u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
 {
 	if (ilog2(a) + ilog2(b) <= 62)
-		return div64_u64(a * b, c);
+		return div64_u64(a * b, d);
 
 #if defined(__SIZEOF_INT128__)
 
@@ -212,37 +212,37 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
 
 #endif
 
-	/* make sure c is not zero, trigger runtime exception otherwise */
-	if (unlikely(c == 0)) {
+	/* make sure d is not zero, trigger runtime exception otherwise */
+	if (unlikely(d == 0)) {
 		unsigned long zero = 0;
 
 		OPTIMIZER_HIDE_VAR(zero);
 		return ~0UL/zero;
 	}
 
-	int shift = __builtin_ctzll(c);
+	int shift = __builtin_ctzll(d);
 
 	/* try reducing the fraction in case the dividend becomes <= 64 bits */
 	if ((n_hi >> shift) == 0) {
 		u64 n = shift ? (n_lo >> shift) | (n_hi << (64 - shift)) : n_lo;
 
-		return div64_u64(n, c >> shift);
+		return div64_u64(n, d >> shift);
 		/*
 		 * The remainder value if needed would be:
-		 *   res = div64_u64_rem(n, c >> shift, &rem);
+		 *   res = div64_u64_rem(n, d >> shift, &rem);
 		 *   rem = (rem << shift) + (n_lo - (n << shift));
 		 */
 	}
 
-	if (n_hi >= c) {
+	if (n_hi >= d) {
 		/* overflow: result is unrepresentable in a u64 */
 		return -1;
 	}
 
 	/* Do the full 128 by 64 bits division */
 
-	shift = __builtin_clzll(c);
-	c <<= shift;
+	shift = __builtin_clzll(d);
+	d <<= shift;
 
 	int p = 64 + shift;
 	u64 res = 0;
@@ -257,8 +257,8 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
 		n_hi <<= shift;
 		n_hi |= n_lo >> (64 - shift);
 		n_lo <<= shift;
-		if (carry || (n_hi >= c)) {
-			n_hi -= c;
+		if (carry || (n_hi >= d)) {
+			n_hi -= d;
 			res |= 1ULL << p;
 		}
 	} while (n_hi);
-- 
2.39.5