+ lib-mul_u64_u64_div_u64-combine-overflow-and-divide-by-zero-checks.patch added to mm-nonmm-unstable branch

+ lib-mul_u64_u64_div_u64-combine-overflow-and-divide-by-zero-checks.patch added to mm-nonmm-unstable branch

[Andrew Morton]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 3794 bytes --]


The patch titled
     Subject: lib: mul_u64_u64_div_u64(): combine overflow and divide by zero checks
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     lib-mul_u64_u64_div_u64-combine-overflow-and-divide-by-zero-checks.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-mul_u64_u64_div_u64-combine-overflow-and-divide-by-zero-checks.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: David Laight <david.laight.linux@gmail.com>
Subject: lib: mul_u64_u64_div_u64(): combine overflow and divide by zero checks
Date: Wed, 5 Nov 2025 20:10:28 +0000

Since the overflow check always triggers when the divisor is zero
move the check for divide by zero inside the overflow check.
This means there is only one test in the normal path.

Link: https://lkml.kernel.org/r/20251105201035.64043-3-david.laight.linux@gmail.com
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-by: Nicolas Pitre <npitre@baylibre.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/math/div64.c |   21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

--- a/lib/math/div64.c~lib-mul_u64_u64_div_u64-combine-overflow-and-divide-by-zero-checks
+++ a/lib/math/div64.c
@@ -212,12 +212,16 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u6
 
 #endif
 
-	/* 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;
+	if (unlikely(n_hi >= d)) {
+		/* trigger runtime exception if divisor is zero */
+		if (d == 0) {
+			unsigned long zero = 0;
+
+			OPTIMIZER_HIDE_VAR(zero);
+			return ~0UL/zero;
+		}
+		/* overflow: result is unrepresentable in a u64 */
+		return ~0ULL;
 	}
 
 	int shift = __builtin_ctzll(d);
@@ -234,11 +238,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u6
 		 */
 	}
 
-	if (n_hi >= d) {
-		/* overflow: result is unrepresentable in a u64 */
-		return -1;
-	}
-
 	/* Do the full 128 by 64 bits division */
 
 	shift = __builtin_clzll(d);
_

Patches currently in -mm which might be from david.laight.linux@gmail.com are

lib-mul_u64_u64_div_u64-rename-parameter-c-to-d.patch
lib-mul_u64_u64_div_u64-combine-overflow-and-divide-by-zero-checks.patch
lib-mul_u64_u64_div_u64-simplify-check-for-a-64bit-product.patch
lib-add-mul_u64_add_u64_div_u64-and-mul_u64_u64_div_u64_roundup.patch
lib-add-tests-for-mul_u64_u64_div_u64_roundup.patch
lib-test_mul_u64_u64_div_u64-test-both-generic-and-arch-versions.patch
lib-mul_u64_u64_div_u64-optimise-multiply-on-32bit-x86.patch
lib-mul_u64_u64_div_u64-optimise-the-divide-code.patch
lib-test_mul_u64_u64_div_u64-test-the-32bit-code-on-64bit.patch