+ mul_u64_u64_div_u64-fix-the-division-by-zero-behavior.patch added to mm-nonmm-unstable branch

+ mul_u64_u64_div_u64-fix-the-division-by-zero-behavior.patch added to mm-nonmm-unstable branch

[Andrew Morton]

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The patch titled
     Subject: mul_u64_u64_div_u64: fix the division-by-zero behavior
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     mul_u64_u64_div_u64-fix-the-division-by-zero-behavior.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mul_u64_u64_div_u64-fix-the-division-by-zero-behavior.patch

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

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------------------------------------------------------
From: Nicolas Pitre <npitre@baylibre.com>
Subject: mul_u64_u64_div_u64: fix the division-by-zero behavior
Date: Mon, 16 Jun 2025 15:22:44 -0400 (EDT)

The current implementation forces a compile-time 1/0 division, which
generates an undefined instruction (ud2 on x86) rather than a proper
runtime division-by-zero exception.

Change to trigger an actual div-by-0 exception at runtime, consistent with
other division operations.  Use a non-1 dividend to prevent the compiler
from optimizing the division into a comparison.

Link: https://lkml.kernel.org/r/q246p466-1453-qon9-29so-37105116009q@onlyvoer.pbz
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Weißschuh <linux@weissschuh.net>
Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Cc: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/math/div64.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

--- a/lib/math/div64.c~mul_u64_u64_div_u64-fix-the-division-by-zero-behavior
+++ a/lib/math/div64.c
@@ -212,12 +212,13 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u6
 
 #endif
 
-	/* make sure c is not zero, trigger exception otherwise */
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wdiv-by-zero"
-	if (unlikely(c == 0))
-		return 1/0;
-#pragma GCC diagnostic pop
+	/* make sure c is not zero, trigger runtime exception otherwise */
+	if (unlikely(c == 0)) {
+		unsigned long zero = 0;
+
+		OPTIMIZER_HIDE_VAR(zero);
+		return ~0UL/zero;
+	}
 
 	int shift = __builtin_ctzll(c);
 
_

Patches currently in -mm which might be from npitre@baylibre.com are

mul_u64_u64_div_u64-fix-the-division-by-zero-behavior.patch