linux-kernel - [PATCH v2] mul_u64_u64_div_u64: fix the division-by-zero behavior

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Message-ID: <q246p466-1453-qon9-29so-37105116009q@onlyvoer.pbz>
Date: Mon, 16 Jun 2025 15:22:44 -0400 (EDT)
From: Nicolas Pitre <npitre@...libre.com>
To: Andrew Morton <akpm@...ux-foundation.org>, 
    David Laight <david.laight.linux@...il.com>
cc: Thomas Weißschuh <linux@...ssschuh.net>, 
    Oleg Nesterov <oleg@...hat.com>, 
    Uwe Kleine-König <u.kleine-koenig@...libre.com>, 
    Peter Zijlstra <peterz@...radead.org>, 
    Biju Das <biju.das.jz@...renesas.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v2] mul_u64_u64_div_u64: fix the division-by-zero behavior

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.

Signed-off-by: Nicolas Pitre <npitre@...libre.com>
---

Change from v1 (http://lore.kernel.org/all/q2o7r916-5601-11pn-30pn-8n5ns6p079o7@onlyvoer.pbz):
- use OPTIMIZER_HIDE_VAR() in place of the open coded incantation.

diff --git a/lib/math/div64.c b/lib/math/div64.c
index 5faa29208bdb..bf77b9843175 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -212,12 +212,13 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
 
 #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);