From: Nicolas Pitre <npitre@baylibre.com>
Date: Mon, 16 Jun 2025 19:22:44 +0000 (-0400)
Subject: mul_u64_u64_div_u64: fix the division-by-zero behavior
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e795000e755c309d1f9bd2a0590eca38b4625f3a;p=thirdparty%2Flinux.git

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.

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>
---

diff --git a/lib/math/div64.c b/lib/math/div64.c
index 5faa29208bdb4..bf77b9843175e 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);