linux-kernel - [PATCH v2 next 2/4] lib: mul_u64_u64_div_u64() Use BUG

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: <20250518133848.5811-3-david.laight.linux@gmail.com>
Date: Sun, 18 May 2025 14:38:46 +0100
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>
Subject: [PATCH v2 next 2/4] lib: mul_u64_u64_div_u64() Use BUG_ON() for divide by zero

Do an explicit BUG_ON(!divisor) instead of hoping the 'undefined
behaviour' the compiler generated for a compile-time 1/0 is in any
way useful.

It may be better to define the function to return ~(u64)0 for
divide by zero.

Signed-off-by: David Laight <david.laight.linux@...il.com>
---

A new change for v2 of the patchset.
Whereas gcc inserts (IIRC) 'ud2' clang is likely to let the code
continue and generate 'random' results for any 'undefined bahaviour'.

 lib/math/div64.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/lib/math/div64.c b/lib/math/div64.c
index a5c966a36836..c426fa0660bc 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -186,6 +186,9 @@ EXPORT_SYMBOL(iter_div_u64_rem);
 #ifndef mul_u64_u64_div_u64
 u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
 {
+	/* Trigger exception if divisor is zero */
+	BUG_ON(!d);
+
 	if (ilog2(a) + ilog2(b) <= 62)
 		return div64_u64(a * b, d);
 
@@ -212,13 +215,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
 
 #endif
 
-	/* make sure d is not zero, trigger exception otherwise */
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wdiv-by-zero"
-	if (unlikely(d == 0))
-		return 1/0;
-#pragma GCC diagnostic pop
-
 	int shift = __builtin_ctzll(d);
 
 	/* try reducing the fraction in case the dividend becomes <= 64 bits */
-- 
2.39.5