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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568b19b32sm4869444f8f.67.2025.06.14.02.54.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 14 Jun 2025 02:54:13 -0700 (PDT) From: David Laight To: Andrew Morton , linux-kernel@vger.kernel.org Cc: David Laight , u.kleine-koenig@baylibre.com, Nicolas Pitre , Oleg Nesterov , Peter Zijlstra , Biju Das Subject: [PATCH v3 next 02/10] lib: mul_u64_u64_div_u64() Use WARN_ONCE() for divide errors. Date: Sat, 14 Jun 2025 10:53:38 +0100 Message-Id: <20250614095346.69130-3-david.laight.linux@gmail.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250614095346.69130-1-david.laight.linux@gmail.com> References: <20250614095346.69130-1-david.laight.linux@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Do an explicit WARN_ONCE(!divisor) instead of hoping the 'undefined behaviour' the compiler generates for a compile-time 1/0 is in any way useful. Return 0 (rather than ~(u64)0) because it is less likely to cause further serious issues. Add WARN_ONCE() in the divide overflow path. 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 behaviour'. v3: Use WARN_ONCE() and return 0 instead of BUG_ON(). Explicitely #include Signed-off-by: David Laight --- lib/math/div64.c | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/lib/math/div64.c b/lib/math/div64.c index a5c966a36836..397578dc9a0b 100644 --- a/lib/math/div64.c +++ b/lib/math/div64.c @@ -19,6 +19,7 @@ */ =20 #include +#include #include #include #include @@ -186,6 +187,15 @@ EXPORT_SYMBOL(iter_div_u64_rem); #ifndef mul_u64_u64_div_u64 u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d) { + if (WARN_ONCE(!d, "%s: division of (%#llx * %#llx) by zero, returning 0", + __func__, a, b)) { + /* + * Return 0 (rather than ~(u64)0) because it is less likely to + * have unexpected side effects. + */ + return 0; + } + if (ilog2(a) + ilog2(b) <=3D 62) return div64_u64(a * b, d); =20 @@ -212,12 +222,10 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d) =20 #endif =20 - /* make sure d is not zero, trigger exception otherwise */ -#pragma GCC diagnostic push -#pragma GCC diagnostic ignored "-Wdiv-by-zero" - if (unlikely(d =3D=3D 0)) - return 1/0; -#pragma GCC diagnostic pop + if (WARN_ONCE(n_hi >=3D d, + "%s: division of (%#llx * %#llx =3D %#llx%016llx) by %#llx overflo= ws, returning ~0", + __func__, a, b, n_hi, n_lo, d)) + return ~(u64)0; =20 int shift =3D __builtin_ctzll(d); =20 @@ -233,11 +241,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d) */ } =20 - if (n_hi >=3D d) { - /* overflow: result is unrepresentable in a u64 */ - return -1; - } - /* Do the full 128 by 64 bits division */ =20 shift =3D __builtin_clzll(d); --=20 2.39.5