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Message-ID: <7rrs9pn1-n266-3013-9q6n-1osp8r8s0rrn@syhkavp.arg>
Date: Fri, 12 Jul 2024 13:41:46 -0400 (EDT)
From: Nicolas Pitre <nico@...xnic.net>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: Uwe Kleine-König <u.kleine-koenig@...libre.com>,
linux-kernel@...r.kernel.org
Subject: [PATCH] mul_u64_u64_div_u64: avoid undefined shift value
From: Nicolas Pitre <npitre@...libre.com>
Shifting a value to its type's size or beyond is undefined. This may
happen if the product of a * b is not more than 64 bits despite
ilog2(a) + ilog2(b) being 63 and c having no trailing 0 bits.
We end up with shift=0 and n_lo >> shift | (n_hi << (64 - shift).
Take care of that case and add such case to the test module.
Using __builtin_ctzll() with 0 is also undefined so take care of that
case too.
Signed-off-by: Nicolas Pitre <npitre@...libre.com>
Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202407121652.69e657c5-oliver.sang@intel.com
---
Andrew: up to you to fold this in the original or queue it as is.
diff --git a/lib/math/div64.c b/lib/math/div64.c
index b7fc752463..5faa29208b 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -212,11 +212,18 @@ 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
+
int shift = __builtin_ctzll(c);
/* try reducing the fraction in case the dividend becomes <= 64 bits */
if ((n_hi >> shift) == 0) {
- u64 n = (n_lo >> shift) | (n_hi << (64 - shift));
+ u64 n = shift ? (n_lo >> shift) | (n_hi << (64 - shift)) : n_lo;
return div64_u64(n, c >> shift);
/*
diff --git a/lib/math/test_mul_u64_u64_div_u64.c b/lib/math/test_mul_u64_u64_div_u64.c
index a25640d349..58d058de4e 100644
--- a/lib/math/test_mul_u64_u64_div_u64.c
+++ b/lib/math/test_mul_u64_u64_div_u64.c
@@ -23,6 +23,7 @@ static test_params test_values[] = {
{ 0x1ffffffff, 0x1ffffffff, 0x4, 0xffffffff00000000 },
{ 0xffff000000000000, 0xffff000000000000, 0xffff000000000001, 0xfffeffffffffffff },
{ 0x3333333333333333, 0x3333333333333333, 0x5555555555555555, 0x1eb851eb851eb851 },
+{ 0x7fffffffffffffff, 0x2, 0x3, 0x5555555555555554 },
{ 0xffffffffffffffff, 0x2, 0x8000000000000000, 0x3 },
{ 0xffffffffffffffff, 0x2, 0xc000000000000000, 0x2 },
{ 0xffffffffffffffff, 0x4000000000000004, 0x8000000000000000, 0x8000000000000007 },
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