The patch titled
Subject: mul_u64_u64_div_u64: avoid undefined shift value
has been added to the -mm mm-nonmm-unstable branch. Its filename is
mul_u64_u64_div_u64-basic-sanity-test-fix.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mul_u64_u64_div_u64-basic-sanity-test-fix.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Nicolas Pitre <npitre@xxxxxxxxxxxx>
Subject: mul_u64_u64_div_u64: avoid undefined shift value
Date: Fri, 12 Jul 2024 13:41:46 -0400 (EDT)
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.
Link: https://lkml.kernel.org/r/7rrs9pn1-n266-3013-9q6n-1osp8r8s0rrn@xxxxxxxxxxx
Signed-off-by: Nicolas Pitre <npitre@xxxxxxxxxxxx>
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202407121652.69e657c5-oliver.sang@xxxxxxxxx
Cc: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
Cc: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/math/div64.c | 9 ++++++++-
lib/math/test_mul_u64_u64_div_u64.c | 1 +
2 files changed, 9 insertions(+), 1 deletion(-)
--- a/lib/math/div64.c~mul_u64_u64_div_u64-basic-sanity-test-fix
+++ a/lib/math/div64.c
@@ -212,11 +212,18 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u6
#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);
/*
--- a/lib/math/test_mul_u64_u64_div_u64.c~mul_u64_u64_div_u64-basic-sanity-test-fix
+++ a/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 },
_
Patches currently in -mm which might be from npitre@xxxxxxxxxxxx are
mul_u64_u64_div_u64-make-it-precise-always.patch
mul_u64_u64_div_u64-basic-sanity-test.patch
mul_u64_u64_div_u64-basic-sanity-test-fix.patch
