+ mul_u64_u64_div_u64-basic-sanity-test-fix.patch added to mm-nonmm-unstable branch

+ mul_u64_u64_div_u64-basic-sanity-test-fix.patch added to mm-nonmm-unstable branch

[Andrew Morton]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 3869 bytes --]


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

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Nicolas Pitre <npitre@baylibre.com>
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@syhkavp.arg
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202407121652.69e657c5-oliver.sang@intel.com
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 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@baylibre.com 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