From ff534e2130f4ab0f297bb08d66b082878483bfc1 Mon Sep 17 00:00:00 2001 From: Alberto Donizetti Date: Tue, 21 Nov 2017 14:16:04 +0100 Subject: [PATCH] math/big: protect against aliasing in nat.divLarge MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit In nat.divLarge (having signature (z nat).divLarge(u, uIn, v nat)), we check whether z aliases uIn or v, but aliasing is currently not checked for the u parameter. Unfortunately, z and u aliasing each other can in some cases cause errors in the computation. The q return parameter (which will hold the result's quotient), is unconditionally initialized as q = z.make(m + 1) When cap(z) ≥ m+1, z.make() will reuse z's backing array, causing q and z to share the same backing array. If then z aliases u, setting q during the quotient computation will then corrupt u, which at that point already holds computation state. To fix this, we add an alias(z, u) check at the beginning of the function, taking care of aliasing the same way we already do for uIn and v. Fixes #22830 Change-Id: I3ab81120d5af6db7772a062bb1dfc011de91f7ad Reviewed-on: https://go-review.googlesource.com/78995 Run-TryBot: Alberto Donizetti Run-TryBot: Robert Griesemer TryBot-Result: Gobot Gobot Reviewed-by: Robert Griesemer --- src/math/big/int_test.go | 20 ++++++++++++++++++++ src/math/big/nat.go | 4 ++-- 2 files changed, 22 insertions(+), 2 deletions(-) diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go index d72cad29a5..270fec6b36 100644 --- a/src/math/big/int_test.go +++ b/src/math/big/int_test.go @@ -1625,6 +1625,26 @@ func TestSqrt(t *testing.T) { } } +// We can't test this together with the other Exp tests above because +// it requires a different receiver setup. +func TestIssue22830(t *testing.T) { + one := new(Int).SetInt64(1) + base, _ := new(Int).SetString("84555555300000000000", 10) + mod, _ := new(Int).SetString("66666670001111111111", 10) + want, _ := new(Int).SetString("17888885298888888889", 10) + + var tests = []int64{ + 0, 1, -1, + } + + for _, n := range tests { + m := NewInt(n) + if got := m.Exp(base, one, mod); got.Cmp(want) != 0 { + t.Errorf("(%v).Exp(%s, 1, %s) = %s, want %s", n, base, mod, got, want) + } + } +} + func BenchmarkSqrt(b *testing.B) { n, _ := new(Int).SetString("1"+strings.Repeat("0", 1001), 10) b.ResetTimer() diff --git a/src/math/big/nat.go b/src/math/big/nat.go index 17d5b8ae2b..3bb818f5f2 100644 --- a/src/math/big/nat.go +++ b/src/math/big/nat.go @@ -621,8 +621,8 @@ func (z nat) divLarge(u, uIn, v nat) (q, r nat) { // determine if z can be reused // TODO(gri) should find a better solution - this if statement // is very costly (see e.g. time pidigits -s -n 10000) - if alias(z, uIn) || alias(z, v) { - z = nil // z is an alias for uIn or v - cannot reuse + if alias(z, u) || alias(z, uIn) || alias(z, v) { + z = nil // z is an alias for u or uIn or v - cannot reuse } q = z.make(m + 1)