cmd/compile: access to negative slice indices improperly permitted #34802

AaronReboot · 2019-10-09T22:16:24Z

What version of Go are you using (`go version`)?

go version go1.13.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (`go env`)?

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/aaron/Library/Caches/go-build"
GOENV="/Users/aaron/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/aaron/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/n0/33wyl5yn1x77xsgjmz3p53zw0000gn/T/go-build007240201=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Accessed a negative slice index. Example: https://play.golang.org/p/Abpv3H1sgol

What did you expect to see?

Expected to see error

What did you see instead?

No error

mvdan · 2019-10-09T22:20:24Z

Interesting. This succeeds on 1.13.1 and master, but fails on 1.12.9. Sounds like a regression somewhere. Perhaps because of the recent improvements to the prove pass?

/cc @randall77 @rasky @zdjones

marigonzes · 2019-10-09T22:53:41Z

Bisected to ddef157

networkimprov · 2019-10-10T03:36:11Z

Don't the trybots test for this?

ianlancetaylor · 2019-10-10T03:38:06Z

@gopherbot Please open a backport to 1.13.

If this is indeed a compiler bug present in 1.13, we should backport it.

gopherbot · 2019-10-10T03:38:12Z

Backport issue(s) opened: #34807 (for 1.13).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

bradfitz · 2019-10-10T04:03:12Z

Don't the trybots test for this?

Trybots only run tests that exist.

randall77 · 2019-10-10T04:45:18Z

There are tests that check that negative indexes panic. Those tests just don't cover this case (induction variable which starts at a negative value).

networkimprov · 2019-10-10T07:09:45Z

@randall77 could a negative index corrupt memory in this case?

zdjones · 2019-10-10T10:25:15Z

I'll look into this in detail tomorrow.

ericlagergren · 2019-10-10T13:40:39Z

@networkimprov yes, it can overwrite values on the stack.

$ cat x.go
package main

import (
	"fmt"
)

var n int

func main() {
	var a [5]int
	var b [5]int
	var c [5]int
	for i := -1; i <= 0; i-- {
		b[i] = i
		n++
		if n > 100 {
			break
		}
	}
	for i := range a {
		fmt.Println(a[i], b[i], c[i])
	}
}
0:1 /tmp $ go run x.go
0 0 -5
0 0 -4
0 0 -3
0 0 -2
0 0 -1

zdjones · 2019-10-11T15:21:21Z

Found the issue, mailing the fix shortly.

gopherbot · 2019-10-11T17:31:46Z

Change https://golang.org/cl/200759 mentions this issue: cmd/compile: make poset use sufficient conditions for orderedOrEqual

gopherbot · 2019-10-14T16:57:00Z

Change https://golang.org/cl/201060 mentions this issue: [release-branch.go1.13] cmd/compile: make poset use sufficient conditions for OrderedOrEqual


        [release-branch.go1.13] cmd/compile: make poset use sufficient condit…

…ions for OrderedOrEqual When assessing whether A <= B, the poset's OrderedOrEqual has a passing condition which permits A <= B, but is not sufficient to infer that A <= B. This CL removes that incorrect passing condition. Having identified that A and B are in the poset, the method will report that A <= B if any of these three conditions are true: (1) A and B are the same node in the poset. - This means we know that A == B. (2) There is a directed path, strict or not, from A -> B - This means we know that, at least, A <= B, but A < B is possible. (3) There is a directed path from B -> A, AND that path has no strict edges. - This means we know that B <= A, but do not know that B < A. In condition (3), we do not have enough information to say that A <= B, rather we only know that B == A (which satisfies A <= B) is possible. The way I understand it, a strict edge shows a known, strictly-ordered relation (<) but the lack of a strict edge does not show the lack of a strictly-ordered relation. The difference is highlighted by the example in #34802, where a bounds check is incorrectly removed by prove, such that negative indexes into a slice succeed: n := make([]int, 1) for i := -1; i <= 0; i++ { fmt.Printf("i is %d\n", i) n[i] = 1 // No Bounds check, program runs, assignment to n[-1] succeeds!! } When prove is checking the negative/failed branch from the bounds check at n[i], in the signed domain we learn (0 > i || i >= len(n)). Because prove can't learn the OR condition, we check whether we know that i is non-negative so we can learn something, namely that i >= len(n). Prove uses the poset to check whether we know that i is non-negative. At this point the poset holds the following relations as a directed graph: -1 <= i <= 0 -1 < 0 In poset.OrderedOrEqual, we are testing for 0 <= i. In this case, condition (3) above is true because there is a non-strict path from i -> 0, and that path does NOT have any strict edges. Because this condition is true, the poset reports to prove that i is known to be >= 0. Knowing, incorrectly, that i >= 0, prove learns from the failed bounds check that i >= len(n) in the signed domain. When the slice, n, was created, prove learned that len(n) == 1. Because i is also the induction variable for the loop, upon entering the loop, prove previously learned that i is in [-1,0]. So when prove attempts to learn from the failed bounds check, it finds the new fact, i > len(n), unsatisfiable given that it previously learned that i <= 0 and len(n) = 1. Fixes #34807 Change-Id: I235f4224bef97700c3aa5c01edcc595eb9f13afc Reviewed-on: https://go-review.googlesource.com/c/go/+/200759 Run-TryBot: Zach Jones <zachj1@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Giovanni Bajo <rasky@develer.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-on: https://go-review.googlesource.com/c/go/+/201060 Run-TryBot: Alexander Rakoczy <alex@golang.org>

mvdan added the NeedsInvestigation label Oct 9, 2019

ianlancetaylor changed the title ~~Access to negative slice indices improperly permitted~~ cmd/compile: access to negative slice indices improperly permitted Oct 10, 2019

ianlancetaylor added this to the Go1.14 milestone Oct 10, 2019

ianlancetaylor added the release-blocker label Oct 10, 2019

gopherbot referenced this issue Oct 10, 2019

cmd/compile: access to negative slice indices improperly permitted [1.13 backport] #34807

Closed

networkimprov referenced this issue Oct 11, 2019

proposal: backports: fix regressions in previous releases for N days #34622

Open

gopherbot closed this in 3c56eb4 Oct 12, 2019

golang/go

cmd/compile: access to negative slice indices improperly permitted #34802

cmd/compile: access to negative slice indices improperly permitted #34802

AaronReboot commented Oct 9, 2019

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ericlagergren commented Oct 10, 2019 •

edited

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gopherbot commented Oct 14, 2019

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cmd/compile: access to negative slice indices improperly permitted #34802

Comments

AaronReboot commented Oct 9, 2019

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

What did you do?

What did you expect to see?

What did you see instead?

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mvdan commented Oct 9, 2019

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marigonzes commented Oct 9, 2019

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networkimprov commented Oct 10, 2019

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ericlagergren commented Oct 10, 2019 • edited

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zdjones commented Oct 11, 2019

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gopherbot commented Oct 11, 2019

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gopherbot commented Oct 14, 2019

What version of Go are you using (`go version`)?

What operating system and processor architecture are you using (`go env`)?

ericlagergren commented Oct 10, 2019 •

edited