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expvar: panic: interface conversion: interface is nil, not expvar.Var #52719

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catenacyber opened this issue May 5, 2022 · 4 comments
Closed

expvar: panic: interface conversion: interface is nil, not expvar.Var #52719

catenacyber opened this issue May 5, 2022 · 4 comments
Labels
FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.19

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

$ go version
go version go1.17.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/catena/Library/Caches/go-build"
GOENV="/Users/catena/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/catena/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/catena/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"
GOVCS=""
GOVERSION="go1.17.6"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/catena/go/src/github.com/catenacyber/go/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/pp/dc1dtf9x2js3v0jx_m010nqr0000gn/T/go-build4237848497=/tmp/go-build -gno-record-gcc-switches -fno-common"
GOROOT/bin/go version: go version go1.17.6 darwin/amd64
GOROOT/bin/go tool compile -V: compile version go1.17.6
uname -v: Darwin Kernel Version 21.3.0: Wed Jan  5 21:37:58 PST 2022; root:xnu-8019.80.24~20/RELEASE_X86_64
ProductName:	macOS
ProductVersion:	12.2.1
BuildVersion:	21D62
lldb --version: lldb-1316.0.9.41
Apple Swift version 5.6 (swiftlang-5.6.0.323.62 clang-1316.0.20.8)
gdb --version: GNU gdb (GDB) 9.1

What did you do?

Run https://go.dev/play/p/L708GRIfgkv

What did you expect to see?

The program finishing and printing hello

What did you see instead?

panic: interface conversion: interface is nil, not expvar.Var

goroutine 1 [running]:
expvar.(*Map).Do(0xc00005e2a0, 0xc00007def8)
	/usr/local/go-faketime/src/expvar/expvar.go:227 +0x13c
expvar.(*Map).String(0xc00005e2a0?)
	/usr/local/go-faketime/src/expvar/expvar.go:117 +0x9c
main.main()
	/tmp/sandbox1378131054/prog.go:14 +0x75

Program exited.

Found by https://github.com/catenacyber/ngolo-fuzzing on oss-fuzz
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=47006

Is this a real bug ?
@mengzhuo
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mengzhuo commented May 5, 2022

A more simpler example.

import (
	"expvar"
)

func main() {
	Map0 := expvar.NewMap("")
	Map0.Set("", nil)
	Map0.String()
}

@mengzhuo
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mengzhuo commented May 5, 2022

I think we should check the type of av Set(key string, av Var) by

switch av.(type) {
case nil:
   panic("can't set nil type")
default:
}

cc @bradfitz per owner, @griesemer

@dr2chase dr2chase added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label May 5, 2022
@dr2chase dr2chase added this to the Backlog milestone May 5, 2022
@ianlancetaylor
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I don't see any deep reason why we should prohibit setting a map element to nil. I think we should fix the String method to not crash.

@ianlancetaylor ianlancetaylor self-assigned this May 28, 2022
@gopherbot
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Change https://go.dev/cl/408677 mentions this issue: expvar: don't crash if map value set to nil

@dmitshur dmitshur added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Jun 15, 2022
@dmitshur dmitshur modified the milestones: Backlog, Go1.19 Jun 15, 2022
@catenacyber catenacyber mentioned this issue Jun 17, 2022
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@golang golang locked and limited conversation to collaborators Jun 22, 2023
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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Milestone
Go1.19
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@mengzhuo @dmitshur @dr2chase @ianlancetaylor @gopherbot @catenacyber