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bytes: Random behavior when reading ssh output with bytes.Buffer #45770

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Canadadry opened this issue Apr 26, 2021 · 7 comments
Closed

bytes: Random behavior when reading ssh output with bytes.Buffer #45770

Canadadry opened this issue Apr 26, 2021 · 7 comments
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FrozenDueToAge WaitingForInfo Issue is not actionable because of missing required information, which needs to be provided.

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

$ go version
go version go1.16.3 darwin/arm64

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="arm64"
GOBIN=""
GOCACHE="/Users/canadadry/Library/Caches/go-build"
GOENV="/Users/canadadry/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/canadadry/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/canadadry/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/canadadry/.brew/Cellar/go/1.16.3/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/canadadry/.brew/Cellar/go/1.16.3/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.16.3"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/canadadry/Prog/ssh/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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/gj/q0mflgrx5tg41qsqh7hbfrm40000gn/T/go-build1246411259=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I want to excecute a shell command over ssh then write the result into a bytes.Buffer and print the content of the Buffer. Here a minimal code reproducing the bug :

package main

import (
	"bytes"
	"flag"
	"fmt"
	"golang.org/x/crypto/ssh"
	"golang.org/x/crypto/ssh/knownhosts"
	"io/ioutil"
	"net"
	"os"
	"path/filepath"
)

type spy struct {
}

func (s *spy) Write(b []byte) (int, error) {
	fmt.Printf("%d : --%v--\n", len(b), b)
	return len(b), nil
}

func main() {
	if err := run(); err != nil {
		fmt.Println(err)
	}
}

func run() error {
	host := flag.String("h", "", "host")
	port := flag.String("p", "22", "port")
	user := flag.String("u", "", "user")
	useSpy := flag.Bool("s", false, "use spy")

	flag.Parse()

	PrivateKey, err := ioutil.ReadFile(filepath.Join(os.Getenv("HOME"), ".ssh", "id_rsa"))
	if err != nil {
		return err
	}
	key, err := ssh.ParsePrivateKey(PrivateKey)
	if err != nil {
		return err
	}
	hostKeyCallback, err := knownhosts.New(filepath.Join(os.Getenv("HOME"), ".ssh", "known_hosts"))
	if err != nil {
		return err
	}
	config := &ssh.ClientConfig{
		User:            *user,
		HostKeyCallback: hostKeyCallback,
		Auth: []ssh.AuthMethod{
			ssh.PublicKeys(key),
		},
	}
	c, err := ssh.Dial("tcp", net.JoinHostPort(*host, *port), config)
	if err != nil {
		return err
	}

	defer c.Close()

	session, err := c.NewSession()
	if err != nil {
		return err
	}

	defer session.Close()
	b := &bytes.Buffer{}
	s := &spy{}
	if *useSpy {
		session.Stdout = s
		session.Stderr = s
	} else {
		session.Stdout = b
		session.Stderr = b
	}
	err = session.Run("ls -la")
	if err != nil {
		return err
	}
	if !*useSpy {
		fmt.Println(b.String())
	}
	fmt.Println("end")
	return nil
}

What did you expect to see?

I want to always see the list of file on my server or a network error.

~/P/ssh > go run main.go -u $USER -h $HOST
total 28
drwxr-xr-x 7 root root 4096 Sep 15  2020 .
drwxr-xr-x 6 root root 4096 Sep 15  2020 ..
drwxr-xr-x 3 usr  usr  4096 Sep 15  2020 .config
drwxr-xr-x 3 usr  usr  4096 Sep 15  2020 public
drwxr-xr-x 2 usr  usr  4096 Sep 15  2020 sessions
drwxr-xr-x 2 root root 4096 Sep 15  2020 .ssh
drwxr-xr-x 2 usr  usr  4096 Apr 20 16:50 tmp

end

What did you see instead?

Some time I see nothing. No error and no output from the command. I suspect this to be linked on how the bytes.Buffer work inside since I am reading and writing in it but I have no idea on how make it more minimal.
Obviously when I use my spy I always see the result of my command.

~/P/ssh > go run main.go -u $USER -h $HOST
total 28
drwxr-xr-x 7 root root 4096 Sep 15  2020 .
drwxr-xr-x 6 root root 4096 Sep 15  2020 ..
drwxr-xr-x 3 usr  usr  4096 Sep 15  2020 .config
drwxr-xr-x 3 usr  usr  4096 Sep 15  2020 public
drwxr-xr-x 2 usr  usr  4096 Sep 15  2020 sessions
drwxr-xr-x 2 root root 4096 Sep 15  2020 .ssh
drwxr-xr-x 2 usr  usr  4096 Apr 20 16:50 tmp
end
~/P/ssh > go run main.go -u $USER -h $HOST

end
~/P/ssh > go run main.go -u $USER -h $HOST

end
~/P/ssh > go run main.go -u $USER -h $HOST
total 28
drwxr-xr-x 7 root root 4096 Sep 15  2020 .
drwxr-xr-x 6 root root 4096 Sep 15  2020 ..
drwxr-xr-x 3 usr  usr  4096 Sep 15  2020 .config
drwxr-xr-x 3 usr  usr  4096 Sep 15  2020 public
drwxr-xr-x 2 usr  usr  4096 Sep 15  2020 sessions
drwxr-xr-x 2 root root 4096 Sep 15  2020 .ssh
drwxr-xr-x 2 usr  usr  4096 Apr 20 16:50 tmp

end
@mvdan
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mvdan commented Apr 26, 2021

I'd suggest that you use https://golang.org/wiki/Questions. This could be an issue with your program, such as a data race. Nothing seems to indicate a specific bug in the bytes package.

@mvdan mvdan added the WaitingForInfo Issue is not actionable because of missing required information, which needs to be provided. label Apr 26, 2021
@ghost
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ghost commented Apr 26, 2021 •
edited by ghost

Try synchronizing writes: https://play.golang.org/p/GE50s3bxCtY

@seankhliao
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There's a race between copying data to the buffer (which may grow) and reading the contents (also between writes from both stdin/stderr)

==================
WARNING: DATA RACE
Write at 0x00c00011e050 by goroutine 16:
  bytes.(*Buffer).ReadFrom()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:200 +0x44
  io.copyBuffer()
      /home/arccy/sdk/gotip/src/io/io.go:409 +0x1b3
  io.Copy()
      /home/arccy/sdk/gotip/src/io/io.go:382 +0xb2
  golang.org/x/crypto/ssh.(*Session).stderr.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:524 +0x2b
  golang.org/x/crypto/ssh.(*Session).start.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:383 +0x35
  golang.org/x/crypto/ssh.(*Session).start·dwrap·23()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:384 +0x47

Previous write at 0x00c00011e050 by goroutine 15:
  bytes.(*Buffer).ReadFrom()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:200 +0x44
  io.copyBuffer()
      /home/arccy/sdk/gotip/src/io/io.go:409 +0x1b3
  io.Copy()
      /home/arccy/sdk/gotip/src/io/io.go:382 +0x90
  golang.org/x/crypto/ssh.(*Session).stdout.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:511 +0x2b
  golang.org/x/crypto/ssh.(*Session).start.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:383 +0x35
  golang.org/x/crypto/ssh.(*Session).start·dwrap·23()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:384 +0x47

Goroutine 16 (running) created at:
  golang.org/x/crypto/ssh.(*Session).start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:382 +0x176
  golang.org/x/crypto/ssh.(*Session).Start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:293 +0x24b
  golang.org/x/crypto/ssh.(*Session).Run()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:310 +0x49
  main.run()
      /home/arccy/tmp/testrepo-394/main.go:80 +0x8f5
  main.main()
      /home/arccy/tmp/testrepo-394/main.go:24 +0x29

Goroutine 15 (running) created at:
  golang.org/x/crypto/ssh.(*Session).start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:382 +0x176
  golang.org/x/crypto/ssh.(*Session).Start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:293 +0x24b
  golang.org/x/crypto/ssh.(*Session).Run()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:310 +0x49
  main.run()
      /home/arccy/tmp/testrepo-394/main.go:80 +0x8f5
  main.main()
      /home/arccy/tmp/testrepo-394/main.go:24 +0x29
==================
==================
WARNING: DATA RACE
Read at 0x00c00011e030 by goroutine 16:
  bytes.(*Buffer).Len()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:73 +0x49
  bytes.(*Buffer).grow()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:118 +0x44
  bytes.(*Buffer).ReadFrom()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:202 +0x65
  io.copyBuffer()
      /home/arccy/sdk/gotip/src/io/io.go:409 +0x1b3
  io.Copy()
      /home/arccy/sdk/gotip/src/io/io.go:382 +0xb2
  golang.org/x/crypto/ssh.(*Session).stderr.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:524 +0x2b
  golang.org/x/crypto/ssh.(*Session).start.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:383 +0x35
  golang.org/x/crypto/ssh.(*Session).start·dwrap·23()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:384 +0x47

Previous write at 0x00c00011e030 by goroutine 15:
  bytes.(*Buffer).grow()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:144 +0x27a
  bytes.(*Buffer).ReadFrom()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:202 +0x65
  io.copyBuffer()
      /home/arccy/sdk/gotip/src/io/io.go:409 +0x1b3
  io.Copy()
      /home/arccy/sdk/gotip/src/io/io.go:382 +0x90
  golang.org/x/crypto/ssh.(*Session).stdout.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:511 +0x2b
  golang.org/x/crypto/ssh.(*Session).start.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:383 +0x35
  golang.org/x/crypto/ssh.(*Session).start·dwrap·23()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:384 +0x47

Goroutine 16 (running) created at:
  golang.org/x/crypto/ssh.(*Session).start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:382 +0x176
  golang.org/x/crypto/ssh.(*Session).Start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:293 +0x24b
  golang.org/x/crypto/ssh.(*Session).Run()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:310 +0x49
  main.run()
      /home/arccy/tmp/testrepo-394/main.go:80 +0x8f5
  main.main()
      /home/arccy/tmp/testrepo-394/main.go:24 +0x29

Goroutine 15 (running) created at:
  golang.org/x/crypto/ssh.(*Session).start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:382 +0x176
  golang.org/x/crypto/ssh.(*Session).Start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:293 +0x24b
  golang.org/x/crypto/ssh.(*Session).Run()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:310 +0x49
  main.run()
      /home/arccy/tmp/testrepo-394/main.go:80 +0x8f5
  main.main()
      /home/arccy/tmp/testrepo-394/main.go:24 +0x29
==================
==================
WARNING: DATA RACE
Read at 0x00c00011e048 by goroutine 16:
  bytes.(*Buffer).Len()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:73 +0x68
  bytes.(*Buffer).grow()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:118 +0x44
  bytes.(*Buffer).ReadFrom()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:202 +0x65
  io.copyBuffer()
      /home/arccy/sdk/gotip/src/io/io.go:409 +0x1b3
  io.Copy()
      /home/arccy/sdk/gotip/src/io/io.go:382 +0xb2
  golang.org/x/crypto/ssh.(*Session).stderr.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:524 +0x2b
  golang.org/x/crypto/ssh.(*Session).start.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:383 +0x35
  golang.org/x/crypto/ssh.(*Session).start·dwrap·23()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:384 +0x47

Previous write at 0x00c00011e048 by goroutine 15:
  bytes.(*Buffer).grow()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:147 +0x364
  bytes.(*Buffer).ReadFrom()
      /home/arccy/sdk/gotip/src/bytes/buffer.go:202 +0x65
  io.copyBuffer()
      /home/arccy/sdk/gotip/src/io/io.go:409 +0x1b3
  io.Copy()
      /home/arccy/sdk/gotip/src/io/io.go:382 +0x90
  golang.org/x/crypto/ssh.(*Session).stdout.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:511 +0x2b
  golang.org/x/crypto/ssh.(*Session).start.func1()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:383 +0x35
  golang.org/x/crypto/ssh.(*Session).start·dwrap·23()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:384 +0x47

Goroutine 16 (running) created at:
  golang.org/x/crypto/ssh.(*Session).start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:382 +0x176
  golang.org/x/crypto/ssh.(*Session).Start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:293 +0x24b
  golang.org/x/crypto/ssh.(*Session).Run()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:310 +0x49
  main.run()
      /home/arccy/tmp/testrepo-394/main.go:80 +0x8f5
  main.main()
      /home/arccy/tmp/testrepo-394/main.go:24 +0x29

Goroutine 15 (running) created at:
  golang.org/x/crypto/ssh.(*Session).start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:382 +0x176
  golang.org/x/crypto/ssh.(*Session).Start()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:293 +0x24b
  golang.org/x/crypto/ssh.(*Session).Run()
      /home/arccy/.data/go/pkg/mod/golang.org/x/crypto@v0.0.0-20210421170649-83a5a9bb288b/ssh/session.go:310 +0x49
  main.run()
      /home/arccy/tmp/testrepo-394/main.go:80 +0x8f5
  main.main()
      /home/arccy/tmp/testrepo-394/main.go:24 +0x29
==================

@mvdan
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mvdan commented Apr 26, 2021

Others confirm my suspicion :) Closing as this is not a bug.

@mvdan mvdan closed this as completed Apr 26, 2021
@Canadadry
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@seankhliao how did you got those error message ?

Thank you, I did not though this was a question. Well I did not except any response from you. May be we can improve the documentation on the crypto/ssh to explicitly warn about possible data race.

@davecheney
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See https://blog.golang.org/race-detector .

@Canadadry
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thank you

@golang golang locked and limited conversation to collaborators Apr 26, 2022
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@davecheney @Canadadry @mvdan @gopherbot @seankhliao