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x/crypto: SSH swallows first character in second connection #41390

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jkohen opened this issue Sep 15, 2020 · 1 comment
Open

x/crypto: SSH swallows first character in second connection #41390

jkohen opened this issue Sep 15, 2020 · 1 comment

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@jkohen
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@jkohen jkohen commented Sep 15, 2020

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

$ go version
go version go1.15 linux/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="/usr/local/google/home/jkohen/.cache/go-build"
GOENV="/usr/local/google/home/jkohen/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/usr/local/google/home/jkohen/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/usr/local/google/home/jkohen/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/google-golang"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/google-golang/pkg/tool/linux_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=/tmp/go-build079540723=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The following assumes a ssh server in localhost set up with SSH_AUTH_SOCK, which is what my environment provides. It should be trivial for you all to adjust it for a different host and hardcoded password as needed:

  1. go run xcrypto-bug.txt
  2. Once you see the "type something" prompt, enter a string and press enter. Note the output.
  3. Repeat for the second attempt.

What did you expect to see?

The output from the second session is exactly what I entered on the keyboard.

What did you see instead?

The output from the second session is missing the first character. If I add more executions of runTest(), I see the same effect in all but the first session.

For instance, in the session below I typed "hi" both times, but the second time only the "i" was registered:

Type something: hi
0000000   h   i  nl
         68  69  0a
0000003
Type something: i
0000000   i  nl
         69  0a
0000002

I tried a few things to no avail:

  • Discard bytes from stdin before and after the session (with bufio.NewReader().Discard()).
  • Different TerminalMode settings.
  • Calling os.Stdin.Sync() and os.Stdin.Seek(0, 2)
  • Execute reset locally before the SSH session.
  • etc.

I also tried a couple different versions of the SSH code in the snippet, and always got the same result. I was unable to reproduce this with a local shell, or with the ssh command-line tool (OpenSSH).

@gopherbot gopherbot added this to the Unreleased milestone Sep 15, 2020
@toothrot
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@toothrot toothrot commented Sep 15, 2020

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