Hacktoberfest 2026：メンテナが10月に向けて印を付けた、オープンで初心者向けの issue。 Hacktoberfest の issue を見る

ShellStream.Expect over-discards on undecodable bytes, driving ArrayBuffer.ActiveLength negative

オープン
  • 何か問題がありますか？
#1,839 コメント 0 件 リアクション 0 件 担当者 0 名 GitHub で見る

まだ誰も着手していません。

評価

難易度
4/5
見積もり時間
3〜5日
初心者へのやさしさ
52/100
issue の種類
バグ
明瞭さ
おおむね明確
活発さ
活発
技術スタック
csharp
領域
networking

調査の方向性

Start with ShellStream.ExpectRegex in src/Renci.SshNet/ShellStream.cs and ArrayBuffer.Discard in src/Renci.SshNet/Common/ArrayBuffer.cs. Reproduce the undecodable-byte arithmetic, then trace the existing Discard call sites and verify that consumed input bytes are counted without making ActiveLength negative. Done means the Release-only failure no longer occurs and the over-discard is surfaced at its origin if it recurs.

索引モデルが issue の本文から書いたものです。

説明

Summary

ShellStream.ExpectRegex decodes the read buffer to a string, matches against that string, and then
discards the byte count of the re-encoded match:

var bufferText = GetString(_readBuffer.ActiveLength);
...
var returnText = bufferText.Substring(0, match.Index + match.Length);

_readBuffer.Discard(_encoding.GetByteCount(returnText));

GetByteCount(returnText) equals the number of buffer bytes returnText was decoded from only when
the decode was lossless. It is not, whenever the consumed region contained a byte the encoding could
not decode: GetString replaces each maximal invalid subpart with U+FFFD, which re-encodes to
3 bytes regardless of how many bytes it replaced.

So Discard is asked for more bytes than the match occupied. And ArrayBuffer.Discard only checks
that bound with a Debug.Assert:

public void Discard(int byteCount)
{
    Debug.Assert(byteCount <= ActiveLength, $"Expected {byteCount} <= {ActiveLength}");
    _activeStart += byteCount;

    if (_activeStart == _availableStart)
    {
        _activeStart = 0;
        _availableStart = 0;
    }
}

In a Release build the assert is gone, _activeStart overshoots past _availableStart, and
ActiveLength (_availableStart - _activeStart) goes negative. The _activeStart == _availableStart
reset cannot fire either, because the overshoot skipped equality — so the buffer never recovers.

The next iteration then calls GetString(_readBuffer.ActiveLength) with a negative length, and throws.

What it looks like in production
System.ArgumentOutOfRangeException: Non-negative number required. (Parameter 'count')
   at Renci.SshNet.ShellStream.ExpectRegex(TimeSpan timeout, Int32 lookback, ExpectAction[] expectActions)
   at <our shell wrapper>.ExecuteCommandAsync(String command)

Reproduced across several runs and several hosts, against Junos network devices, reading a large
configuration and then issuing a second command on the same ShellStream.

Note this is a Release-only symptom. Under Debug the assert fires instead, so a test run would
report the over-discard at its origin rather than as a later crash.

Minimal repro of the arithmetic (no SSH server needed)

The defect reduces to this identity. With the default Encoding.UTF8:

foreach (var bytes in new[]
         {
             new byte[] { 0x61, 0xF8, 0x62 },       // 'a', a lone 0xF8, 'b'
             new byte[] { 0x61, 0xE2, 0x82, 0x62 }, // a truncated 3-byte sequence
             new byte[] { 0x61, 0xFF, 0xFF, 0x62 },
             new byte[] { 0x61, 0xC3, 0xB8, 0x62 }, // valid UTF-8, for contrast
         })
{
    var text = Encoding.UTF8.GetString(bytes);
    Console.WriteLine($"in={bytes.Length} out={Encoding.UTF8.GetByteCount(text)}");
}

in=3 out=5     <- Discard() is asked for 2 bytes more than the match occupied
in=4 out=5     <- 1 byte more
in=4 out=8     <- 4 bytes more
in=4 out=4     <- lossless decode, correct

Every case where the two disagree drives ActiveLength negative by the difference.

ExpectRegex is the only one of the ten Discard(...) call sites in ShellStream that computes its
count by re-encoding; the others count in byte space (ActiveLength, indexOf…, bytesRead) and are
not affected.

Suggested direction

The number of bytes to discard is a fact about the input bytes, so re-encoding the output string
cannot recover it. Options, roughly by intrusiveness:

  • Decode with a Decoder and track how many input bytes produced the consumed prefix.
  • Keep a char-index → byte-offset mapping while building bufferText.
  • Independently, consider making ArrayBuffer.Discard clamp or throw rather than Debug.Assert, so an
    over-discard surfaces where it happens instead of as an ArgumentOutOfRangeException one call later.

Happy to test a patch — we have hosts that reproduce it reliably.

Environment
  • SSH.NET 2026.0.0. The two lines above are byte-identical in 2025.1.0, 2026.0.0 and develop.
  • .NET 10, Release build, Linux container.
主要言語
C#
スター
4.4k
フォーク
994
30日以内にマージされた PR はありません

環境構築

はじめの一歩

  1. issue を最後まで読み、次にプロジェクトのコントリビューションガイドを読みます。
  2. 着手することを issue にコメントします — 二人が同じ作業をするのを防げます。
  3. リポジトリをフォークし、ブランチを切って変更します。
  4. issue 番号を参照したプルリクエストを送ります。

sshnet/SSH.NET のほかの issue

sshnet/SSH.NET の issue をすべて見る

似ている issue

C# の issue をもっと見る

新しい issue をメールで受け取る

初心者向けの GitHub issue を短くまとめたダイジェスト。