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ssh/tailssh: add spec citations and source references to comments

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Add RFC and source references to the comments introduced in the
previous commit:

- SIGHUP: cite OpenSSH session.c:2246 (PTY master close) and POSIX
  General Terminal Interface for terminal disconnect semantics
- Exit codes: cite OpenSSH ssh.c:1693 for the 255 convention, POSIX
  Shell Command Language for the 127 convention
- Exit/Close ordering: cite RFC 4254 Section 6.10 (exit-status before
  CHANNEL_CLOSE) and Section 5.3 (EOF semantics)
- defer ss.Close: explain why Close is deferred and cite RFC 4254

No behavioral changes, only documentation.

Updates #18256

Change-Id: Ice00d19d0462f1d4e6d454df0079c80b3917f5df
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
This commit is contained in:
Kristoffer Dalby 2026-04-08 11:59:52 +00:00
parent 4fd5bcc2b8
commit d8abcce89b
1 changed files with 29 additions and 6 deletions
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35
ssh/tailssh/tailssh.go
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@ -878,7 +878,12 @@ func (ss *sshSession) killProcessOnContextDone() {
// We don't need to Process.Wait here, sshSession.run() does
// the waiting regardless of termination reason.
// Send SIGHUP like a real terminal disconnect would.
// Send SIGHUP to match POSIX terminal disconnect semantics.
// OpenSSH achieves this implicitly by closing the PTY master fd
// (see openssh-portable session.c:2246), which causes the kernel
// to send SIGHUP to the child process group. Since tailssh uses
// pipes for non-PTY sessions, we send SIGHUP explicitly.
// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap11.html#tag_11_01_03
ss.cmd.Process.Signal(syscall.SIGHUP)
})
}
@ -974,10 +979,18 @@ func (ss *sshSession) run() {
metricActiveSessions.Add(1)
defer metricActiveSessions.Add(-1)
defer ss.cancelCtx(errSessionDone)
// Close the channel after all I/O and exit-status have been sent.
// Per RFC 4254 Section 6.10, SSH_MSG_CHANNEL_CLOSE must come after
// exit-status. Since Exit() no longer calls Close() (to allow the
// caller to control ordering), we close explicitly here.
// https://datatracker.ietf.org/doc/html/rfc4254#section-6.10
defer ss.Close()
if attached := ss.conn.srv.attachSessionToConnIfNotShutdown(ss); !attached {
fmt.Fprintf(ss, "Tailscale SSH is shutting down\r\n")
// Exit code 255 matches OpenSSH's convention for SSH protocol/infrastructure
// errors that prevent the session from starting.
// https://github.com/openssh/openssh-portable/blob/master/ssh.c#L1693
ss.Exit(255)
return
}
@ -1000,8 +1013,9 @@ func (ss *sshSession) run() {
if lu.Uid != fmt.Sprint(euid) {
ss.logf("can't switch to user %q from process euid %v", lu.Username, euid)
fmt.Fprintf(ss, "can't switch user\r\n")
// Exit code 255 indicates SSH protocol/permission error,
// matching OpenSSH behavior for fatal errors.
// Exit code 255 matches OpenSSH's convention for SSH permission errors
// that prevent the session from starting.
// https://github.com/openssh/openssh-portable/blob/master/ssh.c#L1693
ss.Exit(255)
return
}
@ -1054,7 +1068,8 @@ func (ss *sshSession) run() {
}
}
} else if isNotFoundOrExecutable(err) {
// Use exit code 127 for "command not found" per shell convention.
// Exit code 127 for "command not found" per POSIX shell convention.
// https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_08_02
exitCode = 127
}
ss.Exit(exitCode)
@ -1083,6 +1098,10 @@ func (ss *sshSession) run() {
if _, err := io.Copy(rec.writer("o", ss), ss.rdStdout); err != nil && !errors.Is(err, io.EOF) {
logf("stdout copy: %v", err)
}
// Send EOF to the SSH client when stdout copying completes.
// Per RFC 4254 Section 5.3, SSH_MSG_CHANNEL_EOF signals that no
// more data will be sent. The channel remains open for exit-status.
// https://datatracker.ietf.org/doc/html/rfc4254#section-5.3
ss.CloseWrite()
}()
@ -1125,8 +1144,12 @@ func (ss *sshSession) run() {
exitCode = 1
}
// Send exit-status before EOF/Close. Per RFC 4254 section 6.10,
// exit-status should be sent before channel close.
// Send exit-status before EOF/Close. Per RFC 4254 Section 6.10,
// exit-status must be sent before SSH_MSG_CHANNEL_CLOSE. By sending
// it here before closing the child pipes (which triggers the stdout
// goroutine to send EOF via CloseWrite), we guarantee the ordering:
// exit-status -> EOF -> CHANNEL_CLOSE
// https://datatracker.ietf.org/doc/html/rfc4254#section-6.10
ss.Exit(exitCode)
// Close process-side of pipes to signal io.Copy goroutines