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draft: prevent LocalCommand 'started' leak on stale SSH master socket #569

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draft: prevent LocalCommand 'started' leak on stale SSH master socket #569

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38 changes: 33 additions & 5 deletions src/libstore/ssh.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,11 @@
#include "nix/util/exec.hh"
#include "nix/util/base-n.hh"

#ifndef _WIN32
# include <sys/wait.h>
# include <cerrno>
#endif

namespace nix {

static std::string parsePublicHostKey(std::string_view host, std::string_view sshPublicHostKey)
Expand Down Expand Up @@ -191,6 +196,16 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(OsStrings && comm
if (verbosity >= lvlChatty)
args.push_back("-v");
args.splice(args.end(), std::move(extraSshArgs));
// Override LocalCommand to no-op on command SSHs; master
// already consumed "started". On fallback, "echo started"
// would leak into the nix protocol stream. #441
if (useMaster) {
for (auto & arg : args) {
if (arg.starts_with(OS_STR("-oLocalCommand="))) {
arg = OS_STR("-oLocalCommand=true");
}
}
}
args.push_back("--");
}

Expand All @@ -206,9 +221,9 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(OsStrings && comm
in.readSide = INVALID_DESCRIPTOR;
out.writeSide = INVALID_DESCRIPTOR;

// Wait for the SSH connection to be established,
// So that we don't overwrite the password prompt with our progress bar.
if (!fakeSSH && !(socketPath && isMasterRunning(*socketPath))) {
// Skip readLine when useMaster: SSH connects via master socket, or
// falls back to direct connection where LocalCommand is now a no-op.
if (!fakeSSH && !useMaster && !(socketPath && isMasterRunning(*socketPath))) {
std::string reply;
try {
reply = readLine(out.readSide.get());
Expand Down Expand Up @@ -237,8 +252,21 @@ std::optional<std::filesystem::path> SSHMaster::startMaster()

auto state(state_.lock());

if (state->sshMaster != INVALID_DESCRIPTOR)
return state->socketPath;
if (state->sshMaster != INVALID_DESCRIPTOR) {
// Check if master is still alive before returning cached socket.
pid_t result = waitpid(state->sshMaster, nullptr, WNOHANG);
if (result == 0)
return state->socketPath;
if (result == -1) {
if (errno == EINTR)
return state->socketPath; // assume alive; worst case SSH falls back
if (errno != ECHILD)
warn("waitpid failed for SSH master %d: %s",
(pid_t) state->sshMaster, strerror(errno));
}
// Master gone — release Pid to avoid kill()/wait() on already-reaped child.
state->sshMaster.release();
}

state->socketPath = tmpDir->path() / "ssh.sock";

Expand Down