docker-exec-sigproxy.c

// The problem is that 'docker exec' does not proxy signals to the container.
// 'docker run' does (--sig-proxy=true is the default)
//
// The consequence of this is that any SIGHUP/SIGINT/SIGTERM to docker-cli
// will keep the process inside the container running.
//
// Details:
// - https://github.com/moby/moby/issues/9098#issuecomment-702524998
// - https://gist.github.com/SkyperTHC/cb4ebb633890ac36ad86e80c6c7a9bb2
//
// This tool solves this problem and proxies the signal.
//
// gcc -Wall -O2 -o docker-exec-sigproxy docker-exec-sigproxy.c
// Example:
//    ./docker-exec-sigproxy exec -it alpine
// Any signal that is send to 'docker-exec-sigproxy' is forwarded ash.
//
// This problem kicked our butts at thc.org/segfault. We spawn user shells using
// 'docker exec'. When sshd detects a network error it sends a SIGHUP to 'docker exec'.
// Docker does not forward the signal to the started container (to 'ash'). 
// Thus we ended up with hundreds of stale 'ash -il' shells that were not
// connected to any sshd.
//
// This hack solves the problem by intercepting the traffic between the docker sockets,
// extracting container id and exec-id and then hocking all signals and forwarding them
// correctly to the container.
//
// Known Problems:
// - docker-exec-sigproxy exits with 0 even if the container's process
//   exits with a different error code. See 'FIXME'
//
// The PID tree
// SSHD +--> docker-exec-sigproxy +--> 'docker exec'
// The STDIN/OUT data flow:
// SSHD -[stdin/out]-> 'docker exec' -[unix-socket]-> docker-exec-sigproxy -[unix-socket]-> dockerd
//
// Segfault specific:
// In our setup the docker-cli is executed by sshd from within another
// docker container. Thus we need to 'break out' of that container to get access
// to the host's pid system and to find out the host-pid of the ash process.
// If you do not do this then comment out "SIGPROXY_INSIDE_CONTAINER"....

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/un.h>
#include <sys/epoll.h>
#include <signal.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <termios.h>

#define DFL_DOCKER_SOCK   "/var/run/docker.sock"
#define DFL_PROXY_SOCK    "/dev/shm/dproxy-%d.sock"
#define DFL_CONTAINER_DIR "/var/run/containerd/io.containerd.runtime.v2.task/moby"
// Comment this line if 'docker-exec-sigproxy exec ...' is run from the HOST 
#define SIGPROXY_INSIDE_CONTAINER   (1)

// #define DEBUG (1)
FILE *dout;
#ifdef DEBUG
# define DEBUGF(a...) do{fprintf(dout, "\033[0;33mDEBUG\033[0m %s:%d ", __func__, __LINE__); fprintf(dout, a); fflush(dout);}while(0)
# define HEXDUMP(a, _len)        do { \
        size_t _n = 0; \
        fprintf(dout, "%s:%d HEX[%zd] ", __FILE__, __LINE__, _len); \
        while (_n < (_len)) fprintf(dout, "%2.2x", ((unsigned char *)a)[_n++]); \
        fprintf(dout, "\n"); \
} while (0)
#else
# define DEBUGF(a...) do{}while(0)
# define HEXDUMP(a, len) do{}while(0)
#endif

static pid_t pid;
static pid_t mypid;
static int lsox;
static int efd;
static struct sockaddr_un addr;
static char *container_id;
static char *exec_id;
int exit_code;
static char sock_path[1024];
static void sig_forward(int sig);

static void
docker_exec(int argc, char *argv[])
{
	pid = fork();

	if (pid != 0)
	{
		// HERE: Parent. sigproxy
		// STDIN/OUT/ERR are directly connected to docker-cli (not sigproxy)
		close(0);
		close(1);
		close(2);
		// Neither child nor parent needs to become session leader (setsid).
		// This way SIGHUP/SIGPIPE will be send to both and I dont need to
		// kill the original 'docker exec' (our child).
		return;
	}

	// HERE: Child.
	// This real 'docker-cli' (my child) is on PROXY_SOCK.
	// The 'real docker-cli' receives STDIN/STDOUT (from SSHD) and pipes them into
	// PROXY_SOCK where we pick it up and forward it to real DOCKER_SOCK.
	char buf[1024];
	snprintf(buf, sizeof buf, "DOCKER_HOST=unix://%.900s", sock_path /* PROXY_SOCK */);
	putenv(buf);
	argv[0] = "docker";
	execvp(argv[0], argv);
	exit(255);
}

#define FL_PEER_CLI         (0x01)
#define FL_PEER_DOCKERD     (0x02)
#define FL_PEER_SHUT_WR     (0x04)
struct _peer
{
	int fd;
	int flags;
	void *buddy;
};

// Add an event to wake if there is something to read on fd.
static void
ev_peer_add(struct _peer *p, int fd, int flags)
{
	p->fd = fd;
	p->flags = flags;
	struct epoll_event ev;

	memset(&ev, 0, sizeof ev);
	ev.events = EPOLLIN;
	ev.data.ptr = p;
	epoll_ctl(efd, EPOLL_CTL_ADD, fd, &ev);
}

static int n_peers;
// Relay between these two peers.
static void
buddy_up(int in /* lsox accept */, int out)
{
	struct _peer *p = malloc(sizeof *p);
	struct _peer *buddy;

	DEBUGF("NEW CLI=%d, DOCKERD=%d\n", in, out);
	ev_peer_add(p, in, FL_PEER_CLI); // Real 'docker exec'

	p->buddy = malloc(sizeof *p);
	buddy = p->buddy;
	buddy->buddy = p;

	n_peers++;
	ev_peer_add(buddy, out, FL_PEER_DOCKERD);
}

static void
peer_del(struct _peer *p)
{
	struct _peer *buddy = (struct _peer *)p->buddy;

	epoll_ctl(efd, EPOLL_CTL_DEL, p->fd, NULL);
	shutdown(buddy->fd, SHUT_WR);
	buddy->flags |= FL_PEER_SHUT_WR;
	if ((p->flags & FL_PEER_SHUT_WR) && (buddy->flags & FL_PEER_SHUT_WR))
	{
		n_peers--;
		close(p->fd);
		free(p);
		close(buddy->fd);
		free(buddy);
		return;
	}
}

// New incoming connection.
// 1. Connect to real socket
// 2. Relay traffic between the two sockets.
static void
ev_accept(void)
{
	int in;
	int out;

	in = accept(lsox, NULL, NULL);
	if (in < 0)
		exit(255);

	signal(SIGCHLD, SIG_IGN);

	// Connect to original socket.
	out = socket(AF_UNIX, SOCK_STREAM| SOCK_CLOEXEC, 0);
	if (connect(out, (struct sockaddr *)&addr, sizeof addr) != 0)
		exit(254);

	buddy_up(in, out);
}

// Dirty: First ID we get is container. Second is exec_id.
static void
parse(char *buf, ssize_t sz)
{
	char *ptr;
	char *next;

	ptr = strstr(buf, "{\"Id\":\"");
	if (ptr != NULL)
	{
		ptr += 7;

		if (exec_id != NULL)
			return;

		next = strchr(ptr, '"');
		if (next == NULL)
			return;
		*next = '\0';

		if (container_id == NULL)
			container_id = strdup(ptr);
		else if (exec_id == NULL)
			exec_id = strdup(ptr);

		return;
	}

	ptr = strstr(buf, "{\"ID\":\"");
	if (ptr == NULL)
		return;
	ptr += 7;	

	ptr = strstr(ptr, "\"ExitCode\":");
	if (ptr == NULL)
		return;
	ptr += 11;

	exit_code = atoi(ptr);
}

static int
dispatch(struct epoll_event *evs, int count)
{
	struct epoll_event *e;
	int n;
	ssize_t sz;
	char buf[4*4096];

	// Relay data between both sockets.
	for (n = 0; n < count; n++)
	{
		e = &evs[n];
		if (e->data.fd == lsox)
		{
			ev_accept();
			continue;
		}

		if (e->events & EPOLLIN)
		{
			struct _peer *p = e->data.ptr;
			struct _peer *buddy = p->buddy;
			sz = read(p->fd, buf, sizeof buf - 1);
			if (sz <= 0)
			{
				// snprintf(buf, sizeof buf, "/dev/null-flag-is-%d-fd-%d", p->flags, p->fd);
				// struct stat sb; stat(buf, &sb);
				peer_del(p);
				if (n_peers > 0)
					continue;

				if (p->flags & FL_PEER_DOCKERD)
					return -1; // dockerd closed?
				return 200; // real 'docker exec' closed.
			}
			buf[sz] = '\0';
			if (write(buddy->fd, buf, sz) != sz)
				return -1;
			if ((exit_code < 0) && (p->flags & FL_PEER_DOCKERD))
				parse(buf, sz);
		}
	}

	return 0;
}


static void
cb_signal(int sig)
{
	DEBUGF("[%d] SIGNAL %d\n", mypid, sig);
	if (sig == SIGCHLD)
	{
		// This will only happen if the real 'docker exec'
		// dies before connecting to PROXY SOCK. Normally we will detect
		// when the child dies by reading on the PROXY_SOCK (EOF).
		exit(255);
	}

	// Forward signal to container/pid
	sig_forward(sig);
}

static void
sig_forward(int sig)
{
	char cmd[4096];

#ifdef SIGPROXY_INSIDE_CONTAINER
	// NOTE: This docker-cli is inside a docker already. Thus we need to break out:
	snprintf(cmd, sizeof cmd, "docker run --rm --pid=host -v "DFL_CONTAINER_DIR"/%s/%s.pid:/pid alpine sh -c '[ -f /pid ] && kill -%d $(cat /pid)'", container_id, exec_id, sig);
#else
	snprintf(cmd, sizeof cmd, "kill -%d $(cat "DFL_CONTAINER_DIR"/%s/%s.pid)", sig, container_id, exec_id);
#endif
	// int ret;
	system(cmd);
}

static struct termios tios;
static int tios_error = -1;

static void
do_exit()
{
	unlink(sock_path);
	if (tios_error == 0)
		tcsetattr(STDIN_FILENO, TCSADRAIN, &tios);
}

int
main(int argc, char *argv[])
{
	mypid = getpid();
#ifdef DEBUG
	char *ptr = getenv("SF_LOG");
	if (ptr != NULL)
		dout = fopen(ptr, "w");
	if (dout == NULL)
		dout = stderr;
#endif

	exit_code = -1;
	tios_error = tcgetattr(STDIN_FILENO, &tios);

	signal(SIGHUP, cb_signal);
	signal(SIGINT, cb_signal);
	signal(SIGQUIT, cb_signal);
	signal(SIGUSR1, cb_signal);
	signal(SIGUSR2, cb_signal);
	signal(SIGPIPE, cb_signal);
	signal(SIGTERM, cb_signal);
	signal(SIGURG, cb_signal);
	signal(SIGWINCH, cb_signal);

	atexit(do_exit);
	// Create listening socket
	lsox = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);

	addr.sun_family = AF_UNIX;
	snprintf(sock_path, sizeof sock_path, DFL_PROXY_SOCK, getpid());
	snprintf(addr.sun_path, sizeof addr.sun_path, "%.107s", sock_path);

	bind(lsox, (struct sockaddr *)&addr, sizeof addr);
	listen(lsox, 5);
	snprintf(addr.sun_path, sizeof addr.sun_path, DFL_DOCKER_SOCK);

	// Start the original docker client
	signal(SIGCHLD, cb_signal);
	docker_exec(argc, argv);

	efd = epoll_create1(EPOLL_CLOEXEC);

	// Event for the listening socket...
	struct epoll_event ev;
	ev.events = EPOLLIN;
	ev.data.fd = lsox;
	epoll_ctl(efd, EPOLL_CTL_ADD, lsox, &ev);

	int nfds;
	int ret = 0;
	struct epoll_event events[4];
	for (;;)
	{
		nfds = epoll_wait(efd, events, 4, -1);
		if (nfds < 0)
		{
			if (errno == EINTR)
				continue;
			break;
		}
		ret = dispatch(events, nfds);
		if (ret != 0)
			break;
	}

	if (ret > 0)
	{
		// HERE: Real 'docker exec' closed connection.
		// -? Received SIGHUP from SSHD?
		sig_forward(SIGHUP);
		exit(209);
	}
	if (ret < 0)
	{
		// HERE: dockerd closed connection (kernel side).
		// Did container/pid terminate?
		// Very rarely does docker-cli call exit(255) without any particular reason. Thus we
		// can not assume that shell exited but must forward signal.
		sig_forward(SIGHUP);
		exit(exit_code);
	}

	// HERE: error in epoll_wait()
	sig_forward(SIGHUP);
	exit(255);
}