ptrace.c

#include <stdlib.h>
#include <stdio.h>
#include <stdarg.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <stdbool.h>
#include <limits.h>
#include <signal.h>

#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/wait.h>

#include "compiler.h"
#include "asm/types.h"
#include "util.h"
#include "ptrace.h"
#include "proc_parse.h"
#include "crtools.h"
#include "seccomp.h"

int unseize_task(pid_t pid, int orig_st, int st)
{
	pr_debug("\tUnseizing %d into %d\n", pid, st);

	if (st == TASK_DEAD) {
		kill(pid, SIGKILL);
		return 0;
	} else if (st == TASK_STOPPED) {
		/*
		 * Task might have had STOP in queue. We detected such
		 * guy as TASK_STOPPED, but cleared signal to run the
		 * parasite code. hus after detach the task will become
		 * running. That said -- STOP everyone regardless of
		 * the initial state.
		 */
		kill(pid, SIGSTOP);
	} else if (st == TASK_ALIVE) {
		/*
		 * Same as in the comment above -- there might be a
		 * task with STOP in queue that would get lost after
		 * detach, so stop it again.
		 */
		if (orig_st == TASK_STOPPED)
			kill(pid, SIGSTOP);
	} else
		pr_err("Unknown final state %d\n", st);

	if (ptrace(PTRACE_DETACH, pid, NULL, NULL)) {
		pr_perror("Unable to detach from %d", pid);
		return -1;
	}

	return 0;
}

int suspend_seccomp(pid_t pid)
{
	if (ptrace(PTRACE_SETOPTIONS, pid, NULL, PTRACE_O_SUSPEND_SECCOMP) < 0) {
		pr_perror("suspending seccomp failed");
		return -1;
	}

	return 0;
}

int seize_catch_task(pid_t pid)
{
	int ret;

	ret = ptrace(PTRACE_SEIZE, pid, NULL, 0);
	if (ret) {
		/*
		 * ptrace API doesn't allow to distinguish
		 * attaching to zombie from other errors.
		 * All errors will be handled in seize_wait_task().
		 */
		pr_warn("Unable to interrupt task: %d (%s)\n", pid, strerror(errno));
		return ret;
	}

	/*
	 * If we SEIZE-d the task stop it before going
	 * and reading its stat from proc. Otherwise task
	 * may die _while_ we're doing it and we'll have
	 * inconsistent seize/state pair.
	 *
	 * If task dies after we seize it but before we
	 * do this interrupt, we'll notice it via proc.
	 */
	ret = ptrace(PTRACE_INTERRUPT, pid, NULL, NULL);
	if (ret < 0) {
		pr_warn("SEIZE %d: can't interrupt task: %s", pid, strerror(errno));
		if (ptrace(PTRACE_DETACH, pid, NULL, NULL))
			pr_perror("Unable to detach from %d", pid);
	}

	return ret;
}

static int skip_sigstop(int pid, int nr_signals)
{
	int i, status, ret;

	/*
	 * 1) SIGSTOP is queued, but isn't handled yet:
	 * SGISTOP can't be blocked, so we need to wait when the kernel
	 * handles this signal.
	 *
	 * Otherwise the process will be stopped immediatly after
	 * starting it.
	 *
	 * 2) A seized task was stopped:
	 * PTRACE_SEIZE doesn't affect signal or group stop state.
	 * Currently ptrace reported that task is in stopped state.
	 * We need to start task again, and it will be trapped
	 * immediately, because we sent PTRACE_INTERRUPT to it.
	 */
	for (i = 0; i < nr_signals; i++) {
		ret = ptrace(PTRACE_CONT, pid, 0, 0);
		if (ret) {
			pr_perror("Unable to start process");
			return -1;
		}

		ret = wait4(pid, &status, __WALL, NULL);
		if (ret < 0) {
			pr_perror("SEIZE %d: can't wait task", pid);
			return -1;
		}

		if (!WIFSTOPPED(status)) {
			pr_err("SEIZE %d: task not stopped after seize\n", pid);
			return -1;
		}
	}
	return 0;
}

/*
 * This routine seizes task putting it into a special
 * state where we can manipulate the task via ptrace
 * interface, and finally we can detach ptrace out of
 * of it so the task would not know if it was saddled
 * up with someone else.
 */
int seize_wait_task(pid_t pid, pid_t ppid, struct proc_status_creds **creds)
{
	siginfo_t si;
	int status, nr_sigstop;
	int ret = 0, ret2, wait_errno = 0;
	struct proc_status_creds cr;

	/*
	 * For the comparison below, let's zero out any padding.
	 */
	memzero(&cr, sizeof(struct proc_status_creds));

	/*
	 * It's ugly, but the ptrace API doesn't allow to distinguish
	 * attaching to zombie from other errors. Thus we have to parse
	 * the target's /proc/pid/stat. Sad, but parse whatever else
	 * we might need at that early point.
	 */

	processes_to_wait--;
try_again:

	ret = wait4(pid, &status, __WALL, NULL);
	if (ret < 0) {
		/*
		 * wait4() can expectedly fail only in a first time
		 * if a task is zombie. If we are here from try_again,
		 * this means that we are tracing this task.
		 *
		 * processes_to_wait should be descrimented only once in this
		 * function if a first wait was success.
		 */
		processes_to_wait++;
		wait_errno = errno;
	}

	ret2 = parse_pid_status(pid, &cr);
	if (ret2)
		goto err;

	if (ret < 0 || WIFEXITED(status) || WIFSIGNALED(status)) {
		if (cr.state != 'Z') {
			if (pid == getpid())
				pr_err("The criu itself is within dumped tree.\n");
			else
				pr_err("Unseizable non-zombie %d found, state %c, err %d/%d\n",
						pid, cr.state, ret, wait_errno);
			return -1;
		}

		return TASK_DEAD;
	}

	if ((ppid != -1) && (cr.ppid != ppid)) {
		pr_err("Task pid reused while suspending (%d: %d -> %d)\n",
				pid, ppid, cr.ppid);
		goto err;
	}

	if (!WIFSTOPPED(status)) {
		pr_err("SEIZE %d: task not stopped after seize\n", pid);
		goto err;
	}

	ret = ptrace(PTRACE_GETSIGINFO, pid, NULL, &si);
	if (ret < 0) {
		pr_perror("SEIZE %d: can't read signfo", pid);
		goto err;
	}

	if (SI_EVENT(si.si_code) != PTRACE_EVENT_STOP) {
		/*
		 * Kernel notifies us about the task being seized received some
		 * event other than the STOP, i.e. -- a signal. Let the task
		 * handle one and repeat.
		 */

		if (ptrace(PTRACE_CONT, pid, NULL,
					(void *)(unsigned long)si.si_signo)) {
			pr_perror("Can't continue signal handling, aborting");
			goto err;
		}

		ret = 0;
		goto try_again;
	}

	if (*creds == NULL) {
		*creds = xzalloc(sizeof(struct proc_status_creds));
		if (!*creds)
			goto err;

		**creds = cr;

	} else if (!proc_status_creds_dumpable(*creds, &cr)) {
		pr_err("creds don't match %d %d\n", pid, ppid);
		goto err;
	}

	if (cr.seccomp_mode != SECCOMP_MODE_DISABLED && suspend_seccomp(pid) < 0)
		goto err;

	nr_sigstop = 0;
	if (cr.sigpnd & (1 << (SIGSTOP - 1)))
		nr_sigstop++;
	if (cr.shdpnd & (1 << (SIGSTOP - 1)))
		nr_sigstop++;
	if (si.si_signo == SIGSTOP)
		nr_sigstop++;

	if (nr_sigstop) {
		if (skip_sigstop(pid, nr_sigstop))
			goto err_stop;

		return TASK_STOPPED;
	}

	if (si.si_signo == SIGTRAP)
		return TASK_ALIVE;
	else {
		pr_err("SEIZE %d: unsupported stop signal %d\n", pid, si.si_signo);
		goto err;
	}

err_stop:
	kill(pid, SIGSTOP);
err:
	if (ptrace(PTRACE_DETACH, pid, NULL, NULL))
		pr_perror("Unable to detach from %d", pid);
	return -1;
}

int ptrace_peek_area(pid_t pid, void *dst, void *addr, long bytes)
{
	unsigned long w;
	if (bytes & (sizeof(long) - 1))
		return -1;
	for (w = 0; w < bytes / sizeof(long); w++) {
		unsigned long *d = dst, *a = addr;
		d[w] = ptrace(PTRACE_PEEKDATA, pid, a + w, NULL);
		if (d[w] == -1U && errno)
			goto err;
	}
	return 0;
err:
	return -2;
}

int ptrace_poke_area(pid_t pid, void *src, void *addr, long bytes)
{
	unsigned long w;
	if (bytes & (sizeof(long) - 1))
		return -1;
	for (w = 0; w < bytes / sizeof(long); w++) {
		unsigned long *s = src, *a = addr;
		if (ptrace(PTRACE_POKEDATA, pid, a + w, s[w]))
			goto err;
	}
	return 0;
err:
	return -2;
}

/* don't swap big space, it might overflow the stack */
int ptrace_swap_area(pid_t pid, void *dst, void *src, long bytes)
{
	void *t = alloca(bytes);

	if (ptrace_peek_area(pid, t, dst, bytes))
		return -1;

	if (ptrace_poke_area(pid, src, dst, bytes)) {
		if (ptrace_poke_area(pid, t, dst, bytes))
			return -2;
		return -1;
	}

	memcpy(src, t, bytes);

	return 0;
}