arch/um/kernel/process.c - linux - Git at Google

 /*
  * Copyright (C) 2000, 2001, 2002 Jeff Dike (jdike@karaya.com)
  * Licensed under the GPL
  */

 #include <stdio.h>
 #include <unistd.h>
 #include <signal.h>
 #include <sched.h>
 #include <errno.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <setjmp.h>
 #include <sys/time.h>
 #include <sys/wait.h>
 #include <sys/mman.h>
 #include <asm/unistd.h>
 #include <asm/page.h>
 #include "user_util.h"
 #include "kern_util.h"
 #include "user.h"
 #include "process.h"
 #include "signal_kern.h"
 #include "signal_user.h"
 #include "sysdep/ptrace.h"
 #include "sysdep/sigcontext.h"
 #include "irq_user.h"
 #include "ptrace_user.h"
 #include "time_user.h"
 #include "init.h"
 #include "os.h"
 #include "uml-config.h"
 #include "choose-mode.h"
 #include "mode.h"
 #include "tempfile.h"
 #ifdef UML_CONFIG_MODE_SKAS
 #include "skas.h"
 #include "skas_ptrace.h"
 #include "registers.h"
 #endif

 void init_new_thread_stack(void *sig_stack, void (*usr1_handler)(int))
 {
 	int flags = 0, pages;

 	if(sig_stack != NULL){
 		pages = (1 << UML_CONFIG_KERNEL_STACK_ORDER);
 		set_sigstack(sig_stack, pages * page_size());
 		flags = SA_ONSTACK;
 	}
 	if(usr1_handler) set_handler(SIGUSR1, usr1_handler, flags, -1);
 }

 void init_new_thread_signals(int altstack)
 {
 	int flags = altstack ? SA_ONSTACK : 0;

 	set_handler(SIGSEGV, (__sighandler_t) sig_handler, flags,
 		    SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
 	set_handler(SIGTRAP, (__sighandler_t) sig_handler, flags,
 		    SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
 	set_handler(SIGFPE, (__sighandler_t) sig_handler, flags,
 		    SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
 	set_handler(SIGILL, (__sighandler_t) sig_handler, flags,
 		    SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
 	set_handler(SIGBUS, (__sighandler_t) sig_handler, flags,
 		    SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
 	set_handler(SIGUSR2, (__sighandler_t) sig_handler,
 		    flags, SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
 	signal(SIGHUP, SIG_IGN);

 	init_irq_signals(altstack);
 }

 struct tramp {
 	int (*tramp)(void *);
 	void *tramp_data;
 	unsigned long temp_stack;
 	int flags;
 	int pid;
 };

 /* See above for why sigkill is here */

 int sigkill = SIGKILL;

 int outer_tramp(void *arg)
 {
 	struct tramp *t;
 	int sig = sigkill;

 	t = arg;
 	t->pid = clone(t->tramp, (void *) t->temp_stack + page_size()/2,
 		       t->flags, t->tramp_data);
 	if(t->pid > 0) wait_for_stop(t->pid, SIGSTOP, PTRACE_CONT, NULL);
 	kill(os_getpid(), sig);
 	_exit(0);
 }

 int start_fork_tramp(void *thread_arg, unsigned long temp_stack,
 		     int clone_flags, int (*tramp)(void *))
 {
 	struct tramp arg;
 	unsigned long sp;
 	int new_pid, status, err;

 	/* The trampoline will run on the temporary stack */
 	sp = stack_sp(temp_stack);

 	clone_flags |= CLONE_FILES | SIGCHLD;

 	arg.tramp = tramp;
 	arg.tramp_data = thread_arg;
 	arg.temp_stack = temp_stack;
 	arg.flags = clone_flags;

 	/* Start the process and wait for it to kill itself */
 	new_pid = clone(outer_tramp, (void *) sp, clone_flags, &arg);
 	if(new_pid < 0)
 		return(new_pid);

 	CATCH_EINTR(err = waitpid(new_pid, &status, 0));
 	if(err < 0)
 		panic("Waiting for outer trampoline failed - errno = %d",
 		      errno);

 	if(!WIFSIGNALED(status) || (WTERMSIG(status) != SIGKILL))
 		panic("outer trampoline didn't exit with SIGKILL, "
 		      "status = %d", status);

 	return(arg.pid);
 }

 static int ptrace_child(void)
 {
 	int ret;
 	int pid = os_getpid(), ppid = getppid();
 	int sc_result;

 	if(ptrace(PTRACE_TRACEME, 0, 0, 0) < 0){
 		perror("ptrace");
 		os_kill_process(pid, 0);
 	}
 	os_stop_process(pid);

 	/*This syscall will be intercepted by the parent. Don't call more than
 	 * once, please.*/
 	sc_result = os_getpid();

 	if (sc_result == pid)
 		ret = 1; /*Nothing modified by the parent, we are running
 			   normally.*/
 	else if (sc_result == ppid)
 		ret = 0; /*Expected in check_ptrace and check_sysemu when they
 			   succeed in modifying the stack frame*/
 	else
 		ret = 2; /*Serious trouble! This could be caused by a bug in
 			   host 2.6 SKAS3/2.6 patch before release -V6, together
 			   with a bug in the UML code itself.*/
 	_exit(ret);
 }

 static int start_ptraced_child(void)
 {
 	int pid, n, status;

 	pid = fork();
 	if(pid == 0)
 		ptrace_child();

 	if(pid < 0)
 		panic("check_ptrace : fork failed, errno = %d", errno);
 	CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
 	if(n < 0)
 		panic("check_ptrace : wait failed, errno = %d", errno);
 	if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGSTOP))
 		panic("check_ptrace : expected SIGSTOP, got status = %d",
 		      status);

 	return(pid);
 }

 /* When testing for SYSEMU support, if it is one of the broken versions, we must
  * just avoid using sysemu, not panic, but only if SYSEMU features are broken.
  * So only for SYSEMU features we test mustpanic, while normal host features
  * must work anyway!*/
 static int stop_ptraced_child(int pid, int exitcode, int mustexit)
 {
 	int status, n, ret = 0;

 	if(ptrace(PTRACE_CONT, pid, 0, 0) < 0)
 		panic("stop_ptraced_child : ptrace failed, errno = %d", errno);
 	CATCH_EINTR(n = waitpid(pid, &status, 0));
 	if(!WIFEXITED(status) || (WEXITSTATUS(status) != exitcode)) {
 		int exit_with = WEXITSTATUS(status);
 		if (exit_with == 2)
 			printk("check_ptrace : child exited with status 2. "
 			       "Serious trouble happening! Try updating your "
 			       "host skas patch!\nDisabling SYSEMU support.");
 		printk("check_ptrace : child exited with exitcode %d, while "
 		      "expecting %d; status 0x%x", exit_with,
 		      exitcode, status);
 		if (mustexit)
 			panic("\n");
 		else
 			printk("\n");
 		ret = -1;
 	}

 	return ret;
 }

 static int force_sysemu_disabled = 0;

 int ptrace_faultinfo = 1;
 int proc_mm = 1;

 static int __init skas0_cmd_param(char *str, int* add)
 {
 	ptrace_faultinfo = proc_mm = 0;
 	return 0;
 }

 static int __init nosysemu_cmd_param(char *str, int* add)
 {
 	force_sysemu_disabled = 1;
 	return 0;
 }

 __uml_setup("skas0", skas0_cmd_param,
 		"skas0\n"
 		"    Disables SKAS3 usage, so that SKAS0 is used, unless you \n"
 		"    specify mode=tt.\n\n");

 __uml_setup("nosysemu", nosysemu_cmd_param,
 		"nosysemu\n"
 		"    Turns off syscall emulation patch for ptrace (SYSEMU) on.\n"
 		"    SYSEMU is a performance-patch introduced by Laurent Vivier. It changes\n"
 		"    behaviour of ptrace() and helps reducing host context switch rate.\n"
 		"    To make it working, you need a kernel patch for your host, too.\n"
 		"    See http://perso.wanadoo.fr/laurent.vivier/UML/ for further information.\n\n");

 static void __init check_sysemu(void)
 {
 	int pid, syscall, n, status, count=0;

 	printk("Checking syscall emulation patch for ptrace...");
 	sysemu_supported = 0;
 	pid = start_ptraced_child();

 	if(ptrace(PTRACE_SYSEMU, pid, 0, 0) < 0)
 		goto fail;

 	CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
 	if (n < 0)
 		panic("check_sysemu : wait failed, errno = %d", errno);
 	if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
 		panic("check_sysemu : expected SIGTRAP, "
 		      "got status = %d", status);

 	n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET,
 		   os_getpid());
 	if(n < 0)
 		panic("check_sysemu : failed to modify system "
 		      "call return, errno = %d", errno);

 	if (stop_ptraced_child(pid, 0, 0) < 0)
 		goto fail_stopped;

 	sysemu_supported = 1;
 	printk("OK\n");
 	set_using_sysemu(!force_sysemu_disabled);

 	printk("Checking advanced syscall emulation patch for ptrace...");
 	pid = start_ptraced_child();
 	while(1){
 		count++;
 		if(ptrace(PTRACE_SYSEMU_SINGLESTEP, pid, 0, 0) < 0)
 			goto fail;
 		CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
 		if(n < 0)
 			panic("check_ptrace : wait failed, errno = %d", errno);
 		if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
 			panic("check_ptrace : expected (SIGTRAP|SYSCALL_TRAP), "
 			      "got status = %d", status);

 		syscall = ptrace(PTRACE_PEEKUSR, pid, PT_SYSCALL_NR_OFFSET,
 				 0);
 		if(syscall == __NR_getpid){
 			if (!count)
 				panic("check_ptrace : SYSEMU_SINGLESTEP doesn't singlestep");
 			n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET,
 				   os_getpid());
 			if(n < 0)
 				panic("check_sysemu : failed to modify system "
 				      "call return, errno = %d", errno);
 			break;
 		}
 	}
 	if (stop_ptraced_child(pid, 0, 0) < 0)
 		goto fail_stopped;

 	sysemu_supported = 2;
 	printk("OK\n");

 	if ( !force_sysemu_disabled )
 		set_using_sysemu(sysemu_supported);
 	return;

 fail:
 	stop_ptraced_child(pid, 1, 0);
 fail_stopped:
 	printk("missing\n");
 }

 void __init check_ptrace(void)
 {
 	int pid, syscall, n, status;

 	printk("Checking that ptrace can change system call numbers...");
 	pid = start_ptraced_child();

 	if (ptrace(PTRACE_OLDSETOPTIONS, pid, 0, (void *)PTRACE_O_TRACESYSGOOD) < 0)
 		panic("check_ptrace: PTRACE_SETOPTIONS failed, errno = %d", errno);

 	while(1){
 		if(ptrace(PTRACE_SYSCALL, pid, 0, 0) < 0)
 			panic("check_ptrace : ptrace failed, errno = %d",
 			      errno);
 		CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
 		if(n < 0)
 			panic("check_ptrace : wait failed, errno = %d", errno);
 		if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP + 0x80))
 			panic("check_ptrace : expected SIGTRAP + 0x80, "
 			      "got status = %d", status);

 		syscall = ptrace(PTRACE_PEEKUSR, pid, PT_SYSCALL_NR_OFFSET,
 				 0);
 		if(syscall == __NR_getpid){
 			n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_NR_OFFSET,
 				   __NR_getppid);
 			if(n < 0)
 				panic("check_ptrace : failed to modify system "
 				      "call, errno = %d", errno);
 			break;
 		}
 	}
 	stop_ptraced_child(pid, 0, 1);
 	printk("OK\n");
 	check_sysemu();
 }

 int run_kernel_thread(int (*fn)(void *), void *arg, void **jmp_ptr)
 {
 	sigjmp_buf buf;
 	int n;

 	*jmp_ptr = &buf;
 	n = sigsetjmp(buf, 1);
 	if(n != 0)
 		return(n);
 	(*fn)(arg);
 	return(0);
 }

 void forward_pending_sigio(int target)
 {
 	sigset_t sigs;

 	if(sigpending(&sigs))
 		panic("forward_pending_sigio : sigpending failed");
 	if(sigismember(&sigs, SIGIO))
 		kill(target, SIGIO);
 }

 extern void *__syscall_stub_start, __syscall_stub_end;

 #ifdef UML_CONFIG_MODE_SKAS

 static inline void check_skas3_ptrace_support(void)
 {
 	struct ptrace_faultinfo fi;
 	int pid, n;

 	printf("Checking for the skas3 patch in the host...");
 	pid = start_ptraced_child();

 	n = ptrace(PTRACE_FAULTINFO, pid, 0, &fi);
 	if (n < 0) {
 		ptrace_faultinfo = 0;
 		if(errno == EIO)
 			printf("not found\n");
 		else {
 			perror("not found");
 		}
 	}
 	else {
 		if (!ptrace_faultinfo)
 			printf("found but disabled on command line\n");
 		else
 			printf("found\n");
 	}

 	init_registers(pid);
 	stop_ptraced_child(pid, 1, 1);
 }

 int can_do_skas(void)
 {
 	printf("Checking for /proc/mm...");
 	if (os_access("/proc/mm", OS_ACC_W_OK) < 0) {
 		proc_mm = 0;
 		printf("not found\n");
 	} else {
 		if (!proc_mm)
 			printf("found but disabled on command line\n");
 		else
 			printf("found\n");
 	}

 	check_skas3_ptrace_support();
 	return 1;
 }
 #else
 int can_do_skas(void)
 {
 	return(0);
 }
 #endif
	/*
	* Copyright (C) 2000, 2001, 2002 Jeff Dike (jdike@karaya.com)
	* Licensed under the GPL
	*/

	#include <stdio.h>
	#include <unistd.h>
	#include <signal.h>
	#include <sched.h>
	#include <errno.h>
	#include <stdarg.h>
	#include <stdlib.h>
	#include <setjmp.h>
	#include <sys/time.h>
	#include <sys/wait.h>
	#include <sys/mman.h>
	#include <asm/unistd.h>
	#include <asm/page.h>
	#include "user_util.h"
	#include "kern_util.h"
	#include "user.h"
	#include "process.h"
	#include "signal_kern.h"
	#include "signal_user.h"
	#include "sysdep/ptrace.h"
	#include "sysdep/sigcontext.h"
	#include "irq_user.h"
	#include "ptrace_user.h"
	#include "time_user.h"
	#include "init.h"
	#include "os.h"
	#include "uml-config.h"
	#include "choose-mode.h"
	#include "mode.h"
	#include "tempfile.h"
	#ifdef UML_CONFIG_MODE_SKAS
	#include "skas.h"
	#include "skas_ptrace.h"
	#include "registers.h"
	#endif

	void init_new_thread_stack(void sig_stack, void (usr1_handler)(int))
	{
	int flags = 0, pages;

	if(sig_stack != NULL){
	pages = (1 << UML_CONFIG_KERNEL_STACK_ORDER);
	set_sigstack(sig_stack, pages * page_size());
	flags = SA_ONSTACK;
	}
	if(usr1_handler) set_handler(SIGUSR1, usr1_handler, flags, -1);
	}

	void init_new_thread_signals(int altstack)
	{
	int flags = altstack ? SA_ONSTACK : 0;

	set_handler(SIGSEGV, (__sighandler_t) sig_handler, flags,
	SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
	set_handler(SIGTRAP, (__sighandler_t) sig_handler, flags,
	SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
	set_handler(SIGFPE, (__sighandler_t) sig_handler, flags,
	SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
	set_handler(SIGILL, (__sighandler_t) sig_handler, flags,
	SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
	set_handler(SIGBUS, (__sighandler_t) sig_handler, flags,
	SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
	set_handler(SIGUSR2, (__sighandler_t) sig_handler,
	flags, SIGUSR1, SIGIO, SIGWINCH, SIGALRM, SIGVTALRM, -1);
	signal(SIGHUP, SIG_IGN);

	init_irq_signals(altstack);
	}

	struct tramp {
	int (tramp)(void );
	void *tramp_data;
	unsigned long temp_stack;
	int flags;
	int pid;
	};

	/* See above for why sigkill is here */

	int sigkill = SIGKILL;

	int outer_tramp(void *arg)
	{
	struct tramp *t;
	int sig = sigkill;

	t = arg;
	t->pid = clone(t->tramp, (void *) t->temp_stack + page_size()/2,
	t->flags, t->tramp_data);
	if(t->pid > 0) wait_for_stop(t->pid, SIGSTOP, PTRACE_CONT, NULL);
	kill(os_getpid(), sig);
	_exit(0);
	}

	int start_fork_tramp(void *thread_arg, unsigned long temp_stack,
	int clone_flags, int (tramp)(void ))
	{
	struct tramp arg;
	unsigned long sp;
	int new_pid, status, err;

	/* The trampoline will run on the temporary stack */
	sp = stack_sp(temp_stack);

	clone_flags \|= CLONE_FILES \| SIGCHLD;

	arg.tramp = tramp;
	arg.tramp_data = thread_arg;
	arg.temp_stack = temp_stack;
	arg.flags = clone_flags;

	/* Start the process and wait for it to kill itself */
	new_pid = clone(outer_tramp, (void *) sp, clone_flags, &arg);
	if(new_pid < 0)
	return(new_pid);

	CATCH_EINTR(err = waitpid(new_pid, &status, 0));
	if(err < 0)
	panic("Waiting for outer trampoline failed - errno = %d",
	errno);

	if(!WIFSIGNALED(status) \|\| (WTERMSIG(status) != SIGKILL))
	panic("outer trampoline didn't exit with SIGKILL, "
	"status = %d", status);

	return(arg.pid);
	}

	static int ptrace_child(void)
	{
	int ret;
	int pid = os_getpid(), ppid = getppid();
	int sc_result;

	if(ptrace(PTRACE_TRACEME, 0, 0, 0) < 0){
	perror("ptrace");
	os_kill_process(pid, 0);
	}
	os_stop_process(pid);

	/*This syscall will be intercepted by the parent. Don't call more than
	* once, please.*/
	sc_result = os_getpid();

	if (sc_result == pid)
	ret = 1; /*Nothing modified by the parent, we are running
	normally.*/
	else if (sc_result == ppid)
	ret = 0; /*Expected in check_ptrace and check_sysemu when they
	succeed in modifying the stack frame*/
	else
	ret = 2; /*Serious trouble! This could be caused by a bug in
	host 2.6 SKAS3/2.6 patch before release -V6, together
	with a bug in the UML code itself.*/
	_exit(ret);
	}

	static int start_ptraced_child(void)
	{
	int pid, n, status;

	pid = fork();
	if(pid == 0)
	ptrace_child();

	if(pid < 0)
	panic("check_ptrace : fork failed, errno = %d", errno);
	CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
	if(n < 0)
	panic("check_ptrace : wait failed, errno = %d", errno);
	if(!WIFSTOPPED(status) \|\| (WSTOPSIG(status) != SIGSTOP))
	panic("check_ptrace : expected SIGSTOP, got status = %d",
	status);

	return(pid);
	}

	/* When testing for SYSEMU support, if it is one of the broken versions, we must
	* just avoid using sysemu, not panic, but only if SYSEMU features are broken.
	* So only for SYSEMU features we test mustpanic, while normal host features
	* must work anyway!*/
	static int stop_ptraced_child(int pid, int exitcode, int mustexit)
	{
	int status, n, ret = 0;

	if(ptrace(PTRACE_CONT, pid, 0, 0) < 0)
	panic("stop_ptraced_child : ptrace failed, errno = %d", errno);
	CATCH_EINTR(n = waitpid(pid, &status, 0));
	if(!WIFEXITED(status) \|\| (WEXITSTATUS(status) != exitcode)) {
	int exit_with = WEXITSTATUS(status);
	if (exit_with == 2)
	printk("check_ptrace : child exited with status 2. "
	"Serious trouble happening! Try updating your "
	"host skas patch!\nDisabling SYSEMU support.");
	printk("check_ptrace : child exited with exitcode %d, while "
	"expecting %d; status 0x%x", exit_with,
	exitcode, status);
	if (mustexit)
	panic("\n");
	else
	printk("\n");
	ret = -1;
	}

	return ret;
	}

	static int force_sysemu_disabled = 0;

	int ptrace_faultinfo = 1;
	int proc_mm = 1;

	static int __init skas0_cmd_param(char str, int add)
	{
	ptrace_faultinfo = proc_mm = 0;
	return 0;
	}

	static int __init nosysemu_cmd_param(char str, int add)
	{
	force_sysemu_disabled = 1;
	return 0;
	}

	__uml_setup("skas0", skas0_cmd_param,
	"skas0\n"
	" Disables SKAS3 usage, so that SKAS0 is used, unless you \n"
	" specify mode=tt.\n\n");

	__uml_setup("nosysemu", nosysemu_cmd_param,
	"nosysemu\n"
	" Turns off syscall emulation patch for ptrace (SYSEMU) on.\n"
	" SYSEMU is a performance-patch introduced by Laurent Vivier. It changes\n"
	" behaviour of ptrace() and helps reducing host context switch rate.\n"
	" To make it working, you need a kernel patch for your host, too.\n"
	" See http://perso.wanadoo.fr/laurent.vivier/UML/ for further information.\n\n");

	static void __init check_sysemu(void)
	{
	int pid, syscall, n, status, count=0;

	printk("Checking syscall emulation patch for ptrace...");
	sysemu_supported = 0;
	pid = start_ptraced_child();

	if(ptrace(PTRACE_SYSEMU, pid, 0, 0) < 0)
	goto fail;

	CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
	if (n < 0)
	panic("check_sysemu : wait failed, errno = %d", errno);
	if(!WIFSTOPPED(status) \|\| (WSTOPSIG(status) != SIGTRAP))
	panic("check_sysemu : expected SIGTRAP, "
	"got status = %d", status);

	n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET,
	os_getpid());
	if(n < 0)
	panic("check_sysemu : failed to modify system "
	"call return, errno = %d", errno);

	if (stop_ptraced_child(pid, 0, 0) < 0)
	goto fail_stopped;

	sysemu_supported = 1;
	printk("OK\n");
	set_using_sysemu(!force_sysemu_disabled);

	printk("Checking advanced syscall emulation patch for ptrace...");
	pid = start_ptraced_child();
	while(1){
	count++;
	if(ptrace(PTRACE_SYSEMU_SINGLESTEP, pid, 0, 0) < 0)
	goto fail;
	CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
	if(n < 0)
	panic("check_ptrace : wait failed, errno = %d", errno);
	if(!WIFSTOPPED(status) \|\| (WSTOPSIG(status) != SIGTRAP))
	panic("check_ptrace : expected (SIGTRAP\|SYSCALL_TRAP), "
	"got status = %d", status);

	syscall = ptrace(PTRACE_PEEKUSR, pid, PT_SYSCALL_NR_OFFSET,
	0);
	if(syscall == __NR_getpid){
	if (!count)
	panic("check_ptrace : SYSEMU_SINGLESTEP doesn't singlestep");
	n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET,
	os_getpid());
	if(n < 0)
	panic("check_sysemu : failed to modify system "
	"call return, errno = %d", errno);
	break;
	}
	}
	if (stop_ptraced_child(pid, 0, 0) < 0)
	goto fail_stopped;

	sysemu_supported = 2;
	printk("OK\n");

	if ( !force_sysemu_disabled )
	set_using_sysemu(sysemu_supported);
	return;

	fail:
	stop_ptraced_child(pid, 1, 0);
	fail_stopped:
	printk("missing\n");
	}

	void __init check_ptrace(void)
	{
	int pid, syscall, n, status;

	printk("Checking that ptrace can change system call numbers...");
	pid = start_ptraced_child();

	if (ptrace(PTRACE_OLDSETOPTIONS, pid, 0, (void *)PTRACE_O_TRACESYSGOOD) < 0)
	panic("check_ptrace: PTRACE_SETOPTIONS failed, errno = %d", errno);

	while(1){
	if(ptrace(PTRACE_SYSCALL, pid, 0, 0) < 0)
	panic("check_ptrace : ptrace failed, errno = %d",
	errno);
	CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
	if(n < 0)
	panic("check_ptrace : wait failed, errno = %d", errno);
	if(!WIFSTOPPED(status) \|\| (WSTOPSIG(status) != SIGTRAP + 0x80))
	panic("check_ptrace : expected SIGTRAP + 0x80, "
	"got status = %d", status);

	syscall = ptrace(PTRACE_PEEKUSR, pid, PT_SYSCALL_NR_OFFSET,
	0);
	if(syscall == __NR_getpid){
	n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_NR_OFFSET,
	__NR_getppid);
	if(n < 0)
	panic("check_ptrace : failed to modify system "
	"call, errno = %d", errno);
	break;
	}
	}
	stop_ptraced_child(pid, 0, 1);
	printk("OK\n");
	check_sysemu();
	}

	int run_kernel_thread(int (fn)(void ), void arg, void *jmp_ptr)
	{
	sigjmp_buf buf;
	int n;

	*jmp_ptr = &buf;
	n = sigsetjmp(buf, 1);
	if(n != 0)
	return(n);
	(*fn)(arg);
	return(0);
	}

	void forward_pending_sigio(int target)
	{
	sigset_t sigs;

	if(sigpending(&sigs))
	panic("forward_pending_sigio : sigpending failed");
	if(sigismember(&sigs, SIGIO))
	kill(target, SIGIO);
	}

	extern void *__syscall_stub_start, __syscall_stub_end;

	#ifdef UML_CONFIG_MODE_SKAS

	static inline void check_skas3_ptrace_support(void)
	{
	struct ptrace_faultinfo fi;
	int pid, n;

	printf("Checking for the skas3 patch in the host...");
	pid = start_ptraced_child();

	n = ptrace(PTRACE_FAULTINFO, pid, 0, &fi);
	if (n < 0) {
	ptrace_faultinfo = 0;
	if(errno == EIO)
	printf("not found\n");
	else {
	perror("not found");
	}
	}
	else {
	if (!ptrace_faultinfo)
	printf("found but disabled on command line\n");
	else
	printf("found\n");
	}

	init_registers(pid);
	stop_ptraced_child(pid, 1, 1);
	}

	int can_do_skas(void)
	{
	printf("Checking for /proc/mm...");
	if (os_access("/proc/mm", OS_ACC_W_OK) < 0) {
	proc_mm = 0;
	printf("not found\n");
	} else {
	if (!proc_mm)
	printf("found but disabled on command line\n");
	else
	printf("found\n");
	}

	check_skas3_ptrace_support();
	return 1;
	}
	#else
	int can_do_skas(void)
	{
	return(0);
	}
	#endif