Re: Again: UML on s390 (31Bit)

[Bodo Stroesser <bstroesser@fujitsu-siemens.com>]

Bodo Stroesser wrote:
> I've prepared and attached a small program that easily can reproduce
> the problem. I hope this will help to find a viable solution.

Here is a slightly modified version of my testtool. The new version
covers the fact, that in certain situations UML must avoid syscall
restarting, even if PSWADDR is not modified.

Hope this helps
		Bodo

/*
 * This is a tool to test syscall invalidation via ptrace on s390.
 * It is based on arch/um/os-Linux/start_up.c
 */

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sched.h>
#include <errno.h>
#include <stdarg.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <asm/unistd.h>
#include <asm/page.h>
#include <linux/ptrace.h>
#include <stddef.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>

#define ERESTARTNOINTR  513

static int ptrace_child(void *arg)
{
	int ret;
	int pid = getpid();
	int sc_result;

	/* Child wants to be ptraced */
	if(ptrace(PTRACE_TRACEME, 0, 0, 0) < 0){
		perror("ptrace");
		kill(pid, SIGKILL);
	}
	/* Child stops itself */
	kill(pid, SIGSTOP);

	/* The following part is run under PTRACE_SYSCALL */

	__asm__ __volatile__ (
		"    svc %b1\n"
		"    lr  %0,2"
		: "=d" (sc_result)
		: "i" (__NR_getpid)
		: "2" );

	/* Here we are back running PTRACE_CONT */
	
	/* Now we check the result of the syscall */
	if (sc_result == -ERESTARTNOINTR)
		ret = 0; /* Expected result: syscall was invalidated, no
			    syscall restart is done */
	else if (sc_result == pid)
		ret = 1; /* This is wrong, as it is the normal result of
			    getpid(). Probably host did a syscall restart! */
	else
		ret = 2; /* We don't know, what happened. There may be a bug in
			    this test tool */

	/* Give father a status indicating success or failure */
	exit(ret);
}

static void errout(char *str, int error)
{
	printf(str, error);
	putchar('\n');
	exit(1);
}

static int start_ptraced_child(void **stack_out)
{
	void *stack;
	unsigned long sp;
	int pid, n, status;
	
	stack = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE | PROT_EXEC,
		     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if(stack == MAP_FAILED)
		errout("start_ptraced_child : mmap failed, errno = %d", errno);
	sp = (unsigned long) stack + PAGE_SIZE - sizeof(void *);
	pid = __clone(ptrace_child, (void *) sp, SIGCHLD, NULL);
	if(pid < 0)
		errout("start_ptraced_child : clone failed, errno = %d", errno);
	n = waitpid(pid, &status, WUNTRACED);
	if(n < 0)
		errout("start_ptraced_child : wait failed, errno = %d", errno);
	if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGSTOP))
		errout("start_ptraced_child : expected SIGSTOP, "
		       "got status = 0x%x", status);

	*stack_out = stack;
	return(pid);
}

static int stop_ptraced_child(int pid, void *stack)
{
	int status, n;

	/* We resume our child and let it check it's result */
	if(ptrace(PTRACE_CONT, pid, 0, 0) < 0)
		errout("stop_ptraced_child : ptrace failed, errno = %d", errno);

	/* Now, we wait for the child to exit */
	n = waitpid(pid, &status, 0);
	if(!WIFEXITED(status))
		errout("\nstop_ptraced_child: error: child didn't exit,"
		       " status 0x%x\n", status);

	if(munmap(stack, PAGE_SIZE) < 0)
		errout("stop_ptraced_child : munmap failed, errno = %d", errno);

	/* Return child's exit status */
	return WEXITSTATUS(status);
}

int main(void)
{
	void *stack;
	int pid, syscall, n, status;
	unsigned long addr;

	printf("Checking if syscall restart handling in host can be skipped...");
	fflush(stdout);

	/* First create a child and wait, until it stops itself */
	pid = start_ptraced_child(&stack);

	/* Now resume the child */
	if(ptrace(PTRACE_SYSCALL, pid, 0, 0) < 0)
		errout("check_restart_skip : ptrace failed, "
		       "errno = %d", errno);

	/* wait, until child does a syscall */
	n = waitpid(pid, &status, WUNTRACED);
	if(n < 0)
		errout("check_restart_skip : wait failed, "
		       "errno = %d", errno);
	if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
		errout("check_restart_skip : expected "
		       "SIGTRAP, got status = %d", status);

	/* Check, if syscall is __NR_getpid */
	syscall = ptrace(PTRACE_PEEKUSR, pid, PT_GPR2, 0);
	if(syscall != __NR_getpid)
		errout("check_restart_skip: unexpected syscall %d\n", syscall);

	/* Modify syscall number to -1 */
	n = ptrace(PTRACE_POKEUSR, pid, PT_GPR2, -1);
	if(n < 0)
		errout("check_restart_skip : failed to "
		       "modify system call, errno = %d", errno);

	/* Resume child and wait for second syscall interception */
	if(ptrace(PTRACE_SYSCALL, pid, 0, 0) < 0)
		errout("check_restart_skip : ptrace failed, "
		       "errno = %d", errno);
	n = waitpid(pid, &status, WUNTRACED);
	if(n < 0)
		errout("check_restart_skip : wait failed, "
		       "errno = %d", errno);
	if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
		errout("check_restart_skip : expected "
		       "SIGTRAP, got status = %d", status);

	/* Set syscall result to -ERESTARTNOINTR */
	n = ptrace(PTRACE_POKEUSR, pid, PT_GPR2, -ERESTARTNOINTR);
	if(n < 0)
		errout("check_restart_skip : failed to modify system "
		       "call result, errno = %d", errno);

	/* Here "accidentally" a signal is queued for the child */
	kill(pid, SIGALRM);

	/* We resume the child again and wait for next interception */
	if(ptrace(PTRACE_SYSCALL, pid, 0, 0) < 0)
		errout("check_restart_skip : ptrace failed, "
		       "errno = %d", errno);
	n = waitpid(pid, &status, WUNTRACED);
	if(n < 0)
		errout("check_restart_skip : wait failed, "
		       "errno = %d", errno);

	/* The interception must be for the signal, not for a syscall
	   Here, UML would do some interrupt processing */
	if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGALRM))
		errout("check_restart_skip : expected "
		       "SIGALRM, got status = %d", status);

	/* At the end of interrupt processing, UML would resume the child
	 * doing ptrace(PTRACE_SYSCALL), but without modifying the regs.
	 * Here we call stop_ptraced_child, which will resume the child
	 * with ptrace(PTRACE_CONT). Then the child will check the "result"
	 * and will exit with
	 *    0 if the result is -ERESTARTNOINTR
	 *    1 if the result is child's pid (host did syscall restart)
	 *    2 if we have an unexpected result
	 */
	n = stop_ptraced_child(pid, stack);
	if (n)
		printf("failed, result = %d\n", n);
	else
		printf("OK\n");

	return n;
}