/*
* 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;
}

Attachments:

check_restart_skip2.c (5.83 kB)

2005-05-31 17:03:40

by Martin Schwidefsky

[permalink] [raw]

Subject: Re: Again: UML on s390 (31Bit)

> > 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.

Ok, Uli convinced me that the original patch to clear regs->traps if
the system call has been canceled on the first call to syscall_trace
is the correct thing to do. If the tracer chooses to invalidate the
system call of the traced process then the complete handling of the
function executed for the system call is done in the tracer. That
includes system call restarting in the case that another system call
is involved to implement the function. The point is that the traced
process did not execute a system call, ergo no system call restarting
may take place.
So after a long discussion I'll just use a slightly modified version
of the original patch:

Index: ptrace.c
===================================================================
RCS file: /home/cvs/linux-2.5/arch/s390/kernel/ptrace.c,v
retrieving revision 1.35
diff -u -r1.35 ptrace.c
--- ptrace.c 6 May 2005 18:59:13 -0000 1.35
+++ ptrace.c 31 May 2005 16:50:50 -0000
@@ -39,6 +39,7 @@
#include <asm/pgalloc.h>
#include <asm/system.h>
#include <asm/uaccess.h>
+#include <asm/unistd.h>

#ifdef CONFIG_S390_SUPPORT
#include "compat_ptrace.h"
@@ -762,6 +763,13 @@
return;
ptrace_notify(SIGTRAP | ((current->ptrace & PT_TRACESYSGOOD)
? 0x80 : 0));
+
+ /*
+ * If the debuffer has set an invalid system call number,
+ * we prepare to skip the system call restart handling.
+ */
+ if (!entryexit && regs->gprs[2] >= NR_syscalls)
+ regs->trap = -1;

/*
* this isn't the same as continuing with a signal, but it will do

===================================================================

regs->trap should be reset for any invalid system call, not just for
negative system call numbers.

blue skies,
Martin

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH