Re: ptrace single-stepping change breaks Wine

Davide Libenzi
2004-12-30T01:15:23Z

On Wed, 29 Dec 2004, Linus Torvalds wrote:

> On Wed, 29 Dec 2004, Jesse Allen wrote:
> > >
> > > So instead of removing the setting of TIF_SINGLESTEP in set_singlestep(),
> > > can you test whether removing the _testing_ of it in do_syscall_trace()
> > > makes things happier for you? Hmm?
> > >
> >
> > Yes, doing that does work. But I still have to remove the conditional
> > TF clear. Here's the diff now to show you.
>
> Ok. That's a good piece of information.
>
> I don't actually understand why do_syscall_trace() does that
> TIF_SINGLESTEP test in the first place, since the ptrace_notify() should
> happen as part of the _normal_ TIF_SINGLESTEP handling, afaiks. No
> apparent need to do it in syscall tracing, and do_syscall_trace() does
> that bogus fake signal sending.
>
> Hmm.. That TF_SINGLESTEP test was added by Davide Libenzi in August, and I
> think Davide had some test for exactly this. Davide? Can you recall why
> you did this in the system call tracing code, rather than depend on the
> regular TIF_SINGLESTEP handling in arch/i386/kernel/signal.c:
> do_notify_resume()?

That test went in to be able to have ptrace single step, to see even the
instruction following the #int instruction (this was the target of the
patch itself). I just verified that, in 2.6.8 that does not have such test
anymore, the single-step-after-int capability is lost. Below I inlining a
simple test program (that I already sent you time ago) to test this. In my
2.6.8 I get this output:

waiting ...
done: pid=21398 status=1407
sig=5
EIP=0x080485d5
waiting ...
done: pid=21398 status=1407
sig=5
EIP=0x080485da
waiting ...
done: pid=21398 status=1407
sig=5
EIP=0x080485df
waiting ...
done: pid=21398 status=1407
sig=5
EIP=0x080485d5
waiting ...
done: pid=21398 status=1407
sig=5
EIP=0x080485da
waiting ...
done: pid=21398 status=1407
sig=5
EIP=0x080485df

in front of this code:

80485d5: b8 14 00 00 00 mov $0x14,%eax
80485da: cd 80 int $0x80
80485dc: 89 45 ec mov %eax,0xffffffec(%ebp)
80485df: eb f4 jmp 80485d5 <main+0x85>

You can see that the "mov %eax,0xffffffec(%ebp)" instruction at 0x80485dc
is not seen by ptrace single step. With the test in place, it will show up
again in your traces.

- Davide

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <linux/user.h>
#include <linux/unistd.h>

int main(int ac, char **av) {
int i, status, res;
long start, end;
pid_t cpid, pid;
struct user_regs_struct ur;
struct sigaction sa;

sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
sa.sa_handler = SIG_DFL;
sigaction(SIGCHLD, &sa, NULL);

printf("nargs=%d\n", ac);
if (ac == 1)
goto tracer;

printf("arg=%s\n", av[1]);
loop:
__asm__ volatile ("int $0x80"
: "=a" (res)
: "0" (__NR_getpid));
goto loop;
endloop:
exit(0);

tracer:
if ((cpid = fork()) != 0)
goto parent;

printf("child=%d\n", getpid());
ptrace(PTRACE_TRACEME, 0, NULL, NULL);

execl(av[0], av[0], "child", NULL);

exit(0);

parent:
start = (long) &&loop;
end = (long) &&endloop;

printf("pchild=%d\n", cpid);

for (;;) {
pid = wait(&status);
if (pid != cpid)
continue;
res = WSTOPSIG(status);
if (ptrace(PTRACE_GETREGS, pid, NULL, &ur)) {
printf("[%d] error: ptrace(PTRACE_GETREGS, %d)\n",
pid, pid);
return 1;
}

if (ptrace(PTRACE_SINGLESTEP, pid, NULL, res != SIGTRAP ? res: 0)) {
perror("ptrace(PTRACE_SINGLESTEP)");
return 1;
}

if (ur.eip >= start && ur.eip <= end)
break;
}

for (i = 0; i < 15; i++) {
printf("waiting ...\n");
pid = wait(&status);
printf("done: pid=%d status=%d\n", pid, status);
if (pid != cpid)
continue;
res = WSTOPSIG(status);
printf("sig=%d\n", res);
if (ptrace(PTRACE_GETREGS, pid, NULL, &ur)) {
printf("[%d] error: ptrace(PTRACE_GETREGS, %d)\n",
pid, pid);
return 1;
}

printf("EIP=0x%08x\n", ur.eip);

if (ptrace(PTRACE_SINGLESTEP, pid, NULL, res != SIGTRAP ? res: 0)) {
perror("ptrace(PTRACE_SINGLESTEP)");
return 1;
}
}

if (ptrace(PTRACE_CONT, cpid, NULL, SIGKILL)) {
perror("ptrace(PTRACE_SINGLESTEP)");
return 1;
}

return 0;
}