Re: ptrace single-stepping change breaks Wine

Davide Libenzi
2004-11-22T13:50:16Z

On Sun, 21 Nov 2004, Linus Torvalds wrote:

> Ok, how about this patch?
>
> It does basically two things:
>
> - it makes the x86 version of ptrace be a lot more careful about the TF
> bit in eflags, and in particular it never touches it _unless_ the
> tracer has explicitly asked for it (ie we set TF only when doing a
> PTRACE_SINGESTEP, and we clear it only when it has been set by us, not
> if it has been set by the program itself).
>
> This patch also cleans up the codepaths by doing all the common stuff
> in set_singlestep()/clear_singlestep().
>
> - It clarifies signal handling, and makes it clear that we always push
> the full eflags onto the signal stack, _except_ if the TF bit was set
> by an external ptrace user, in which case we hide it so that the tracee
> doesn't see it when it looks at its stack contents.
>
> It also adds a few comments, and makes it clear that the signal handler
> itself is always set up with TF _clear_. But if we were single-stepped
> into it, we will have notified the debugger, so the debugger obviously
> can (and often will) decide to continue single-stepping.

Looks like a nice cleanup. What does the test program below print for you?

- 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;
}