Re: ptrace single-stepping change breaks Wine

[Davide Libenzi]

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;
}