This patch permits a ptrace process on x86 to "see" the instruction following
the INT #80h op.  This has been tested on 2.6.6 using the appended test
source.  Running over this:

80485a9:       b8 14 00 00 00          mov    $0x14,%eax
80485ae:       cd 80                   int    $0x80
80485b0:       89 45 ec                mov    %eax,0xffffffec(%ebp)
80485b3:       eb f4                   jmp    80485a9 <main+0x85>

it produces:

waiting ...
done: pid=12387  status=1407
sig=5
EIP=0x080485a9
waiting ...
done: pid=12387  status=1407
sig=5
EIP=0x080485ae
waiting ...
done: pid=12387  status=1407
sig=5
EIP=0x080485b0
waiting ...
done: pid=12387  status=1407
sig=5
EIP=0x080485b3

(Andi says: "I think this patch is a bad idea.  The ptrace handling is
traditionally fragile (I remember when merging a rather simple patch from IBM
for DR allocation long ago into the suse it broke several debuggers).  If you
really want to do that wait for 2.7.")


test-program.c:

#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;
}
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>