From owner-freebsd-questions  Mon Feb  4 11: 5:44 2002
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Date: Mon, 4 Feb 2002 14:04:20 -0500 (EST)
From: Zhihui Zhang <zzhang@cs.binghamton.edu>
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To: freebsd-questions@freebsd.org
Subject: A simple ptrace program
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Yesterday I read the book "Unix operating system" for a while and decided
to give the ptrace example in the book a try on FreeBSD 4.4. This program
is very simple, it basically tries to write into an array of its child
process before its child has a chance to run.  But the result is not what
I expected.  Maybe the semantics of ptrace changes over the time?  Please
help me with this code:

mymachine# cat trace.c
/*
 * Filename: trace.c
 */
int data[8] = { 0, 1, 2, 3, 4, 5, 6, 7 };
main()
{
	int i;
	for (i = 0; i < 8; i++)
		printf("trace: addr = 0x%x: data[%d] = %d\n", 
				&data[i], i, data[i]);
	printf("ptrace data addr 0x%x\n", &data[0]);
}

/*
 * Filename: debug.c
 */
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ptrace.h>

main(int argc, char * argv[])
{
	caddr_t addr;
	int i, pid, ret, data, status;
	
	if (argc != 2) {
		printf("Usage: debug address\n");
		exit(0);
	}

	sscanf(argv[1], "%x", &addr);
	printf("Child array address is 0x%x\n", addr);
	if ((pid = fork()) == 0) {

		ptrace(PT_TRACE_ME, 0, 0, 0);
		execl("./trace", "trace", 0);	/* l - list */
		printf("Fail to execl() child process!\n");
		exit(1);
	}

	ret = waitpid(pid, &status, 0);
	printf("ret = %d, status = %d\n", ret, status);

	for (i = 0; i < 8; i++) {
		/*
		 * Write value of i into address addr in proc pid.
		 */
		if (ptrace(PT_WRITE_D, pid, addr, i) == -1)
			exit(1);

		if (ptrace(PT_READ_D, pid, addr, data) == -1)
			exit(1);
		printf("debug: addr = 0x%x, data[%d] = %d\n", addr, i, data);

		addr += sizeof(int);
	}
	/*
	 * Traced process should resume execution.
	 */
	if (ptrace(PT_CONTINUE, pid, (caddr_t)1, 0) == -1) {
		printf("ptrace error = %d\n", errno);
		exit(0);
	}

} /* end of main() */
mymachine# cc -o trace trace.c
mymachine# cc -o debug debug.c
mymachine# trace
trace: addr = 0x80495c0: data[0] = 0
trace: addr = 0x80495c4: data[1] = 1
trace: addr = 0x80495c8: data[2] = 2
trace: addr = 0x80495cc: data[3] = 3
trace: addr = 0x80495d0: data[4] = 4
trace: addr = 0x80495d4: data[5] = 5
trace: addr = 0x80495d8: data[6] = 6
trace: addr = 0x80495dc: data[7] = 7
ptrace data addr 0x80495c0
mymachine# debug 0x80495c0
Child array address is 0x80495c0
ret = 15512, status = 1407
debug: addr = 0x80495c0, data[0] = -1077937164
debug: addr = 0x80495c4, data[1] = -1077937164
debug: addr = 0x80495c8, data[2] = -1077937164
debug: addr = 0x80495cc, data[3] = -1077937164
debug: addr = 0x80495d0, data[4] = -1077937164
debug: addr = 0x80495d4, data[5] = -1077937164
debug: addr = 0x80495d8, data[6] = -1077937164
debug: addr = 0x80495dc, data[7] = -1077937164

As you can see, the child (trace) does not seem to be running at all.

-Zhihui


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