Date: Mon, 4 Feb 2002 18:43:05 -0500 (EST) From: Zhihui Zhang <zzhang@cs.binghamton.edu> To: freebsd-questions@FreeBSD.ORG Subject: Re: A simple ptrace program Message-ID: <Pine.SOL.4.21.0202041839220.26634-100000@opal> In-Reply-To: <Pine.SOL.4.21.0202041356170.28423-100000@onyx>
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On Mon, 4 Feb 2002, Zhihui Zhang wrote:
>
> 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);
Sorry, this program has a bug. This line should be
data = ptrace(PT_READ_D, pid, addr);
However, after the parent exits, the child can not write to the terminal
for some reason. I am wondering whether an orphaned process can write
to the terminal.
-Zhihui
> 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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