ptrace problems

[Shourya Sarcar <sarcar@xxxxxxxx>]

Programmerz,
This is a little pair of programs I wrote. The father tries to behave
like a debugger and get the register values of the child. But I keep on
getting incorrect results. Rather no results at all.
Help, someone. Anyone .

I am giving below the souce codes of the two programs and a typescript
showing strace debug and the output. Note how strace debug shows weird
stuff for the second ptrace.

Thanks for your concern.
Shourya


-------------------------------------------
/*
	Debugger, father , debug1.c
*/

#include <stdio.h>
#include <sys/ptrace.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <asm/ptrace.h>
#include <sys/user.h>


void fatal_error(const char* s)
{
	printf("%s\n",s);
	exit(-1);
}
	
pid_t  fork_child()
{
	/*
	  Fork a child and return PID 
	  Take care of errors
	*/
	pid_t child_pid;
	
	child_pid=fork();
	if (child_pid==-1) 
	{
		fatal_error("Could not fork in file");
	}
	
	/* If I am the child ... */
	if (child_pid==0)
	{
		printf("I am the child !\n");
		execv("./new",NULL);
	}
	getchar();
	return child_pid;
}

int main()
{
	int child_pid,err;
	//struct user q;
	
	unsigned long* gpregs=(unsigned long*)malloc(sizeof(long)*17);
	
	bzero((char*)gpregs,sizeof(long)*17);
	child_pid=fork_child();
	
	err=ptrace(PTRACE_ATTACH,child_pid,0,0);
	if (err<0) 
		fatal_error("Invalid Ptrace in PTRACE_ATTACH");
	else
		printf("Successful ptrace ATTACH\n");
		 
	/*Get the regs */
	err=ptrace(PT_GETREGS,child_pid,NULL,gpregs);
	printf ("Err = %d\n",err);
	if (err<0) fatal_error("Invalid Ptrace in GETREGS");
	printf("EAX : 0x%lx\n",gpregs[EAX]);
	printf("CS  : 0x%lx\n",gpregs[CS]);
	printf("EIP : 0x%lx\n",gpregs[EIP]);
	
				
	
	return 0;
}

-------------------------------------------
/*
 * new.c, child
*/ 	


#include <unistd.h>
#include <stdio.h>
#include <sys/ptrace.h>


int main()
{
	//int ch;
	int i;
	printf("I am the new image : PID = %d .. Entering loop !\n",getpid());
	/*
	ch=ptrace(PTRACE_TRACEME,0,0,0);
	if(ch==-1) {
		printf ("No tracing !\n");
		return 1;
	}
	else
	{
		printf("Tracing on \n");
	}
	*/
		
	for (i=0;i<1000000000L;i+=5);
	printf("Program new stopped\n");
	
	return 0;
}

------------------------------------------
Script started on Mon Feb 28 22:53:19 2000
[sarcar@localhost progs]$ gcc -o debug debug1.c -Wall -g
[sarcar@localhost progs]$ gcc -o new new.c -Wall -g
[sarcar@localhost progs]$ debug
I am the child !
I am the new image : PID = 1145 .. Entering loop !

Successful ptrace ATTACH
Err = -1
Invalid Ptrace in GETREGS
[sarcar@localhost progs]$ ma  strace debug|more
execve("./debug", ["debug"], [/* 24 vars */]) = 0
brk(0)                                  = 0x80498fc
open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat(3, {st_mode=0, st_size=0, ...})   = 0
mmap(0, 17006, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40014000
close(3)                                = 0
open("/lib/libc.so.6", O_RDONLY)        = 3
fstat(3, {st_mode=0, st_size=0, ...})   = 0
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3"..., 4096) = 4096
mmap(0, 974392, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) = 0x40019000
mprotect(0x400ff000, 32312, PROT_NONE)  = 0
mmap(0x400ff000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0xe5000) = 0x400ff000
mmap(0x40104000, 11832, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x40104000
close(3)                                = 0
munmap(0x40014000, 17006)               = 0
personality(PER_LINUX)                  = 0
getpid()                                = 1148
brk(0)                                  = 0x80498fc
brk(0x8049954)                          = 0x8049954
brk(0x804a000)                          = 0x804a000
fork()                                  = 1149
fstat(0, {st_mode=S_ISVTX|0401, st_size=0, ...}) = 0
mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40014000
ioctl(0, TCGETS, {B38400 opost isig -icanon -echo ...}) = 0
read(0, "\n", 4096)                     = 1
ptrace(PTRACE_ATTACH, 1149, 0, 0)       = 0
fstat(1, {st_mode=0, st_size=0, ...})   = 0
mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40015000
ptrace(PTRACE_???, 1149, 0, 0x8049908)  = -1 ESRCH (No such process)
write(1, "Successful ptrace ATTACH\nErr = "..., 60) = 60
munmap(0x40015000, 4096)                = 0
_exit(-1)                               = ?
Successful ptrace ATTACH
Err = -1
Invalid Ptrace in GETREGS

[sarcar@localhost progs]$ 
Script done on Mon Feb 28 22:54:43 2000



			  



 --
_______________________________________________________________
Shourya Sarcar 	       <sarcar@xxxxxxxx>  <Tel:91-033-4710477>
Department of Computer Science and Engineering
Jadavpur University    Calcutta, India 700 032

All the world's a stage..
And I am acting tonight
C - the difference : http://www.eskimo.com/~scs/C-faq/top.html
--
_______________________________________________________________
Shourya Sarcar 	       <sarcar@xxxxxxxx>  <Tel:91-033-4710477>
Department of Computer Science and Engineering
Jadavpur University    Calcutta, India 700 032

All the world's a stage..
And I am acting tonight
C - the difference : http://www.eskimo.com/~scs/C-faq/top.html