problem with registers

mr.cracker · 05-11-2012, 10:58 AM

i try to copy one string to another using strcpy() in c.
and in gdb i typed--info reg. But it says that

The program has no registers now.

Why i get this msg.Because in my opinion all process use registers?? Then why do i can't access registers??

Is strecpy() doesn't use registers?? pls tell me about that.

markush · 05-12-2012, 02:51 AM

Hi,

please show us the code you've written, I think this would help us to find out more about your problem.

Also please describe in detail how you've compiled the program.

I'll report this thread and ask a Moderator to move it to the "Programming" forum of LQ.

Markus

colucix · 05-12-2012, 09:23 AM

Moved: This thread is more suitable in Programming and has been moved accordingly to help your thread/question get the exposure it deserves. Thank you markush for reporting.

mr.cracker · 05-13-2012, 10:38 AM

Quote:

Originally Posted by markush

Hi,

please show us the code you've written, I think this would help us to find out more about your problem.

Also please describe in detail how you've compiled the program.

I'll report this thread and ask a Moderator to move it to the "Programming" forum of LQ.

Markus

i didn't get u. where can i find the programming forum???
my code is like this,

#include <stdio.h>

int main(int argc, char **argv[]) {

char b[256];

if(argc == 1) {
printf("Usage: %s input\n", argv[0]);
exit(0);
}

strcpy(b,argv[1]);
printf("%s", b);

}

and i compiled like this,

$gcc name.c -o name
$./name AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
$ gdb -c core ./name
(gdb) info reg

ntubski · 05-13-2012, 11:18 AM

Quote:

Originally Posted by mr.cracker

i didn't get u. where can i find the programming forum???

This thread has been moved there (here).

Quote:

my code is like this,

Please use [code][/code] tags.

Regarding the original question, you can't look at register values until the program starts running:

Code:

~/tmp$ make name
gcc -Wall -Wextra -Wformat=2 -std=gnu99 -g    name.c   -o name
name.c:3:5: warning: second argument of ‘main’ should be ‘char **’ [-Wmain]
name.c: In function ‘main’:
name.c:8:9: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘char **’ [-Wformat]
name.c:9:9: warning: implicit declaration of function ‘exit’ [-Wimplicit-function-declaration]
name.c:9:9: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]
name.c:12:5: warning: implicit declaration of function ‘strcpy’ [-Wimplicit-function-declaration]
name.c:12:5: warning: incompatible implicit declaration of built-in function ‘strcpy’ [enabled by default]
name.c:12:5: warning: passing argument 2 of ‘strcpy’ from incompatible pointer type [enabled by default]
name.c:12:5: note: expected ‘const char *’ but argument is of type ‘char **’
~/tmp$ ./name AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA~/tmp$ 
~/tmp$ gdb ./name
GNU gdb (GDB) 7.4.1-debian
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/npostavs/tmp/name...done.
(gdb) break main
Breakpoint 1 at 0x40059c: file name.c, line 7.
(gdb) run
Starting program: /home/npostavs/tmp/name 

Breakpoint 1, main (argc=1, argv=0x7fffffffea08)
    at name.c:7
7	    if(argc == 1) {
(gdb) info reg
rax            0x7ffff7dd9ee8	140737351884520
rbx            0x0	0
rcx            0x0	0
rdx            0x7fffffffea18	140737488349720
rsi            0x7fffffffea08	140737488349704
rdi            0x1	1
rbp            0x7fffffffe920	0x7fffffffe920
rsp            0x7fffffffe810	0x7fffffffe810
r8             0x7ffff7dd8320	140737351877408
r9             0x7ffff7deb060	140737351954528
r10            0x0	0
r11            0x7ffff7a74db0	140737348324784
r12            0x4004a0	4195488
r13            0x7fffffffea00	140737488349696
r14            0x0	0
r15            0x0	0
rip            0x40059c	0x40059c <main+24>
eflags         0x202	[ IF ]
cs             0x33	51
ss             0x2b	43
ds             0x0	0
es             0x0	0
fs             0x0	0
gs             0x0	0

mr.cracker · 05-14-2012, 11:53 PM

Quote:

Originally Posted by ntubski

This thread has been moved there (here).

Please use [code][/code] tags.

Regarding the original question, you can't look at register values until the program starts running:

Code:

~/tmp$ make name
gcc -Wall -Wextra -Wformat=2 -std=gnu99 -g    name.c   -o name
name.c:3:5: warning: second argument of ‘main’ should be ‘char **’ [-Wmain]
name.c: In function ‘main’:
name.c:8:9: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘char **’ [-Wformat]
name.c:9:9: warning: implicit declaration of function ‘exit’ [-Wimplicit-function-declaration]
name.c:9:9: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]
name.c:12:5: warning: implicit declaration of function ‘strcpy’ [-Wimplicit-function-declaration]
name.c:12:5: warning: incompatible implicit declaration of built-in function ‘strcpy’ [enabled by default]
name.c:12:5: warning: passing argument 2 of ‘strcpy’ from incompatible pointer type [enabled by default]
name.c:12:5: note: expected ‘const char *’ but argument is of type ‘char **’
~/tmp$ ./name AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA~/tmp$ 
~/tmp$ gdb ./name
GNU gdb (GDB) 7.4.1-debian
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/npostavs/tmp/name...done.
(gdb) break main
Breakpoint 1 at 0x40059c: file name.c, line 7.
(gdb) run
Starting program: /home/npostavs/tmp/name 

Breakpoint 1, main (argc=1, argv=0x7fffffffea08)
    at name.c:7
7	    if(argc == 1) {
(gdb) info reg
rax            0x7ffff7dd9ee8	140737351884520
rbx            0x0	0
rcx            0x0	0
rdx            0x7fffffffea18	140737488349720
rsi            0x7fffffffea08	140737488349704
rdi            0x1	1
rbp            0x7fffffffe920	0x7fffffffe920
rsp            0x7fffffffe810	0x7fffffffe810
r8             0x7ffff7dd8320	140737351877408
r9             0x7ffff7deb060	140737351954528
r10            0x0	0
r11            0x7ffff7a74db0	140737348324784
r12            0x4004a0	4195488
r13            0x7fffffffea00	140737488349696
r14            0x0	0
r15            0x0	0
rip            0x40059c	0x40059c <main+24>
eflags         0x202	[ IF ]
cs             0x33	51
ss             0x2b	43
ds             0x0	0
es             0x0	0
fs             0x0	0
gs             0x0	0

sir.thank u.its working. BUt if i give more number of "A" as argument (suppose i give 500 "A" as argument). Then the esp will overwrite with A(0x41). But i can't view the registers that are overwriten by "A". What i want to do for that. If u know pls help me sir.

pan64 · 05-15-2012, 12:58 AM

you need to step over. It means you enter the command next in gdb and you will see the lines as they executed. you need to step further to see what will happen....

mr.cracker · 05-15-2012, 03:42 AM

Quote:

Originally Posted by pan64

you need to step over. It means you enter the command next in gdb and you will see the lines as they executed. you need to step further to see what will happen....

i didn't get u. in gdb i first create a break point and run it. Then when i use next command that u had said,the running program exit.I didn't see any overwritten registers.
what i will do.??

pan64 · 05-15-2012, 03:53 AM

you can try step, next, and also you can set breakpoint on the line strcpy or printf.

ntubski · 05-15-2012, 11:25 AM

You probably want to run gdb like this:

Code:

gdb --args ./name AAAAAAAAA...

That way you'll actually be debugging the failing case. Also, you see those warnings I got when I compiled your code? You should fix those.

mr.cracker · 05-16-2012, 12:07 PM

Quote:

Originally Posted by ntubski

You probably want to run gdb like this:

Code:

gdb --args ./name AAAAAAAAA...

That way you'll actually be debugging the failing case. Also, you see those warnings I got when I compiled your code? You should fix those.

Its working. BUt only edi is overwritten by "0x41" . The esp,eip is not overwritten why??
if any mistake in the code pls. help me??

the commands i used are,
(gdb) break main
(gdb) run
(gdb) step
(gdb) info reg

Then the it will exit from my c program. why??

pan64 · 05-16-2012, 12:35 PM

strcpy will not overwrite registers. it will overwrite the area where those register points (that is the stack or heap itself).
I do not really understand why do you expect that?