A question on debugging libraries
Hi,
I'm not sure if this is a bug or something I have misunderstood about
the debugging libraries. I want to show an example of debugging a
libc call, so just simply overflowed a buffer with strcpy.
---
#include stdio.h
#include string.h
char *b = A long string;
int main(void)
{
char a[1];
strcpy(a, b);
}
---
Which when I run on i386 under GDB with LD_LIBRARY_PATH=/usr/lib/debug
I get
--- gdb output on i386 ---
$ gcc -g -o test test.c
$ echo $LD_LIBRARY_PATH
/usr/lib/debug
$ gdb ./test
GNU gdb 6.0 (etc)
(gdb) r
Starting program: /home/ianw/test
Program received signal SIGSEGV, Segmentation fault.
0x74732067 in ?? ()
(gdb) back
#0 0x74732067 in ?? ()
#1 0x676e6972 in ?? ()
#2 0xba00 in ?? ()
#3 0xba0c in ?? ()
#4 0x40016c20 in ?? () from /lib/ld-linux.so.2
#5 0x0001 in ?? ()
#6 0x080482a0 in ?? ()
---
I don't belive the strcpy has been inlined, for example it shows up in ltrace
$ ltrace ./test
__libc_start_main(0x08048364, 1, 0xba04, 0x08048390, 0x080483f0
unfinished ...
strcpy(0xb9b7, A long string) = 0xb9b7
--- SIGSEGV (Segmentation fault) ---
+++ killed by SIGSEGV +++
I would have expected that this would give me a good backtrace. Is
this wrong?
-i
[EMAIL PROTECTED]
http://www.gelato.unsw.edu.au
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Re: A question on debugging libraries
On Tue, Feb 10, 2004 at 11:39:31AM +1100, Ian Wienand wrote: Hi, I'm not sure if this is a bug or something I have misunderstood about the debugging libraries. I want to show an example of debugging a libc call, so just simply overflowed a buffer with strcpy. There's your problem. You are trying to get a backtrace. A walk up the stack frame, yes? So you generated a crash by overwriting the stack; naturally we can not backtrace. #0 0x74732067 in ?? () #1 0x676e6972 in ?? () If the top frames are lost, then there is nothing you can expect from the below frames. Besides, you're in main; there is only one or at most two frames on the stack anyway. -- Daniel Jacobowitz MontaVista Software Debian GNU/Linux Developer -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
A question on debugging libraries
Hi,
I'm not sure if this is a bug or something I have misunderstood about
the debugging libraries. I want to show an example of debugging a
libc call, so just simply overflowed a buffer with strcpy.
---
#include stdio.h
#include string.h
char *b = A long string;
int main(void)
{
char a[1];
strcpy(a, b);
}
---
Which when I run on i386 under GDB with LD_LIBRARY_PATH=/usr/lib/debug
I get
--- gdb output on i386 ---
$ gcc -g -o test test.c
$ echo $LD_LIBRARY_PATH
/usr/lib/debug
$ gdb ./test
GNU gdb 6.0 (etc)
(gdb) r
Starting program: /home/ianw/test
Program received signal SIGSEGV, Segmentation fault.
0x74732067 in ?? ()
(gdb) back
#0 0x74732067 in ?? ()
#1 0x676e6972 in ?? ()
#2 0xba00 in ?? ()
#3 0xba0c in ?? ()
#4 0x40016c20 in ?? () from /lib/ld-linux.so.2
#5 0x0001 in ?? ()
#6 0x080482a0 in ?? ()
---
I don't belive the strcpy has been inlined, for example it shows up in ltrace
$ ltrace ./test
__libc_start_main(0x08048364, 1, 0xba04, 0x08048390, 0x080483f0
unfinished ...
strcpy(0xb9b7, A long string) = 0xb9b7
--- SIGSEGV (Segmentation fault) ---
+++ killed by SIGSEGV +++
I would have expected that this would give me a good backtrace. Is
this wrong?
-i
[EMAIL PROTECTED]
http://www.gelato.unsw.edu.au
pgpVMksDxYvrr.pgp
Description: PGP signature
Re: A question on debugging libraries
On Tue, Feb 10, 2004 at 11:39:31AM +1100, Ian Wienand wrote: Hi, I'm not sure if this is a bug or something I have misunderstood about the debugging libraries. I want to show an example of debugging a libc call, so just simply overflowed a buffer with strcpy. There's your problem. You are trying to get a backtrace. A walk up the stack frame, yes? So you generated a crash by overwriting the stack; naturally we can not backtrace. #0 0x74732067 in ?? () #1 0x676e6972 in ?? () If the top frames are lost, then there is nothing you can expect from the below frames. Besides, you're in main; there is only one or at most two frames on the stack anyway. -- Daniel Jacobowitz MontaVista Software Debian GNU/Linux Developer
Re: A question on debugging libraries
On Mon, Feb 09, 2004 at 07:49:53PM -0500, Daniel Jacobowitz wrote:
You are trying to get a backtrace. A walk up the stack frame, yes? So
you generated a crash by overwriting the stack; naturally we can not
backtrace.
doh, you are of course right.
Just for the archives, don't use a stack variable and try something
like
--- new program ---
#include stdio.h
#include string.h
char *b = A long string;
char *a;
int main(void)
{
strcpy(a, b);
}
---
and you'll correctly be able to debug it
Program received signal SIGSEGV, Segmentation fault.
strcpy (dest=0x0, src=0x80484a4 A long string) at
../sysdeps/generic/strcpy.c:40
40 ../sysdeps/generic/strcpy.c: No such file or directory.
in ../sysdeps/generic/strcpy.c
(gdb) info args
dest = 0x0
src = 0x80484a4 A long string
(gdb)
-i
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