Segmentation fault

This is a discussion on Segmentation fault within the C Programming forums, part of the General Programming Boards category; Here's the deal I'm not getting a segmentation fault when I'm suppose to. Code: #include <stdio.h> int call_me(void) { char ...

  1. #1
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    Segmentation fault

    Here's the deal I'm not getting a segmentation fault when I'm suppose to.

    Code:
    #include <stdio.h> 
     
    int call_me(void) 
    { 
      char buffer[10]; 
      scanf("%s", buffer); 
      printf("%s\n", buffer); 
      return 0; 
    } 
     
    int main(void) 
    { 
      call_me(); 
      return 0; 
    }
    Memory is accessed in words (4 bytes or 32 bits) so buffer[10] is really 12 bytes. Ok, so anything over 12 should overflow it and give a segmentation fault, correct? Well, this is not the case....

    Code:
    [nouse@localhost nouse]$ gcc -o vuln vuln.c
    [nouse@localhost nouse]$ ./vuln
    AAAABBBBCCCCDDDD
    AAAABBBBCCCCDDDD
    [nouse@localhost nouse]$ ./vuln
    AAAABBBBCCCCDDDDEEEE
    AAAABBBBCCCCDDDDEEEE
    [nouse@localhost nouse]$ ./vuln
    AAAABBBBCCCCDDDDEEEEFFFF
    AAAABBBBCCCCDDDDEEEEFFFF
    Segmentation fault
    [nouse@localhost nouse]$
    So the first attempt I use 16 bytes which should overflow the buffer (12 bytes) and sfp (saved frame pointer (4 bytes)) there after. Doesn't do it. Next, I use 20 bytes, which should overflow the buffer, spf, and ret (saved ip (4 bytes)). Finally, after 24 bytes I get a seg fault. 24 bytes to overflow a 12 byte buffer? What's going on?

    Also, gdb isn't working with me either. I try to check my registers to see what's going on but it says there arn't any.

    Code:
    [nouse@localhost nouse]$ gdb vuln
    GNU gdb 5.3-22mdk (Mandrake Linux)
    Copyright 2002 Free Software Foundation, Inc.
    GDB is free software, covered by the GNU General Public License, and you are
    welcome to change it and/or distribute copies of it under certain conditions.
    Type "show copying" to see the conditions.
    There is absolutely no warranty for GDB.  Type "show warranty" for details.
    This GDB was configured as "i586-mandrake-linux-gnu"...
    (gdb) info registers
    The program has no registers now.
    (gdb)
    I must be missing something? Thanks for any help.
    Last edited by NoUse; 03-26-2005 at 01:05 PM.

  2. #2
    Gawking at stupidity
    Join Date
    Jul 2004
    Location
    Oregon, USA
    Posts
    3,170
    It's not guaranteed to generate a segfault if you write past the end of a buffer. It's just guaranteed to give "undefined results". Why are you trying to define an undefined result? There's no purpose in this type of exercise unless you're trying to do something malicious or some pointless trick.

    EDIT: BTW, the reason "gdb isn't working" is because you're using it incorrectly. The program has to be running to get register information. Try using breakpoints.

    Something like this:
    Code:
    itsme@itsme:~/C$ gdb malicious
    GNU gdb 6.1.1
    Copyright 2004 Free Software Foundation, Inc.
    GDB is free software, covered by the GNU General Public License, and you are
    welcome to change it and/or distribute copies of it under certain conditions.
    Type "show copying" to see the conditions.
    There is absolutely no warranty for GDB.  Type "show warranty" for details.
    This GDB was configured as "i486-slackware-linux"...Using host libthread_db library "/lib/libthread_db.so.1".
    
    (gdb) list
    4       {
    5         char buffer[10];
    6         scanf("%s", buffer);
    7         printf("%s\n", buffer);
    8         return 0;
    9       }
    10
    11      int main(void)
    12      {
    13        call_me();
    (gdb) break 6
    Breakpoint 1 at 0x80483ba: file malicious.c, line 6.
    (gdb) break 7
    Breakpoint 2 at 0x80483cd: file malicious.c, line 7.
    (gdb) run
    Starting program: /home/itsme/C/malicious
    
    Breakpoint 1, call_me () at malicious.c:6
    6         scanf("%s", buffer);
    (gdb) info registers
    eax            0x10     16
    ecx            0x4      4
    edx            0xb7fd0b40       -1208153280
    ebx            0xb7fcf60c       -1208158708
    esp            0xbffff720       0xbffff720
    ebp            0xbffff748       0xbffff748
    esi            0xb8000900       -1207957248
    edi            0xbffff7b4       -1073743948
    eip            0x80483ba        0x80483ba
    eflags         0x282    642
    cs             0x73     115
    ss             0x7b     123
    ds             0x7b     123
    es             0x7b     123
    fs             0x0      0
    gs             0x0      0
    (gdb) continue
    Continuing.
    foo
    
    Breakpoint 2, call_me () at malicious.c:7
    7         printf("%s\n", buffer);
    (gdb) info registers
    eax            0x1      1
    ecx            0x0      0
    edx            0x1      1
    ebx            0xb7fcf60c       -1208158708
    esp            0xbffff720       0xbffff720
    ebp            0xbffff748       0xbffff748
    esi            0xb8000900       -1207957248
    edi            0xbffff7b4       -1073743948
    eip            0x80483cd        0x80483cd
    eflags         0x246    582
    cs             0x73     115
    ss             0x7b     123
    ds             0x7b     123
    es             0x7b     123
    fs             0x0      0
    gs             0x0      0
    (gdb) quit
    Last edited by itsme86; 03-26-2005 at 01:39 PM.
    If you understand what you're doing, you're not learning anything.

  3. #3
    Registered User
    Join Date
    Feb 2005
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    38
    Quote Originally Posted by itsme86
    It's not guaranteed to generate a segfault if you write past the end of a buffer. It's just guaranteed to give "undefined results". Why are you trying to define an undefined result? There's no purpose in this type of exercise unless you're trying to do something malicious or some pointless trick.

    EDIT: BTW, the reason "gdb isn't working" is because you're using it incorrectly. The program has to be running to get register information. Try using breakpoints.

    Something like this:
    Code:
    itsme@itsme:~/C$ gdb malicious
    GNU gdb 6.1.1
    Copyright 2004 Free Software Foundation, Inc.
    GDB is free software, covered by the GNU General Public License, and you are
    welcome to change it and/or distribute copies of it under certain conditions.
    Type "show copying" to see the conditions.
    There is absolutely no warranty for GDB.  Type "show warranty" for details.
    This GDB was configured as "i486-slackware-linux"...Using host libthread_db library "/lib/libthread_db.so.1".
    
    (gdb) list
    4       {
    5         char buffer[10];
    6         scanf("%s", buffer);
    7         printf("%s\n", buffer);
    8         return 0;
    9       }
    10
    11      int main(void)
    12      {
    13        call_me();
    (gdb) break 6
    Breakpoint 1 at 0x80483ba: file malicious.c, line 6.
    (gdb) break 7
    Breakpoint 2 at 0x80483cd: file malicious.c, line 7.
    (gdb) run
    Starting program: /home/itsme/C/malicious
    
    Breakpoint 1, call_me () at malicious.c:6
    6         scanf("%s", buffer);
    (gdb) info registers
    eax            0x10     16
    ecx            0x4      4
    edx            0xb7fd0b40       -1208153280
    ebx            0xb7fcf60c       -1208158708
    esp            0xbffff720       0xbffff720
    ebp            0xbffff748       0xbffff748
    esi            0xb8000900       -1207957248
    edi            0xbffff7b4       -1073743948
    eip            0x80483ba        0x80483ba
    eflags         0x282    642
    cs             0x73     115
    ss             0x7b     123
    ds             0x7b     123
    es             0x7b     123
    fs             0x0      0
    gs             0x0      0
    (gdb) continue
    Continuing.
    foo
    
    Breakpoint 2, call_me () at malicious.c:7
    7         printf("%s\n", buffer);
    (gdb) info registers
    eax            0x1      1
    ecx            0x0      0
    edx            0x1      1
    ebx            0xb7fcf60c       -1208158708
    esp            0xbffff720       0xbffff720
    ebp            0xbffff748       0xbffff748
    esi            0xb8000900       -1207957248
    edi            0xbffff7b4       -1073743948
    eip            0x80483cd        0x80483cd
    eflags         0x246    582
    cs             0x73     115
    ss             0x7b     123
    ds             0x7b     123
    es             0x7b     123
    fs             0x0      0
    gs             0x0      0
    (gdb) quit
    I wasn't saying "gdb isn't working" I was saying "gdb isn't working for me" implying that I'm doing something wrong, so no need to be so condescending. I appreciate you help though. Also, I figured out that it isn't entirely necessary to set breakpoints, all you have to do is "run" it.

    Also, I'm not doing anything malicious. Just learning.

    Good day. Thanks again for your help.
    Last edited by NoUse; 03-26-2005 at 02:09 PM.

  4. #4
    Gawking at stupidity
    Join Date
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    Location
    Oregon, USA
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    3,170
    I wasn't trying to be condescending. I suggested breakpoints so you could check the registers at specific lines in your code. Just make sure you compile your program with the -g option.
    If you understand what you're doing, you're not learning anything.

  5. #5
    Registered User
    Join Date
    Feb 2005
    Posts
    38
    Oh I see. Thanks.
    I like to play pocket pool.

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