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How static and dynamically linked processes use system memory
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The following command line shows how you would use the GNU compiler to compile the source code file zapc with the debug options enabled Notice that this is accomplished by adding the -g option to the command line
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gcc -g zapc -o zapdebug
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There are three debug levels that display increasing amounts of information The default is level 2 Depending on the debug level, GCC may produce information to facilitate backtraces, descriptions of functions and external variables, local variables, and macro definitions The following is a listing of a directory that contains the log-wiping tool zap compiled dynamically, statically, and with debug options
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root@conan zap]# ls -al mtotal 1604 drwxr-xr-x 2 root root drwxr-xr-x 3 root root -rwxr-xr-x 1 root root -rwxr-xr-x 1 root root -rwxr-xr-x 1 root root -rwxr-xr-x 1 root root
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1024 1024 1972 25657 13217 1587273
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zapc zapdebug zapdynamic zapstatic
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Investigating Hacker Tools
Notice the size of each version The dynamically compiled zap is 13,217 bytes, and the static zap is 1,587,273 bytes in size The static zap binary file is more than 120 times larger than the dynamic zap binary file The debug version contains additional data, making it nearly twice the size of the dynamically compiled zap
Stripped Programs
Strip is a function that discards all symbols from the object code to make a file much smaller and perhaps more optimal for execution Since stripped, dynamically compiled programs result in the smallest size executable, these types of files are usually the most difficult for an investigator to analyze when using string and symbol extraction techniques For example, if the file has not been stripped and contains symbols, the nm command will display them Conversely, the strip command will remove that information The following command line demonstrates using the GNU version of strip and shows how much smaller the dynamically compiled, stripped version of zap is compared to the files created with other types of compilation Most utilities generate a new file, but strip modifies the actual content of the object file specified on the command line
[root@conan zap]# strip zapdynamic [root@conan zap]# ls -al total 1595 drwxr-xr-x 2 root root 1024 drwxr-xr-x 3 root root 1024 -rwxr-xr-x 1 root root 1972 -rwxr-xr-x 1 root root 25657 -rwxr-xr-x 1 root root 4400 -rwxr-xr-x 1 root root 1587273
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zapc zapdebug zapdynamic zapstatic
Notice that stripping the dynamically linked zap program (zapdynamic) shrinks the file size from its original size of 13,217 bytes (as shown in the previous section) to 4,400 bytes
Programs Packed with UPX
UPX, or the Ultimate Packer for eXecutables, is becoming increasingly popular as an effective compression tool for executable files Perhaps another reason for its popularity is that attackers can use it to obscure their illicit programs from signature-based IDS UPX will pack and unpack Linux and Win32 applications, as well as DOS 16-bit executable and com files, DOS 32-bit COFF files, DOS 32-bit executables, and Atari TOS/MiNT executables A review of the ASCII-formatted strings within the rogue code will show whether UPX was used to compress the executable, as shown in the example in Figure 15-2 If you
Incident Response & Computer Forensics
Symbol Extraction
If a file has not been stripped (with the strip command), an investigator may be able to analyze it using string and symbol extraction techniques To extract symbols from a Unix object file, use the nm command (-a means list all):
root@conan zap]# nm -a zap ---Truncated for brevity--08049a20 A __bss_start U bzero@@GLIBC_20 08048474 t call_gmon_start U close@@GLIBC_20 08049a20 b completed1 00000000 a zapc
In the nm command output, the first column is the symbol value in hexadecimal, followed by the symbol type, and then the symbol name For symbol types, if the character is lowercase, it represents a local variable Uppercase characters represent global (external) variables Here are some examples of symbol types: M I I I I I I L A indicates an absolute value (it won t be changed by further linking) B indicates an uninitialized data section C indicates a common section (uninitialized data) D indicates an initialized data section N indicates a debugging symbol R indicates a symbol in a read-only data section T indicates a symbol in a text or code data section U indicates an undefined symbol
When debugging information is included, nm s list line numbers command-line option, -l, may provide valuable information:
root@conan ---Excerpt 0804872a T 08048500 T 080485e0 T zap]# nm -al zapdebug --kill_lastlog /home/johndoe/zapc:59 kill_utmp /home/johndoe/zapc:17 kill_wtmp /home/johndoe/zapc:33
Compare this to the previous non-debug output, and you will notice that the kill_utmp function started at line 17 of the file zapc, which was in the directory /home/johndoe The kill_wtmp function started at line 33 of the source code, and
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