Every program (a compiled binary) is just a combination of code, data and metadata (i.e. data defining code & data). The code which is bunch of low level instructions deal with the data to produce some output. Since the code and data are embedded inside a single binary file, there should be a way for the machine to distinguish between them and the inability of the machine to do so may result in disastrous effects (which indeed is the root cause of code injection attacks).
When the source code is compiled, machine code generated is termed as 'object code' which may intermediately be stored in a file (termed as object file). Linker combines one or more of such object code files to produce a final executable binary. Object code generated is one of the blocks of the final executable (which may be made up of many such blocks).
NOTE: It may be helpful to know the details regarding translation of source code to executable form, some of which are covered in Memory Malware Part 0x1 — Introduction to the binary world.
For a machine to execute a binary program, it must know in advance how the code and data is laid out in the file , i.e. it needs to know the format of the binary program. A binary file format describes the arrangement of various parts of binary program and gives various interfaces for a smooth run of any program. Some of the most common binary file formats are:
- Executable and Linkable Fromat (ELF) for Linux
- Portable Eexecutable (PE) for Windows (Extensions -
.exe,.dll,.sys,.ocx,.cpl) - Mach-O for OSX.
For now, we will be focusing on the ELF executable format and perhaps later in this course we'll cover Mach-O and PE file format too.
Let's look at the simple binary file main compiled from source code present in this directory main.c. We'll use the tool hexdump to see the raw binary file.
Hexdump is a command line utility which formats the user supplied input files into a user specified format.
critical@d3ad:~/DISECTING_BINARIES_COURSE/Introduction$ echo "int main() {}" > main.c
critical@d3ad:~/DISECTING_BINARIES_COURSE/Introduction$ cat main.c
int main() {}
critical@d3ad:~/DISECTING_BINARIES_COURSE/Introduction$ gcc main.c -o main
critical@d3ad:~/DISECTING_BINARIES_COURSE/Introduction$ hexdump main
0000000 457f 464c 0102 0001 0000 0000 0000 0000
0000010 0003 003e 0001 0000 04f0 0000 0000 0000
0000020 0040 0000 0000 0000 18e0 0000 0000 0000
0000030 0000 0000 0040 0038 0009 0040 001c 001b
0000040 0006 0000 0004 0000 0040 0000 0000 0000
0000050 0040 0000 0000 0000 0040 0000 0000 0000
...Each output line constitutes of 16 bytes of the file.
0000000 457f 464c 0102 0001 0000 0000 0000 0000
0000010 0003 003e 0001 0000 04f0 0000 0000 0000A binary file is usually represented in hexadecimal format since a series of 0's and 1's probably don't make much sense to us humans.
- FIRST LINE : In the output above,
0000000is the offset to the first byte, i.e. 0x7f (not 0x45 - due to little endian byte order) in file. FILE OFFSET here means the distance (in bytes) of any byte from first byte of the file, which accounts to 0 since the first byte of this line (0x7f) itself is the first byte of the file. After this there are 8 groups (of 2 bytes each) which accounts for 16 bytes in the file. - SECOND LINE : The offset to the first byte in this line is
0x0000010(i.e. 16 bytes in decimal), therefore0x03(not0x00- due to little endian byte order) is 16 bytes away from the first byte in the file. This makes sense as each line of hexdump displays 16 bytes.
LITTLE ENDIAN byte order : It specifies the order in which the data bytes are stored in memory. Little Endian byte ordering explains that the least significant byte will be stored first and then the more significant bytes.
BIG ENDIAN byte order : It specifies that the most significant byte is stored first and then the lower significant bytes.
For Example : In the number 169, the least significant digit is 9 (at 1's place), then 6 (at 10's place) and the digit 1 here has the most significant place (100's), therefore the most significant digit. Similarly, suppose the hexadecimal number 0x080400ab in memory will be stored in little endian byte order i.e. - ab 00 04 08. Intel CPU's are little endian and will interpret this value 0x080400ab stored in memory as 0xab000408 (i.e. in revere order).
NOTE : These hexadecimal bytes would make more sense ahead.
Now that you know how to reach out to any offset/location in a binary and examine content, let's move furthur and understand the file type of this binary.
critical@d3ad:~/COURSE_DISECTING_BINARIES/Introduction$ file main
main: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=0d072f7bfdfee55c728743f9ac6e7e08309ed581, not strippedLet's make sense out of this information -
main: refers to name of the binaryELF 64-bit LSB shared object: It is a 64-bit compatible shared object file. Executable files and shared object files are similar to each other, having a few differences which I'll explain ahead. LSB says that it is encoded in 2's complement, little endian format.x86-64: It is compiled to run on a 64-bit platform (processor architecture).version 1 (SYSV): It is some field set to 0 in ELF header. (Not relevant right now)dynamically linked: Tells how is the linking of object files done i.e. static or dynamic. Linking is covered later in the course.interpreter /lib64/ld-linux-x86-64.so.2: Specifies location of the dynamic linker a.k.a program interpreter (more on this later).GNU/Linux 3.2.0: The kernel version for which the C library is compiled.BuildID[sha1]=0d072f7bfdfee55c728743f9ac6e7e08309ed581: Identifier for build session producing the binary. Useful while debugging.not stripped: Stripping a binary removes all the information that is useless for the binary to execute. It also makes a binary difficult to reverse engineer or debug.
This was just a glimpse of what comes ahead. Next, we'll move on to understanding the ELF's. Don't worry if any/some of the above fields does'nt make sense right now. Just move ahead and trust me you'll get more comfortable :)