elf - Difference between p_filesz and p_memsz of Elf32_Phdr - Stack Overflow
As you've already stated in your comment, the p_filesz field corresponds to the segment's size in bytes in the file, whereas the p_memsz is the. int $0x80 is indeed executed by some of the C library functions in the statically linked binary, but not by nearly as many library functions as one. What is the relationship between these two functions, if any? We can see that quite a bit of information is simply not present in optimized code.
The ELF format also allows executable files to be dynamically linked.
Instead of linking all the source files that contain subroutines into the final binaries, separate binaries are organized in libraries that can be loaded per use case, on demand.
Essentially, the libraries are loaded only once into memory and when a program instance requires a subroutine from a specific library. In this case, it inquires a special OS component about it and new resources are allocated only for the volatile parts of the library image. ELF header readelf -h: ELF sections readelf -S: The ELF segments are missing, as they are built during linking.
What symbols are there in the symbol table? Additionally, object files have a relocation table, i. Let's look at hello.
ELF - OSDev Wiki
Offset Info Type Sym. We notice that one of the external symbols is puts. Since that is part of the C library, the linker must resolve its location and replace all occurences with the symbol's address. This is where the difference between static and dynamic linking shows.
Executable and Linkable Format 101 Part 3: Relocations
During static linking, the actual symbol address is filled in; while dynamic linking uses dynamic relocation tables the Global Offset Table and the Procedure Linkage Table whose values are resolved by the loader. More details on this in the next lab. What does -M do? In general we encourage you to check out the manpages to find out.
Sometimes however it is possible that the code we are dealing with doesn't have any useful metadata associated with it, e. Let's take for example the hello2 binary generated from hello2.
S in the lab archive: In this case however, objdump does not assume any target architecture, so we must pass it explicitly using -m. S source file, we notice that the disassembled code maps almost directly. The last part of the binary does not contain any meaningful code, because here objdump attempts to also disassemble data. Code is also data! This has interesting security implications, as we will see throughout the course. To obtain raw data we can just dump the binary using hexdump or xxd: Shellcode [2p] The purpose of this task is to get you acquainted with some tools that can be used to manipulate ELF files.
Inspect the source code of shellcode.
LOAD 0xe28 0xe28 0xe28 0x 0x RW a trick that might work is making the stack executable execstack -s. Compile run and save the generated shellcode Compile gcc -O0 -o shellcode shellcode. Is it a false positive? File is reading some magic bytes, this is misleading Try to execute. Who is throwing the error? The loader, which resides in the operating system How to actually run the generated shellcode.
The problem so far is that the shellcode SC ends in a segment that does not have the executable bit set. One solution to this is, at runtime, remap the segment page with the exec flag — this solution requires writing some code.
We can focus on another solution: ELF object file from the raw binary objcopy -I binary -O elfx Where are the segments? It should be WA! The segments are linktime info, we didn't link yet Adjust the. How many arguments are passed in a function call? The even less basics: Deducing function argument types from disassembly of object code is referred to as type recovery, and is closely related to variable recovery. Both are difficult problems and the subject of research.
The notion of variables and types does not exist in object code. A variable name is a label that is given a memory address which corresponds to the data located at that address. While type information is necessary for the compiler to evaluate syntatical and semantic correctness of source code, object code that is executed directly by the CPU does not preserve this information at least not directly.
The type recovery task, which gives a high-level type to each variable, is more challenging. Type recovery is challenging because high-level types are typically thrown away by the compiler early on in the compilation process.
Lab 03 - Executables. Static Analysis [CS Open CourseWare]
Within the compiled code itself we have byte-addressable memory and registers. For example, if a variable is put into eax, it is easy to conclude that it is of a type compatible with bit register, but difficult to infer high-level types such as signed integers, pointers, unions, and structures. Current solutions to type recovery take either a dynamic approach, which results in poor program coverage, or use unprincipled heuristics, which often given incorrect results.
I suggest you do give them different physical pages too if you don't want to end up with modifiable code.
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- The even less basics: variable type recovery
The most relevant sections for this project are 1. The steps involved in identifying the sections of the ELF file are: Read the ELF Header. The ELF header contains information about how the rest of the file is laid out. You are interested only in the program headers. Find the Program Headers, which specify where in the file to find the text and data sections and where they should end up in the executable image.
There are a few simplifying assumptions you can make about the types and location of program headers. In the files you will be working with, there will always be one text header and one data header. The text header will be the first program header and the data header will be the second program header.