The document discusses creating a minimal ELF (Executable and Linkable Format) file by hand. It explains that an ELF file contains a general header with metadata like the system architecture and endianness. It also contains a program header specifying the memory layout and permissions. The document gives an example of writing a "Hello World" program manually by writing the string to memory, setting registers, and calling syscalls to write to stdout and exit. It then introduces the concept of self-modifying code where the program writes instructions directly to memory at runtime.

1. ELF 101
cmj
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2. • If you know how to
• create a minimal ELF file by hand
• self-modified code (SMC)
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3. You can pass this slides !
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4. ELF
• Executable and Linkable Format
• Means it can
• Execute as a binary
• Linked by another binary
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5. • Build a minimal ELF by hand
• General header
• Program header
• Machine code
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6. • General Header - Basic ELF information
• System - 32 / 64 bits
• Architecture - AMD / ARM / PPC / …
• Class - Big / Little Endian
• … etc
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7. • Program Header - Running and Code Location
• Memory type - RWX
• Memory address location from
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8. • Machine Code - Translate Instruction to Code
• Register - RAX / ESP / RIP
• Memory - 0x1000178
• syscall table - syscall / int
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9. See Hex
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10. General Header
x86_64 / executable
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11. Program Header
only one program segment
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12. Give an Example
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13. • How to say hello world
• Need write something to stdout
• Write string into men
• Setup register
• Call syscall
• Need to exit normally
• Setup register
• Call syscall
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14. • How to say hello world
• Need write something to stdout
• Write string into mem
• Setup register
• Call syscall
• Need to exit normally
• Setup register
• Call syscall
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15. • How to say hello world
• Need write something to stdout
• Write string into mem
• Setup register
• Call syscall
• Need to exit normally
• Setup register
• Call syscall
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16. Hello World
X86_64 instruction set
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17. This is TRIVIAL part
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18. Next Part
~ Self-Modify ~
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19. How and Why
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20. • How
• The normal way - Write something into men
• The abnormal way - Write something into mem
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21. • Code are store in memory
• mov data from register into mem
• mov 0x1000689(rax) rax
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22. @bnormal Way
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23. • Buffer Overflow
• Stack Variable/Function are store as stack.
• Write something to variable === Write to mem
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24. Conclusion
• SMC in ASM is the supported method, if
• Find out where can write to
• How many you can write to
• Then, just write the code into memory
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25. Thanks for your attention ~
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