FEAT Added subroutines to the list of skills taught in this tutorial.
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@ -6,3 +6,5 @@ hello32
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hello64
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hello64
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counted-hello32
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counted-hello32
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counted-hello64
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counted-hello64
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subroutine-hello32
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subroutine-hello64
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8
Makefile
8
Makefile
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@ -23,5 +23,13 @@ counted-hello64: counted-hello64.s ## Build the 32 bit version of Project 3
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nasm -f elf64 counted-hello64.s
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nasm -f elf64 counted-hello64.s
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ld -o counted-hello64 counted-hello64.o
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ld -o counted-hello64 counted-hello64.o
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subroutine-hello32: subroutine-hello32.s ## Build the 32 bit version of Project 3
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nasm -f elf subroutine-hello32.s
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ld -m elf_i386 -o subroutine-hello32 subroutine-hello32.o
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subroutine-hello64: subroutine-hello64.s ## Build the 32 bit version of Project 3
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nasm -f elf64 subroutine-hello64.s
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ld -o subroutine-hello64 subroutine-hello64.o
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clean: ## Delete all built and intermediate features
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clean: ## Delete all built and intermediate features
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rm -f hello32 hello64 counted-hello32 counted-hello64 *.o
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rm -f hello32 hello64 counted-hello32 counted-hello64 *.o
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@ -0,0 +1,66 @@
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;; Hello World Program #1
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;; Compile with: nasm -f elf hello.s
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;; Link with: ld -m elf_i386 -o hello hello.o
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;; Run with: ./hello
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;; sys/unistd_32.h
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%define SYS_write 4
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%define SYS_exit 1
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;; unistd.h
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%define STDOUT 1
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section .data
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msg db "Okay, so we're doing 32-bit subroutines now, huh?", 0Ah, 00h
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section .text
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global _start
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_start:
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mov eax, msg ; Put the address of our message into eax.
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call strlen ; call the function strlen. We're using EAX as our argument.
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mov edx, eax ; We know printit wants these two
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mov eax, msg
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call printit
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call exit
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strlen:
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push ebx ; Push ebx onto the stack, since the calling scope
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; will probably want its state restored correctly, right?
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mov ebx, eax
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strlen_next:
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cmp byte [eax], 0
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jz strlen_done
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inc eax
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jmp strlen_next
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strlen_done:
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sub eax, ebx ; Straight from the counted-hello file
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pop ebx ; restore EBX for the calling scope, which is
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; expecting its answer in eax
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ret
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;; Takes EAX as the address of the message and EDX as the
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;; length, and prints them. Restores all used registers
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;; when finished.
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printit:
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push eax
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push ebx
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push ecx
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mov ecx, eax
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mov ebx, STDOUT
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mov eax, SYS_write
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int 80h
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pop ecx
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pop ebx
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pop eax
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ret
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;; Since this terminates the program, I'm not worried about
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;; managing the stack correctly.
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exit:
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mov ebx, 0
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mov eax, SYS_exit
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int 80h
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@ -0,0 +1,66 @@
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;; Hello World Program #1
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;; Compile with: nasm -f elf hello.s
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;; Link with: ld -m elf_i386 -o hello hello.o
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;; Run with: ./hello
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;; sys/unistd_32.h
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%define SYS_write 1
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%define SYS_exit 60
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;; unistd.h
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%define STDOUT 1
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section .data
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msg db "Okay, so we're doing 64-bit subroutines now, huh?", 0Ah, 00h
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section .text
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global _start
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_start:
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mov rsi, msg ; Put the address of our message into rax.
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call strlen ; call the function strlen. We're using RAX as our argument.
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;; At this point, RDX is length and RSI still points to the source.
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;; The differences here make me wonder if my understanding of the 32-bit
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;; version are incorrect, since now the two fields that matter are
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;; already populated.
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;;
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;; Note that this is non-standard. RAX is usually assumed to hold
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;; the return value, but I'm cheating by knowing that printit
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;; wants the result of strlen in RDX. Whether this is exploitable
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;; in a compiler (you know, that whole "choosing registers"
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;; question) is something to ponder.
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call printit
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call exit
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strlen:
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mov rdx, rsi
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strlen_next:
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cmp byte [rdx], 0
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jz strlen_done
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inc rdx
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jmp strlen_next
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strlen_done:
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sub rdx, rsi
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ret
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;; Takes RDI as the address of the message and RDX as the length,
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;; and prints them. Restores all used registers when finished.
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printit:
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push rdi
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push rax
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mov rdi, STDOUT ; using STDOUT (see definition above)
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mov rax, SYS_write ; Using WRITE in 32-bit mode?
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syscall
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pop rax
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pop rdx
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pop rdi
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;; Since this terminates the program, I'm not worried about
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;; managing the stack correctly.
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exit:
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mov rdi, 0
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mov rax, SYS_exit
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syscall
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