126 lines
3.8 KiB
Plaintext
126 lines
3.8 KiB
Plaintext
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Title: Slim Summer Elf
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Brief: Making of minimal x86 (Linux) ELF executable.
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Date: 1684666702
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Tags: Programming, Linux, C
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CSS: /style.css
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Code below was composed for [4mb-jam](https://itch.io/jam/4mb-jam-2023) which I didn't finish.
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It was tested on `Zorin OS 16.2 x86_64`, but should be compatible with any x86/i386
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operating system using ELF executable format (besides the system call part)
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What's going on:
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* Amalgamation of shell script and C source, it's possible because of them both having
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`#if` style directives and similar comment syntax. Also note the usage of shebang starting with `/`.
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* ELF header is created via `GNU Assembler`.
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* Custom link script allows for drop of unnecessary data produced by `GNU Linker` by default. There's *a lot* of it it turns out.
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* One can pass a `DEBUG` argument when compiling so that debug symbols would be generated.
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### Source ###
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```c
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//bin/sh
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#if 0
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set -e
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cat << ELF_HEADER_SOURCE > ./elf-header.as
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elfoff = 0x08048000
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vaddr = 0x08048000
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.text
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ehdrstart:
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.byte 0x7F # e_ident
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.ascii "ELF"
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.skip 3, 1
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.skip 9, 0
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.word 2 # e_type
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.word 3 # e_machine
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.long 1 # e_version
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.long entry # e_entry
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.long phdrstart - elfoff # e_phoff
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.long 0 # e_shoff
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.long 0 # e_flags
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.word ehdrsize # e_ehsize
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.word 32 # e_phentsize
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.word 1 # e_phnum
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.skip 6, 0
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ehdrsize = . - ehdrstart
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phdrstart:
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#PT_LOAD
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.long 1 # p_type
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.long 0 # p_offset
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.long vaddr # p_vaddr
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.long 0 # p_paddr
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.long filesz # p_filesz
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.long memsz # p_memsz
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.long 7 # p_flags
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.long 0x0000 # p_align
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ELF_HEADER_SOURCE
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cat << LINKER_SCRIPT_SOURCE > ./ld.scr
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SECTIONS {
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. = 0x08048000;
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filestart = .;
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.elf : { ./elf-header.o (.text) }
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.text ALIGN(0x1) : SUBALIGN(0x1) { *(.text*) *(.rodata*) }
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.data ALIGN(0x1) : SUBALIGN(0x1) { *(.data*) }
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filesz = . - filestart;
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.bss : { *(.bss*) }
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memsz = . - filestart;
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/DISCARD/ : {
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*(.note.*)
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*(.gnu*)
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*(.gcc*)
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*(.comment)
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*(.eh_frame*)
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}
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}
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OUTPUT_FORMAT(binary)
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LINKER_SCRIPT_SOURCE
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CFLAGS="-x c -std=gnu99 $0 -m32 -nostdlib -fno-pie -DELF -Wall -Wextra -Wpedantic -Werror"
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LDFLAGS="-m elf_i386"
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if [ ! -z "$1" ] && [ $1 = DEBUG ];
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then
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CFLAGS="$CFLAGS -O0 -g3 -ggdb -DDEBUG"
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LDFLAGS="$LDFLAGS -eentry"
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else
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CFLAGS="$CFLAGS -Os"
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LDFLAGS="$LDFLAGS -T ld.scr -s"
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fi
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as --32 -o elf-header.o elf-header.as
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cc -c -o ./elf.o $CFLAGS
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ld -o elf ./elf-header.o ./elf.o $LDFLAGS
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./elf
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exit;
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#endif
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#ifdef ELF
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/* https://man7.org/linux/man-pages/man2/exit.2.html */
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#define SYS_EXIT(p_return_code) \
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{ \
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asm volatile("int $0x80" : : "a"(1), "b"(p_return_code)); \
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__builtin_unreachable(); \
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}
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/* https://man7.org/linux/man-pages/man2/write.2.html */
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#define SYS_WRITE(p_fd, p_msg, p_msg_len) \
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asm volatile("int $0x80" \
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: "=a"(sys_result) \
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: "a"(4), "b"(p_fd), "c"(p_msg), "d"(p_msg_len))
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__attribute((naked)) void entry(void) {
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static int sys_result;
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SYS_WRITE(1, "hello world!\n", 13);
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SYS_EXIT(0);
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}
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#endif /* #ifdef ELF */
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```
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