tiny-elf article

2023-05-21 19:25:19 +05:00 · 2023-05-21 19:25:19 +05:00 · f6c52abd83
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+Title:  Slim Summer Elf
+Brief:  Making of minimal x86 (Linux) ELF executable.
+Date:   1684666702
+Tags:   Programming, Linux, C
+CSS:    /style.css
+
+Code below was composed for [4mb-jam](https://itch.io/jam/4mb-jam-2023) which I didn't finish.
+
+It was tested on `Zorin OS 16.2 x86_64`, but should be compatible with any x86/i386
+operating system using ELF executable format (besides the system call part)
+
+What's going on:
+* Amalgamation of shell script and C source, it's possible because of them both having
+`#if` style directives and similar comment syntax. Also note the usage of shebang starting with `/`.
+* ELF header is created via `GNU Assembler`.
+* Custom link script allows for drop of unnecessary data produced by `GNU Linker` by default. There's *a lot* of it it turns out.
+* One can pass a `DEBUG` argument when compiling so that debug symbols would be generated.
+
+### Source ###
+
+```c
+//bin/sh
+#if 0
+set -e
+cat << ELF_HEADER_SOURCE > ./elf-header.as
+elfoff = 0x08048000
+vaddr = 0x08048000
+.text
+ehdrstart:
+  .byte      0x7F                       # e_ident
+  .ascii     "ELF"
+  .skip      3, 1
+  .skip      9, 0
+  .word      2                          # e_type
+  .word      3                          # e_machine
+  .long      1                          # e_version
+  .long      entry                      # e_entry
+  .long      phdrstart - elfoff         # e_phoff
+  .long      0                          # e_shoff
+  .long      0                          # e_flags
+  .word      ehdrsize                   # e_ehsize
+  .word      32                         # e_phentsize
+  .word      1                          # e_phnum
+  .skip      6, 0
+ehdrsize = . - ehdrstart
+
+phdrstart:
+#PT_LOAD
+  .long      1                          # p_type
+  .long      0                          # p_offset
+  .long      vaddr                      # p_vaddr
+  .long      0                          # p_paddr
+  .long      filesz                     # p_filesz
+  .long      memsz                      # p_memsz
+  .long      7                          # p_flags
+  .long      0x0000                     # p_align
+ELF_HEADER_SOURCE
+
+cat << LINKER_SCRIPT_SOURCE > ./ld.scr
+SECTIONS {
+  . = 0x08048000;
+  filestart = .;
+
+  .elf : { ./elf-header.o (.text) }
+  .text ALIGN(0x1) : SUBALIGN(0x1) { *(.text*) *(.rodata*) }
+  .data ALIGN(0x1) : SUBALIGN(0x1) { *(.data*) }
+
+  filesz = . - filestart;
+  .bss : { *(.bss*) }
+  memsz = . - filestart;
+
+  /DISCARD/ : {
+    *(.note.*)
+    *(.gnu*)
+    *(.gcc*)
+    *(.comment)
+    *(.eh_frame*)
+  }
+}
+OUTPUT_FORMAT(binary)
+LINKER_SCRIPT_SOURCE
+
+CFLAGS="-x c -std=gnu99 $0 -m32 -nostdlib -fno-pie -DELF -Wall -Wextra -Wpedantic -Werror"
+LDFLAGS="-m elf_i386"
+
+if [ ! -z "$1" ] && [ $1 = DEBUG ];
+then
+  CFLAGS="$CFLAGS -O0 -g3 -ggdb -DDEBUG"
+  LDFLAGS="$LDFLAGS -eentry"
+else
+  CFLAGS="$CFLAGS -Os"
+  LDFLAGS="$LDFLAGS -T ld.scr -s"
+fi
+
+as --32 -o elf-header.o elf-header.as
+cc -c -o ./elf.o $CFLAGS
+ld -o elf ./elf-header.o ./elf.o $LDFLAGS
+./elf
+exit;
+#endif
+
+#ifdef ELF
+
+/* https://github.com/Jorengarenar/CMObALL/blob/master/cmoball.h */
+
+/* https://man7.org/linux/man-pages/man2/exit.2.html */
+#define SYS_EXIT(p_return_code)                                                \
+  {                                                                            \
+    asm volatile("int $0x80" : : "a"(1), "b"(p_return_code));                  \
+    __builtin_unreachable();                                                   \
+  }
+
+/* https://man7.org/linux/man-pages/man2/write.2.html */
+#define SYS_WRITE(p_fd, p_msg, p_msg_len)                                      \
+  asm volatile("int $0x80"                                                     \
+               : "=a"(sys_result)                                              \
+               : "a"(4), "b"(p_fd), "c"(p_msg), "d"(p_msg_len))
+
+__attribute((naked)) void entry(void) {
+    static int sys_result;
+    SYS_WRITE(1, "hello world!\n", 13);
+    SYS_EXIT(0);
+}
+
+#endif /* #ifdef ELF */
+
+```