From d89df1906d0805a1465e052ae80339c38f25db1b Mon Sep 17 00:00:00 2001
From: Nathan Reiner <nathan@nathanreiner.xyz>
Date: Thu, 26 Dec 2024 16:15:22 +0100
Subject: init commit

---
 bfc.S | 159 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 159 insertions(+)
 create mode 100644 bfc.S

(limited to 'bfc.S')

diff --git a/bfc.S b/bfc.S
new file mode 100644
index 0000000..44ee7ec
--- /dev/null
+++ b/bfc.S
@@ -0,0 +1,159 @@
+.global _start
+
+.data
+
+v_elf_header:
+	.byte 0x7f       # Magic Number
+	.ascii "ELF"     #
+	.byte 0x2        # 64-Bit
+	.byte 0x1        # little-endian
+	.byte 0x1        # ELFv1
+	.byte 0x0        # System V ABI
+	.quad 0x0        #
+	.hword 0x02      # Executable
+	.hword 0x3e      # AMD x86-64
+	.int 0x1         # ELFv1
+	.quad 0x401000   # Entry
+	.quad 0x40       # Program Header Offset
+	.quad 0x100      # TODO: Section Header Offset
+	.int 0x0         #
+	.hword 0x40      # ELF Header Size
+	.hword 0x38      # Program Header Table Entry Size
+	.hword 0x3       # TODO: Number Of Entries in Program Header Table
+	.hword 0x40      # Section Header Table Entry Size
+	.hword 0x3       # TODO: Number Of Entries in Section Header Table
+	.hword 0x2       # TODO: Index of Section Header Table Entry containing section names
+
+v_usage_message:
+	.asciz "bfc [input file] [output file]\n"
+
+v_input_fd:
+	.int 0x0
+
+v_output_fd:
+	.int 0x0
+
+v_buffer_start:
+	.quad 0x0
+
+v_buffer_end:
+	.quad 0x0
+
+v_buffer_cursor:
+	.quad 0x0
+
+
+.text
+_start:
+	mov %rsp, %rbp
+	call f_check_args
+	call f_open_files
+	call f_init_buffer
+
+	mov $v_buffer_cursor, %rbx
+	mov (%rbx), %rsi
+	add $0x300, %rsi
+	mov %rsi, (%rbx)
+	call f_check_buffer
+
+	mov $v_buffer_cursor, %rsi
+	mov (%rbx), %rsi
+	add $0x300, %rsi
+	mov %rsi, (%rbx)
+	call f_check_buffer
+
+	# ----
+
+	mov $1, %rax
+	mov %rcx, %rdi
+	mov $v_elf_header, %rsi
+	mov $0x40, %rdx
+	syscall
+
+	# ----
+
+	call f_close_files
+	mov $0, %rdi
+	jmp f_exit
+
+f_exit:
+	mov $60, %rax
+	syscall
+
+f_check_args:
+	mov (%rbp), %rax
+	cmp $3, %rax
+	je l_check_args_success
+
+	mov $1, %rax
+	mov $2, %rdi
+	mov $v_usage_message, %rsi
+	mov $32, %rdx
+	syscall
+
+	mov $-1, %rdi
+	jmp f_exit
+
+l_check_args_success:
+	ret
+
+f_open_files:
+	mov $2, %rax
+	mov 16(%rbp), %rdi
+	mov $0, %rsi
+	mov $0, %rdx
+	syscall
+	mov $v_input_fd, %rbx
+	mov %eax, (%rbx)
+
+	mov $2, %rax
+	mov 24(%rbp), %rdi
+	mov $0x41, %rsi
+	mov $0744, %rdx
+	syscall
+	mov $v_output_fd, %rbx
+	mov %eax, (%rbx)
+	ret
+
+f_close_files:
+	mov $3, %rax
+	mov %rbx, %rsi
+	syscall
+
+	mov $3, %rax
+	mov %rcx, %rsi
+	syscall
+	ret
+
+f_init_buffer:
+	mov $12, %rax
+	mov $0, %rdi
+	syscall
+	mov $v_buffer_start, %rbx
+	mov %rax, (%rbx)
+	mov $v_buffer_end, %rbx
+	mov %rax, (%rbx)
+	mov $v_buffer_cursor, %rbx
+	mov %rax, (%rbx)
+	/*
+	 * We directly skip to f_resize_buffer
+	 * since we want to resize it to a usable amount
+	 */
+f_resize_buffer:
+	mov $12, %rax
+	mov $v_buffer_end, %rbx
+	mov (%rbx), %rdi
+	add $0x400, %rdi
+	syscall
+	mov %rax, (%rbx)
+	ret
+
+/* %rsi = needed end */
+f_check_buffer:
+	mov $v_buffer_end, %rbx
+	mov (%rbx), %rax
+	cmp %rax, %rsi
+	jl l_check_buffer_large_enough
+	call f_resize_buffer
+l_check_buffer_large_enough:
+	ret
-- 
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