diff options
| -rw-r--r-- | gdb-object-file.go | 702 |
1 files changed, 702 insertions, 0 deletions
diff --git a/gdb-object-file.go b/gdb-object-file.go new file mode 100644 index 0000000..2f777e3 --- /dev/null +++ b/gdb-object-file.go @@ -0,0 +1,702 @@ +// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64 +// with debugging information mapping instructions onto an IR dump. +// gofmt has been tried, with disappointing results. +// codegen{} is also pretty ugly in the way it works but damn convenient. +package main + +import ( + "encoding/binary" + "errors" + "fmt" + "io/ioutil" + "log" + "os" + "strconv" + + // Let's not repeat all those constants here onstants + "debug/dwarf" + "debug/elf" +) + +const ( RIGHT = iota; LEFT; INC; DEC; IN; OUT; BEGIN; END ) + +var info = []struct { + grouped bool + name string +}{ + {true, "RIGHT"}, + {true, "LEFT"}, + {true, "INC"}, + {true, "DEC"}, + {false, "IN"}, + {false, "OUT"}, + {false, "BEGIN"}, + {false, "END"}, +} + +type instruction struct { + command int + arg int +} + +// Dump internal representation to a file for debugging purposes +func dump(filename string, irb []instruction) error { + out, err := os.Create(filename) + if err != nil { + return err + } + + indent := 0 + for _, x := range irb { + if x.command == END { + indent-- + } + for i := 0; i < indent; i++ { + out.WriteString(" ") + } + out.WriteString(info[x.command].name) + if info[x.command].grouped { + fmt.Fprintf(out, " %d", x.arg) + } + out.WriteString("\n") + if x.command == BEGIN { + indent++ + } + } + if err = out.Close(); err != nil { + return err + } + return nil +} + +// Decode a Brainfuck program into internal representation, +// coalescing identical commands together as the most basic optimization +func decode(program []byte) (irb []instruction) { + for _, c := range program { + var command int + switch c { + case '>': command = RIGHT + case '<': command = LEFT + case '+': command = INC + case '-': command = DEC + case '.': command = OUT + case ',': command = IN + case '[': command = BEGIN + case ']': command = END + default: continue + } + + if len(irb) == 0 || !info[command].grouped || + irb[len(irb)-1].command != command { + irb = append(irb, instruction{command, 1}) + } else { + irb[len(irb)-1].arg++ + } + } + return +} + +// Match loop commands so that we know where to jump +func pairLoops(irb []instruction) error { + nesting := 0 + stack := make([]int, len(irb)) + for i, x := range irb { + switch x.command { + case BEGIN: + stack[nesting] = i + nesting++ + case END: + if nesting <= 0 { + return errors.New("unbalanced loops") + } + nesting-- + irb[stack[nesting]].arg = i + 1 + irb[i].arg = stack[nesting] + 1 + } + } + if nesting != 0 { + return errors.New("unbalanced loops") + } + return nil +} + +// --- Code generation --------------------------------------------------------- + +type codegen struct { + buf []byte +} + +// Convert an arbitrary integral value up to 8 bytes long to little endian +func le(unknown interface{}) []byte { + // Trying hard to avoid reflect.Value.Int/Uint + formatted := fmt.Sprintf("%d", unknown) + + b := make([]byte, 8) + if unsigned, err := strconv.ParseUint(formatted, 10, 64); err == nil { + binary.LittleEndian.PutUint64(b, unsigned) + } else if signed, err := strconv.ParseInt(formatted, 10, 64); err == nil { + binary.LittleEndian.PutUint64(b, uint64(signed)) + } else { + panic("cannot convert to number") + } + return b +} + +func (a *codegen) append(v []byte) { a.buf = append(a.buf, v...) } +func (a *codegen) code(v string) *codegen { a.append([]byte(v)); return a } +func (a *codegen) db(v interface{}) *codegen { a.append(le(v)[:1]); return a } +func (a *codegen) dw(v interface{}) *codegen { a.append(le(v)[:2]); return a } +func (a *codegen) dd(v interface{}) *codegen { a.append(le(v)[:4]); return a } +func (a *codegen) dq(v interface{}) *codegen { a.append(le(v)[:8]); return a } + +const ( + ElfCodeAddr = 0x400000 // Where the code is loaded in memory + ElfDataAddr = 0x800000 // Where the tape is placed in memory +) + +const ( + SYS_READ = 0 + SYS_WRITE = 1 + SYS_EXIT = 60 +) + +func codegenAmd64(irb []instruction) (code []byte, offsets []int, tapeoff int) { + offsets = make([]int, len(irb)+1) + a := codegen{} + + // The linker may _add_ to the offset even with explicit addends (.rela) + tapeoff = 1 + a.code("\xB8").dd(0) // mov rax, "tape" + a.code("\x30\xDB") // xor bl, bl + + for i, x := range irb { + offsets[i] = len(a.buf) + if x.command == LEFT || x.command == RIGHT { + a.code("\x88\x18") // mov [rax], bl + } + switch x.command { + case RIGHT: a.code("\x48\x05").dd(x.arg) // add rax, "arg" + case LEFT: a.code("\x48\x2D").dd(x.arg) // sub rax, "arg" + case INC: a.code("\x80\xC3").db(x.arg) // add bl, "arg" + case DEC: a.code("\x80\xEB").db(x.arg) // sub bl, "arg" + case OUT: a.code("\xE8").dd(0) // call "write" + case IN: a.code("\xE8").dd(0) // call "read" + case BEGIN: + // test bl, bl; jz "offsets[arg]" + a.code("\x84\xDB" + "\x0F\x84").dd(0) + case END: + // test bl, bl; jnz "offsets[arg]" + a.code("\x84\xDB" + "\x0F\x85").dd(0) + } + if x.command == LEFT || x.command == RIGHT { + a.code("\x8A\x18") // mov bl, [rax] + } + } + // When there is a loop at the end we need to be able to jump past it + offsets[len(irb)] = len(a.buf) + + // Write an epilog which handles all the OS interfacing + // + // System V x86-64 ABI: + // rax <-> both syscall number and return value + // args -> rdi, rsi, rdx, r10, r8, r9 + // trashed <- rcx, r11 + + a.code("\xB8").dd(SYS_EXIT) // mov eax, 0x3c + a.code("\x48\x31\xFF") // xor rdi, rdi + a.code("\x0F\x05") // syscall + + fatal := len(a.buf) + a.code("\x48\x89\xF7") // mov rdi, rsi -- use the string in rsi + a.code("\x30\xC0") // xor al, al -- look for the nil byte + a.code("\x48\x31\xC9") // xor rcx, rcx + a.code("\x48\xF7\xD1") // not rcx -- start from -1 + a.code("\xFC" + "\xF2\xAE") // cld; repne scasb -- decrement until found + a.code("\x48\xF7\xD1") // not rcx + a.code("\x48\x8D\x51\xFF") // lea rdx, [rcx-1] -- save length in rdx + a.code("\xB8").dd(SYS_WRITE) // mov eax, "SYS_WRITE" + a.code("\xBF").dd(2) // mov edi, "STDERR_FILENO" + a.code("\x0F\x05") // syscall + + a.code("\xB8").dd(SYS_EXIT) // mov eax, "SYS_EXIT" + a.code("\xBF").dd(1) // mov edi, "EXIT_FAILURE" + a.code("\x0F\x05") // syscall + + read := len(a.buf) + a.code("\x50") // push rax -- save tape position + a.code("\xB8").dd(SYS_READ) // mov eax, "SYS_READ" + a.code("\x48\x89\xC7") // mov rdi, rax -- STDIN_FILENO + a.code("\x66\x6A\x00") // push word 0 -- the default value for EOF + a.code("\x48\x89\xE6") // mov rsi, rsp -- the char starts at rsp + a.code("\xBA").dd(1) // mov edx, 1 -- count + a.code("\x0F\x05") // syscall + a.code("\x66\x5B") // pop bx + + a.code("\x48\x83\xF8\x00") // cmp rax, 0 + a.code("\x48\x8D\x35").dd(4) // lea rsi, [rel read_message] + a.code("\x7C") // jl "fatal_offset" -- write failure message + a.db(fatal - len(a.buf) - 1) + a.code("\x58") // pop rax -- restore tape position + a.code("\xC3") // ret + a.code("fatal: read failed\n\x00") + + write := len(a.buf) + a.code("\x50") // push rax -- save tape position + a.code("\xB8").dd(SYS_WRITE) // mov eax, "SYS_WRITE" + a.code("\x48\x89\xC7") // mov rdi, rax -- STDOUT_FILENO + a.code("\x66\x53") // push bx + a.code("\x48\x89\xE6") // mov rsi, rsp -- the char starts at rsp + a.code("\xBA").dd(1) // mov edx, 1 -- count + a.code("\x0F\x05") // syscall + a.code("\x66\x5B") // pop bx + + a.code("\x48\x83\xF8\x00") // cmp rax, 0 + a.code("\x48\x8D\x35").dd(4) // lea rsi, [rel write_message] + a.code("\x7C") // jl "fatal_offset" -- write failure message + a.db(fatal - len(a.buf) - 1) + a.code("\x58") // pop rax -- restore tape position + a.code("\xC3") // ret + a.code("fatal: write failed\n\x00") + + // Now that we know where each instruction is, fill in relative jumps + for i, x := range irb { + // This must accurately reflect the code generators + target, fixup := 0, offsets[i] + if x.command == BEGIN || x.command == END { + fixup += 4 + target = offsets[x.arg] + } else if x.command == IN { + fixup += 1 + target = read + } else if x.command == OUT { + fixup += 1 + target = write + } else { + continue + } + copy(a.buf[fixup:], le(target - fixup - 4)[:4]) + } + return a.buf, offsets, tapeoff +} + +// --- Main -------------------------------------------------------------------- + +func main() { + var err error + if len(os.Args) > 3 { + log.Fatalf("usage: %s [INPUT-FILE] [OUTPUT-FILE]", os.Args[0]) + } + + input := os.Stdin + if len(os.Args) > 1 { + if input, err = os.Open(os.Args[1]); err != nil { + log.Fatalf("%s", err) + } + } + + outputPath := "a.out" + if len(os.Args) > 2 { + outputPath = os.Args[2] + } + + program, err := ioutil.ReadAll(input) + input.Close() + if err != nil { + log.Fatalf("can't read program: %s", err) + } + + irb := decode(program) + // ... various optimizations could be performed here if we give up brevity + pairLoops(irb) + dump("ir-dump.txt", irb) + code, offsets, tapeoff := codegenAmd64(irb) + +// - - ELF generation - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + // Now that we know how long the machine code is, we can write the header. + // Note that for PIE we would need to depend on the dynamic linker, so no. + // + // Recommended reading: + // http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html + // man 5 elf + // + // In case of unexpected gdb problems, also see: + // DWARF4.pdf + // https://sourceware.org/elfutils/DwarfLint + // http://wiki.osdev.org/DWARF + + const ( + ElfHeaderSize = 64 // Size of the ELF header + ElfProgramEntrySize = 56 // Size of a program header + ElfSectionEntrySize = 64 // Size of a section header + ElfDataSize = 1 << 20 // Tape length + ) + +// - - Program headers - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + ph := codegen{} + phCount := 2 + + codeOffset := ElfHeaderSize + phCount*ElfProgramEntrySize + codeEndOffset := codeOffset + len(code) + + // Program header for code + // The entry point address seems to require alignment, so map start of file + ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_X) + ph.dq(0) // Offset within the file + ph.dq(ElfCodeAddr) // Address in virtual memory + ph.dq(ElfCodeAddr) // Address in physical memory + ph.dq(codeEndOffset) // Length within the file + ph.dq(codeEndOffset) // Length within memory + ph.dq(4096) // Segment alignment + + // Program header for the tape + ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_W) + ph.dq(0) // Offset within the file + ph.dq(ElfDataAddr) // Address in virtual memory + ph.dq(ElfDataAddr) // Address in physical memory + ph.dq(0) // Length within the file + ph.dq(ElfDataSize) // One megabyte of memory + ph.dq(4096) // Segment alignment + + // Now that the rigid part has been generated, we can append sections + pieces := [][]byte{ph.buf, code} + position := codeEndOffset + +// - - Sections - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + sh := codegen{} + shCount := 0 + + // This section is created on the go as we need to name other sections + stringTable := codegen{} + + // A null section is needed by several GNU tools + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code("\x00") + sh.dd(elf.SHT_NULL).dq(0).dq(0) // Type, no flags, no memory address + sh.dq(0).dq(0) // Byte offset, byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shCount++ + +// - - Text - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".text\x00") + sh.dd(elf.SHT_PROGBITS) + sh.dq(elf.SHF_ALLOC | elf.SHF_EXECINSTR) + sh.dq(ElfCodeAddr + codeOffset) // Memory address + sh.dq(codeOffset) // Byte offset + sh.dq(len(code)) // Byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shTextIndex := shCount + shCount++ + +// - - BSS - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".bss\x00") + sh.dd(elf.SHT_NOBITS) + sh.dq(elf.SHF_ALLOC | elf.SHF_WRITE) + sh.dq(ElfDataAddr) // Memory address + sh.dq(0) // Byte offset + sh.dq(ElfDataSize) // Byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shBSSIndex := shCount + shCount++ + +// - - Symbol table - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + symtab := codegen{} + symstrtab := codegen{} + + // A null symbol is needed by several GNU tools + + symtab.dd(len(symstrtab.buf)) // Index for symbol name + symstrtab.code("\x00") + symtab.db(elf.ST_INFO(elf.STB_LOCAL, elf.STT_NOTYPE)) + symtab.db(elf.STV_DEFAULT) // Default visibility rules + symtab.dw(0).dq(0).dq(0) // No section, no offset, no length + + symtab.dd(len(symstrtab.buf)) // Index for symbol name + symstrtab.code("tape\x00") + symtab.db(elf.ST_INFO(elf.STB_LOCAL, elf.STT_OBJECT)) + symtab.db(elf.STV_DEFAULT) // Default visibility rules + symtab.dw(shBSSIndex) // Relative to section .bss + symtab.dq(0) // Right at the start of BSS + symtab.dq(ElfDataSize) // Span the entire section + + symtab.dd(len(symstrtab.buf)) // Index for symbol name + symstrtab.code("_start\x00") + symtab.db(elf.ST_INFO(elf.STB_GLOBAL, elf.STT_FUNC)) + symtab.db(elf.STV_DEFAULT) // Default visibility rules + symtab.dw(shTextIndex) // Relative to section .text + symtab.dq(0) // Right at the start of code + symtab.dq(len(code)) // Span the entire section + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".symstrtab\x00") + sh.dd(elf.SHT_STRTAB).dq(0).dq(0) // Type, no flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(symstrtab.buf)) // Byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shSymstrtabIndex := shCount + shCount++ + + pieces = append(pieces, symstrtab.buf) + position += len(symstrtab.buf) + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".symtab\x00") + sh.dd(elf.SHT_SYMTAB).dq(0).dq(0) // Type, no flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(symtab.buf)) // Byte size + sh.dd(shSymstrtabIndex).dd(2) // Link, info: index of first non-local + sh.dq(0).dq(24) // No alignment, entry size + shSymtabIndex := shCount + shCount++ + + pieces = append(pieces, symtab.buf) + position += len(symtab.buf) + +// - - Text relocation records - - - - - - - - - - - - - - - - - - - - - - - - - + + textRel := codegen{} + // Relocation record for code[tapeoff] += &tape + textRel.dq(tapeoff).dq(elf.R_INFO(1, uint32(elf.R_X86_64_32))) + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".rel.text\x00") + sh.dd(elf.SHT_REL) // Type + sh.dq(elf.SHF_INFO_LINK).dq(0) // Flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(textRel.buf)) // Byte size + sh.dd(shSymtabIndex).dd(shTextIndex)// Link, info + sh.dq(0).dq(16) // No alignment, entry size + shCount++ + + pieces = append(pieces, textRel.buf) + position += len(textRel.buf) + +// - - Debug line - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + const ( + opcodeBase = 13 // Offset by DWARF4 standard opcodes + lineBase = 0 // We don't need negative line indexes + lineRange = 2 // Either we advance a line or not (we always do) + ) + + // FIXME: we use db() a lot instead of a proper un/signed LEB128 encoder; + // that means that values > 127/63 or < 0 would break it; + // see Appendix C to DWARF4.pdf for an algorithm + + lineProgram := codegen{} + // Extended opcode DW_LNE_set_address to reset the PC to the start of code + lineProgram.db(0).db(1 + 8).db(2) + lineAddressOff := len(lineProgram.buf) + lineProgram.dq(0) + if len(irb) > 0 { + lineProgram.db(opcodeBase + offsets[0] * lineRange) + } + // The epilog, which is at the very end of the offset array, is included + for i := 1; i <= len(irb); i++ { + size := offsets[i] - offsets[i - 1] + lineProgram.db(opcodeBase + (1 - lineBase) + size * lineRange) + } + // Extended opcode DW_LNE_end_sequence is mandatory at the end + lineProgram.db(0).db(1).db(1) + + lineHeader := codegen{} + lineHeader.db(1) // Minimum instruction length + lineHeader.db(1) // Maximum operations per instruction + lineHeader.db(1) // default_is_stmt + lineHeader.db(lineBase) + lineHeader.db(lineRange) + + lineHeader.db(opcodeBase) + // Number of operands for all standard opcodes (1..opcodeBase-1) + opcodeLengths := []byte{0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1} + lineHeader.buf = append(lineHeader.buf, opcodeLengths...) + + // include_directories []string \x00 + lineHeader.db(0) + // file_names []struct{base string; dir u8; modified u8; length u8} \x00 + lineHeader.code("ir-dump.txt\x00").db(0).db(0).db(0).db(0) + + lineEntry := codegen{} + lineEntry.dw(4) // .debug_line version number + lineEntry.dd(len(lineHeader.buf)) + lineEntry.buf = append(lineEntry.buf, lineHeader.buf...) + lineAddressOff += len(lineEntry.buf) + lineEntry.buf = append(lineEntry.buf, lineProgram.buf...) + + debugLine := codegen{} + debugLine.dd(len(lineEntry.buf)) + lineAddressOff += len(debugLine.buf) + debugLine.buf = append(debugLine.buf, lineEntry.buf...) + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".debug_line\x00") + sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(debugLine.buf)) // Byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shLineIndex := shCount + shCount++ + + pieces = append(pieces, debugLine.buf) + position += len(debugLine.buf) + +// - - Debug line relocation records - - - - - - - - - - - - - - - - - - - - - - + + lineRel := codegen{} + // Relocation record for debug_line[lineAddressOff] += &_start + lineRel.dq(lineAddressOff).dq(elf.R_INFO(2, uint32(elf.R_X86_64_64))) + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".rel.debug_line\x00") + sh.dd(elf.SHT_REL) // Type + sh.dq(elf.SHF_INFO_LINK).dq(0) // Flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(lineRel.buf)) // Byte size + sh.dd(shSymtabIndex).dd(shLineIndex)// Link, info + sh.dq(0).dq(16) // No alignment, entry size + shCount++ + + pieces = append(pieces, lineRel.buf) + position += len(lineRel.buf) + +// - - Debug abbreviations - - - - - - - - - - - - - - - - - - - - - - - - - - - + + const ( + formAddr = 0x01 // Pointer size + formSecOffset = 0x17 // DWARF size + ) + + debugAbbrev := codegen{} + debugAbbrev.db(1) // Our abbreviation code + debugAbbrev.db(dwarf.TagCompileUnit) + debugAbbrev.db(0) // DW_CHILDREN_no + debugAbbrev.db(dwarf.AttrLowpc).db(formAddr) + debugAbbrev.db(dwarf.AttrHighpc).db(formAddr) + debugAbbrev.db(dwarf.AttrStmtList).db(formSecOffset) + debugAbbrev.db(0).db(0) // End of attributes + debugAbbrev.db(0) // End of abbreviations + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".debug_abbrev\x00") + sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(debugAbbrev.buf)) // Byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shCount++ + + pieces = append(pieces, debugAbbrev.buf) + position += len(debugAbbrev.buf) + +// - - Debug info - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + cuEntry := codegen{} + cuEntry.dw(4) // .debug_info version number + cuEntry.dd(0) // Offset into .debug_abbrev + cuEntry.db(8) // Pointer size + + // Single compile unit as per .debug_abbrev + cuEntry.db(1) + infoStartOff := len(cuEntry.buf) + cuEntry.dq(0) + infoEndOff := len(cuEntry.buf) + cuEntry.dq(len(code)) + cuEntry.dd(0) + + debugInfo := codegen{} + debugInfo.dd(len(cuEntry.buf)) + infoStartOff += len(debugInfo.buf) + infoEndOff += len(debugInfo.buf) + debugInfo.buf = append(debugInfo.buf, cuEntry.buf...) + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".debug_info\x00") + sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(debugInfo.buf)) // Byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shInfoIndex := shCount + shCount++ + + pieces = append(pieces, debugInfo.buf) + position += len(debugInfo.buf) + +// - - Debug info relocation records - - - - - - - - - - - - - - - - - - - - - - + + infoRel := codegen{} + // Relocation record for debug_info[info{Start,End}Off] += &_start + infoRel.dq(infoStartOff).dq(elf.R_INFO(2, uint32(elf.R_X86_64_64))) + infoRel.dq(infoEndOff).dq(elf.R_INFO(2, uint32(elf.R_X86_64_64))) + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".rel.debug_info\x00") + sh.dd(elf.SHT_REL) // Type + sh.dq(elf.SHF_INFO_LINK).dq(0) // Flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(infoRel.buf)) // Byte size + sh.dd(shSymtabIndex).dd(shInfoIndex)// Link, info + sh.dq(0).dq(16) // No alignment, entry size + shCount++ + + pieces = append(pieces, infoRel.buf) + position += len(infoRel.buf) + +// - - Section names and section table - - - - - - - - - - - - - - - - - - - - - + + sh.dd(len(stringTable.buf)) // Index for the name of the section + stringTable.code(".shstrtab\x00") + sh.dd(elf.SHT_STRTAB).dq(0).dq(0) // Type, no flags, no memory address + sh.dq(position) // Byte offset + sh.dq(len(stringTable.buf)) // Byte size + sh.dd(0).dd(0) // No link, no info + sh.dq(0).dq(0) // No alignment, no entry size + shCount++ + + pieces = append(pieces, stringTable.buf) + position += len(stringTable.buf) + + pieces = append(pieces, sh.buf) + // Don't increment the position, we want to know where section headers start + +// - - Final assembly of parts - - - - - - - - - - - - - - - - - - - - - - - - - + + bin := codegen{} + + // ELF header + bin.code("\x7FELF\x02\x01\x01") // ELF, 64-bit, little endian, v1 + // Unix System V ABI, v0, padding + bin.code("\x00\x00" + "\x00\x00\x00\x00\x00\x00\x00") + // The BFD linker will happily try to link ET_EXEC though + bin.dw(elf.ET_REL).dw(elf.EM_X86_64).dd(elf.EV_CURRENT) + bin.dq(ElfCodeAddr + codeOffset) // Entry point address + bin.dq(ElfHeaderSize) // Program header offset + bin.dq(position) // Section header offset + bin.dd(0) // No processor-specific flags + bin.dw(ElfHeaderSize) // ELF header size + bin.dw(ElfProgramEntrySize) // Program header table entry size + bin.dw(phCount) // Program header table entry count + bin.dw(ElfSectionEntrySize) // Section header table entry size + bin.dw(shCount) // Section header table entry count + bin.dw(shCount - 1) // Section index for strings + + for _, x := range pieces { + bin.buf = append(bin.buf, x...) + } + if err = ioutil.WriteFile(outputPath, bin.buf, 0777); err != nil { + log.Fatalf("%s", err) + } +} |