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Diffstat (limited to 'gdb-experiment.go')
-rw-r--r--gdb-experiment.go291
1 files changed, 243 insertions, 48 deletions
diff --git a/gdb-experiment.go b/gdb-experiment.go
index 893d5b8..cb976b9 100644
--- a/gdb-experiment.go
+++ b/gdb-experiment.go
@@ -1,14 +1,21 @@
-// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64;
-// gofmt has been tried, with disappointing results
+// 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 )
@@ -124,16 +131,15 @@ func le(unknown interface{}) []byte {
// Trying hard to avoid reflect.Value.Int/Uint
formatted := fmt.Sprintf("%d", unknown)
- var v uint64
+ b := make([]byte, 8)
if unsigned, err := strconv.ParseUint(formatted, 10, 64); err == nil {
- v = unsigned
+ binary.LittleEndian.PutUint64(b, unsigned)
} else if signed, err := strconv.ParseInt(formatted, 10, 64); err == nil {
- v = uint64(signed)
+ binary.LittleEndian.PutUint64(b, uint64(signed))
} else {
panic("cannot convert to number")
}
- return []byte{byte(v), byte(v >> 8), byte(v >> 16), byte(v >> 24),
- byte(v >> 32), byte(v >> 40), byte(v >> 48), byte(v >> 56)}
+ return b
}
func (a *codegen) append(v []byte) { a.buf = append(a.buf, v...) }
@@ -154,8 +160,8 @@ const (
SYS_EXIT = 60
)
-func codegenAmd64(irb []instruction) []byte {
- offsets := make([]int, len(irb)+1)
+func codegenAmd64(irb []instruction) (code []byte, offsets []int) {
+ offsets = make([]int, len(irb)+1)
a := codegen{}
a.code("\xB8").dd(ElfDataAddr) // mov rax, "ElfCodeAddr"
@@ -268,7 +274,7 @@ func codegenAmd64(irb []instruction) []byte {
}
copy(a.buf[fixup:], le(target - fixup - 4)[:4])
}
- return a.buf
+ return a.buf, offsets
}
// --- Main --------------------------------------------------------------------
@@ -301,55 +307,244 @@ func main() {
// ... various optimizations could be performed here if we give up brevity
pairLoops(irb)
dump("ir-dump.txt", irb)
+ code, offsets := codegenAmd64(irb)
- code := codegenAmd64(irb)
- a := codegen{}
+// - - ELF generation - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- // TODO: also use the constants in package "debug/elf"
+ // 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
- ElfPrologSize = ElfHeaderSize + 2*ElfProgramEntrySize
+ ElfHeaderSize = 64 // Size of the ELF header
+ ElfProgramEntrySize = 56 // Size of a program header
+ ElfSectionEntrySize = 64 // Size of a section header
)
- // ELF header
- a.code("\x7FELF\x02\x01\x01") // ELF, 64-bit, little endian, v1
- // Unix System V ABI, v0, padding
- a.code("\x00\x00" + "\x00\x00\x00\x00\x00\x00\x00")
- a.dw(2).dw(62).dd(1) // executable, x86-64, v1
- a.dq(ElfCodeAddr + ElfPrologSize) // entry point address
-
- // We only append section headers with debugging info with DEBUG
- a.dq(ElfHeaderSize).dq(0) // program, section header offset
- a.dd(0) // no processor-specific flags
- a.dw(ElfHeaderSize) // ELF header size
- a.dw(ElfProgramEntrySize).dw(2) // program hdr tbl entry size, count
- a.dw(ElfSectionEntrySize).dw(0) // section hdr tbl entry size, count
- a.dw(0) // no section index for strings
+// - - 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
- a.dd(1).dd(5) // PT_LOAD, PF_R | PF_X
- a.dq(0) // offset within the file
- a.dq(ElfCodeAddr) // address in virtual memory
- a.dq(ElfCodeAddr) // address in physical memory
- a.dq(ElfPrologSize + len(code)) // length within the file
- a.dq(ElfPrologSize + len(code)) // length within memory
- a.dq(4096) // segment alignment
+ 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
- a.dd(1).dd(6) // PT_LOAD, PF_R | PF_W
- a.dq(0) // offset within the file
- a.dq(ElfDataAddr) // address in virtual memory
- a.dq(ElfDataAddr) // address in physical memory
- a.dq(0) // length within the file
- a.dq(1 << 20) // one megabyte of memory
- a.dq(4096) // segment alignment
-
- a.buf = append(a.buf, code...)
- if err = ioutil.WriteFile(outputPath, a.buf, 0777); err != nil {
+ 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(1 << 20) // 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{}
+
+// - - 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) - codeOffset) // Byte size
+ sh.dd(0).dd(0) // No link, no info
+ sh.dq(0).dq(0) // No alignment, no entry size
+ shCount++
+
+// - - 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).dq(ElfCodeAddr + codeOffset)
+ 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...)
+ lineEntry.buf = append(lineEntry.buf, lineProgram.buf...)
+
+ debugLine := codegen{}
+ debugLine.dd(len(lineEntry.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
+ shCount++
+
+ pieces = append(pieces, debugLine.buf)
+ position += len(debugLine.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)
+ cuEntry.dq(ElfCodeAddr + codeOffset)
+ cuEntry.dq(ElfCodeAddr + codeEndOffset)
+ cuEntry.dd(0)
+
+ debugInfo := codegen{}
+ debugInfo.dd(len(cuEntry.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
+ shCount++
+
+ pieces = append(pieces, debugInfo.buf)
+ position += len(debugInfo.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")
+ bin.dw(elf.ET_EXEC).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)
}
}
generated by cgit v1.2.3-70-g09d2 (git 2.47.0) at 2024-11-24 06:16:50 +0000