Browse Source

gdb-experiment.go: add DWARF debugging information

Now we can debug compiled Brainfuck!
master
Přemysl Janouch 2 years ago
parent
commit
def3218714
Signed by: Přemysl Janouch <p.janouch@gmail.com> GPG Key ID: B715679E3A361BE6
3 changed files with 258 additions and 48 deletions
  1. +3
    -0
      .gitignore
  2. +12
    -0
      README.adoc
  3. +243
    -48
      gdb-experiment.go

+ 3
- 0
.gitignore View File

@@ -1,6 +1,9 @@
a.out
gdb-experiment
bfc-amd64-*

ir-dump.txt

bfc.creator*
bfc.config
bfc.files

+ 12
- 0
README.adoc View File

@@ -35,6 +35,18 @@ When no input file is specified, standard input is used. Similarly, the default
output filename is `a.out`. After the compilation, the resulting file can be
run on the target platform.

gdb
---
You may have noticed the `gdb-experiment.go` file. It is a non-optimizing
version of the compiler targeting Linux only that adds DWARF debugging
information mapping code locations onto lines in the `ir-dump.txt` byproduct
output file. It's been rewritten in Go since managing all those binary buffers
required to build the symbol table proved to be too painful in C.

$ go run gdb-experiment.go [INPUT-FILE] [OUTPUT-FILE]

Use `break *0x4000b7` to get a breakpoint at the first Brainfuck instruction.

Contributing and Support
------------------------
Use this project's GitHub to report any bugs, request features, or submit pull

+ 243
- 48
gdb-experiment.go View File

@@ -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)
}
}

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