gdb-object-file.go: discard program headers
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@ -1,4 +1,4 @@
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// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64
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// Non-optimizing Brainfuck compiler generating object files for Linux on x86-64
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// with debugging information mapping instructions onto an IR dump.
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// gofmt has been tried, with disappointing results.
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// codegen{} is also pretty ugly in the way it works but damn convenient.
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@ -149,11 +149,6 @@ func (a *codegen) dw(v interface{}) *codegen { a.append(le(v)[:2]); return a }
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func (a *codegen) dd(v interface{}) *codegen { a.append(le(v)[:4]); return a }
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func (a *codegen) dq(v interface{}) *codegen { a.append(le(v)[:8]); return a }
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const (
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ElfCodeAddr = 0x400000 // Where the code is loaded in memory
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ElfDataAddr = 0x800000 // Where the tape is placed in memory
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)
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const (
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SYS_READ = 0
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SYS_WRITE = 1
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@ -332,36 +327,9 @@ func main() {
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ElfDataSize = 1 << 20 // Tape length
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)
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// - - Program headers - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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ph := codegen{}
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phCount := 2
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codeOffset := ElfHeaderSize + phCount*ElfProgramEntrySize
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codeEndOffset := codeOffset + len(code)
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// Program header for code
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// The entry point address seems to require alignment, so map start of file
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ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_X)
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ph.dq(0) // Offset within the file
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ph.dq(ElfCodeAddr) // Address in virtual memory
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ph.dq(ElfCodeAddr) // Address in physical memory
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ph.dq(codeEndOffset) // Length within the file
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ph.dq(codeEndOffset) // Length within memory
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ph.dq(4096) // Segment alignment
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// Program header for the tape
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ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_W)
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ph.dq(0) // Offset within the file
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ph.dq(ElfDataAddr) // Address in virtual memory
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ph.dq(ElfDataAddr) // Address in physical memory
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ph.dq(0) // Length within the file
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ph.dq(ElfDataSize) // One megabyte of memory
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ph.dq(4096) // Segment alignment
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// Now that the rigid part has been generated, we can append sections
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pieces := [][]byte{ph.buf, code}
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position := codeEndOffset
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codeOffset := ElfHeaderSize
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pieces := [][]byte{code}
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position := codeOffset + len(code)
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// - - Sections - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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@ -387,7 +355,7 @@ func main() {
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stringTable.code(".text\x00")
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sh.dd(elf.SHT_PROGBITS)
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sh.dq(elf.SHF_ALLOC | elf.SHF_EXECINSTR)
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sh.dq(ElfCodeAddr + codeOffset) // Memory address
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sh.dq(0) // Memory address
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sh.dq(codeOffset) // Byte offset
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sh.dq(len(code)) // Byte size
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sh.dd(0).dd(0) // No link, no info
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@ -401,7 +369,7 @@ func main() {
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stringTable.code(".bss\x00")
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sh.dd(elf.SHT_NOBITS)
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sh.dq(elf.SHF_ALLOC | elf.SHF_WRITE)
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sh.dq(ElfDataAddr) // Memory address
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sh.dq(0) // Memory address
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sh.dq(0) // Byte offset
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sh.dq(ElfDataSize) // Byte size
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sh.dd(0).dd(0) // No link, no info
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@ -466,6 +434,10 @@ func main() {
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// - - Text relocation records - - - - - - - - - - - - - - - - - - - - - - - - -
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// ld.gold doesn't support SHT_REL, with SHT_RELA it overrides the target.
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// ld.bfd addends to the target even with SHT_RELA.
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// Thus, with SHT_RELA the target needs to be all zeros to be portable.
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textRel := codegen{}
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// Relocation record for code[tapeoff] += &tape
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textRel.dq(tapeoff).dq(elf.R_INFO(1, uint32(elf.R_X86_64_32)))
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@ -682,13 +654,13 @@ func main() {
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bin.code("\x00\x00" + "\x00\x00\x00\x00\x00\x00\x00")
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// The BFD linker will happily try to link ET_EXEC though
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bin.dw(elf.ET_REL).dw(elf.EM_X86_64).dd(elf.EV_CURRENT)
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bin.dq(ElfCodeAddr + codeOffset) // Entry point address
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bin.dq(ElfHeaderSize) // Program header offset
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bin.dq(0) // Entry point address
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bin.dq(0) // Program header offset
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bin.dq(position) // Section header offset
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bin.dd(0) // No processor-specific flags
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bin.dw(ElfHeaderSize) // ELF header size
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bin.dw(ElfProgramEntrySize) // Program header table entry size
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bin.dw(phCount) // Program header table entry count
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bin.dw(0) // Program header table entry count
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bin.dw(ElfSectionEntrySize) // Section header table entry size
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bin.dw(shCount) // Section header table entry count
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bin.dw(shCount - 1) // Section index for strings
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