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gdb-object-file.go: discard program headers

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This commit is contained in:
Přemysl Eric Janouch
2017-06-25 03:51:03 +02:00
parent 03957f0069
commit eaa1125b7c
1 changed files with 13 additions and 41 deletions
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54
gdb-object-file.go
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@@ -1,4 +1,4 @@
// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64
// Non-optimizing Brainfuck compiler generating object files 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.
@@ -149,11 +149,6 @@ 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
@@ -332,36 +327,9 @@ func main() {
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
codeOffset := ElfHeaderSize
pieces := [][]byte{code}
position := codeOffset + len(code)
// - - Sections - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
@@ -387,7 +355,7 @@ func main() {
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(0) // Memory address
sh.dq(codeOffset) // Byte offset
sh.dq(len(code)) // Byte size
sh.dd(0).dd(0) // No link, no info
@@ -401,7 +369,7 @@ func main() {
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) // Memory address
sh.dq(0) // Byte offset
sh.dq(ElfDataSize) // Byte size
sh.dd(0).dd(0) // No link, no info
@@ -466,6 +434,10 @@ func main() {
// - - Text relocation records - - - - - - - - - - - - - - - - - - - - - - - - -
// ld.gold doesn't support SHT_REL, with SHT_RELA it overrides the target.
// ld.bfd addends to the target even with SHT_RELA.
// Thus, with SHT_RELA the target needs to be all zeros to be portable.
textRel := codegen{}
// Relocation record for code[tapeoff] += &tape
textRel.dq(tapeoff).dq(elf.R_INFO(1, uint32(elf.R_X86_64_32)))
@@ -682,13 +654,13 @@ func main() {
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(0) // Entry point address
bin.dq(0) // 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(0) // 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