From eaa1125b7cbffd55a22c20cd417ff0ccebb64db0 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?P=C5=99emysl=20Janouch?= Date: Sun, 25 Jun 2017 03:51:03 +0200 Subject: [PATCH] gdb-object-file.go: discard program headers --- gdb-object-file.go | 54 +++++++++++----------------------------------- 1 file changed, 13 insertions(+), 41 deletions(-) diff --git a/gdb-object-file.go b/gdb-object-file.go index 2f777e3..f849be6 100644 --- a/gdb-object-file.go +++ b/gdb-object-file.go @@ -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