322 lines
8.8 KiB
Go
322 lines
8.8 KiB
Go
// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
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// SPDX-License-Identifier: BSD-3-Clause
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package asm
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import (
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"encoding/binary"
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"fmt"
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)
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// This file emits ELF64 relocatable objects (ET_REL) from an assembled
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// Image: a .text section holding the function bodies, a .data section
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// holding the GLOBL initialisers, a symbol table with one symbol per TEXT
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// and GLOBL (file-local <> symbols are STB_LOCAL, the rest STB_GLOBAL), and
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// a .rela.text relocation table — one R_X86_64_PC32 entry per static-symbol
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// reference, internal references resolving against the local data symbols
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// and external ones against undefined globals. The output links with the
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// system toolchain (cc/ld) the way a hand-assembled .o would.
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const (
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elfClass64 = 2
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elfDataLSB = 1
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elfVersion = 1
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etREL = 1 // relocatable object
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emX8664 = 62
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shtNull = 0
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shtProgbits = 1
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shtSymtab = 2
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shtStrtab = 3
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shtRela = 4
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shfWrite = 1
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shfAlloc = 2
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shfExecInstr = 4
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stbGlobal = 1
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sttObject = 1
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sttFunc = 2
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sttSection = 3
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stInfoShift = 4
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rX8664PC32 = 2
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)
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// elfSym is one symbol-table entry in construction.
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type elfSym struct {
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name string
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info byte
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shndx uint16
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value uint64
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size uint64
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}
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// ELFObject returns the image as an ELF64 relocatable object file, ready for
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// the system linker. Symbol names are the TEXT and GLOBL identifiers as
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// written (the middle dot stripped); a package prefix, when present, is
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// joined with a dot. Every static-symbol reference becomes an
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// R_X86_64_PC32 relocation, so the code is position-independent and links
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// at any address.
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func (img *Image) ELFObject() ([]byte, error) {
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le := binary.LittleEndian
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// Section indices: 0 NULL, 1 .text, 2 .data; the tables follow.
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const (
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secText = 1
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secData = 2
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)
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// Build the symbol table: the null entry and the two section symbols
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// come first, then the local symbols (static TEXT and GLOBL), then the
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// globals (exported TEXT and GLOBL, and the undefined externals) — ELF
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// requires every local to precede every global, and sh_info records the
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// boundary. symIdx maps a symbol name to its index for the relocations.
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var locals, globals []elfSym
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for _, fn := range img.Funcs {
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s := elfSym{
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name: objectName(fn.Pkg, fn.Name),
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info: sttFunc,
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shndx: secText,
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value: uint64(fn.Offset),
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size: uint64(fn.Size),
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}
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if fn.Static {
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locals = append(locals, s)
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} else {
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s.info |= stbGlobal << stInfoShift
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globals = append(globals, s)
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}
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}
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for _, d := range img.DataSyms {
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s := elfSym{
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name: objectName(d.Pkg, d.Name),
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info: sttObject,
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shndx: secData,
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value: uint64(d.Offset),
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size: uint64(d.Size),
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}
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if d.Static {
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locals = append(locals, s)
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} else {
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s.info |= stbGlobal << stInfoShift
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globals = append(globals, s)
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}
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}
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for _, name := range img.Externals {
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globals = append(globals, elfSym{name: name, info: stbGlobal << stInfoShift})
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}
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syms := []elfSym{
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{}, // the mandatory null entry
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{name: ".text", info: sttSection, shndx: secText},
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{name: ".data", info: sttSection, shndx: secData},
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}
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syms = append(syms, locals...)
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shInfo := len(syms) // first global symbol
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syms = append(syms, globals...)
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symIdx := map[string]int{}
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for i, s := range syms {
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symIdx[s.name] = i
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}
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// Build the relocations.
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type elfRela struct {
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off uint64
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sym int
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addend int64
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}
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var relas []elfRela
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for _, fn := range img.Funcs {
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for _, r := range fn.Relocs {
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idx, ok := symIdx[r.Name]
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if !ok {
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return nil, fmt.Errorf("relocation references unknown symbol %q", r.Name)
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}
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relas = append(relas, elfRela{
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off: uint64(fn.Offset + r.Off),
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sym: idx,
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// R_X86_64_PC32 computes S + A − P with P the patch site; the
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// assembler measures the symbol from the instruction end,
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// After − Off bytes past the field, so the addend carries
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// that distance with a negative sign.
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addend: r.Addend - int64(r.After-r.Off),
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})
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}
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}
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// Serialise the string tables.
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stNames := newElfStrtab()
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for _, s := range syms {
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stNames.add(s.name)
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}
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stSections := newElfStrtab()
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for _, n := range []string{".text", ".data", ".symtab", ".strtab", ".rela.text", ".shstrtab"} {
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stSections.add(n)
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}
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for _, n := range dwarfSectionNames {
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stSections.add(n)
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}
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// Section presence: .rela.text only when there are relocations.
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hasRela := len(relas) > 0
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nSections := 6 // NULL, .text, .data, .symtab, .strtab, .shstrtab
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if hasRela {
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nSections = 7
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}
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secSymtab, secStrtab := 3, 4
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secShstr := nSections - 1
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// Lay the file out: header, section data, section headers.
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var out []byte
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out = append(out, make([]byte, 64)...) // ELF header, filled last
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align := func(n int) {
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for len(out)%n != 0 {
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out = append(out, 0)
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}
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}
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align(16)
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textOff := len(out)
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out = append(out, img.Code...)
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align(16)
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dataOff := len(out)
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out = append(out, img.Data...)
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align(8)
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symtabOff := len(out)
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for _, s := range syms {
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var b [24]byte
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le.PutUint32(b[0:], uint32(stNames.at(s.name)))
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b[4] = s.info
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b[5] = 0 // st_other
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le.PutUint16(b[6:], s.shndx)
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le.PutUint64(b[8:], s.value)
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le.PutUint64(b[16:], s.size)
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out = append(out, b[:]...)
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}
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strtabOff := len(out)
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out = append(out, stNames.bytes()...)
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var relaOff int
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if hasRela {
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align(8)
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relaOff = len(out)
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for _, r := range relas {
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var b [24]byte
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le.PutUint64(b[0:], r.off)
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le.PutUint64(b[8:], uint64(r.sym)<<32|rX8664PC32)
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le.PutUint64(b[16:], uint64(r.addend))
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out = append(out, b[:]...)
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}
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}
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shstrOff := len(out)
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out = append(out, stSections.bytes()...)
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// DWARF debug sections (no relocations — the linker resolves DWARF fixups).
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dwAlign := func(n int) {
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for len(out)%n != 0 {
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out = append(out, 0)
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}
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}
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dw := appendDWARFSections(&out, img, "gasm.s", symIdx, dwAlign)
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if dw != nil {
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nSections += 4 // .debug_abbrev, .debug_info, .debug_line, .debug_line_str
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}
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align(8)
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shoff := len(out)
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// Section headers.
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putSh := func(name string, typ int, flags uint64, off, size int, link, info int, alignV, entsize uint64) {
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var b [64]byte
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le.PutUint32(b[0:], uint32(stSections.at(name)))
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le.PutUint32(b[4:], uint32(typ))
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le.PutUint64(b[8:], flags)
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le.PutUint64(b[16:], 0) // sh_addr
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le.PutUint64(b[24:], uint64(off))
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le.PutUint64(b[32:], uint64(size))
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le.PutUint32(b[40:], uint32(link))
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le.PutUint32(b[44:], uint32(info))
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le.PutUint64(b[48:], alignV)
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le.PutUint64(b[56:], entsize)
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out = append(out, b[:]...)
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}
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putSh("", shtNull, 0, 0, 0, 0, 0, 0, 0)
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putSh(".text", shtProgbits, shfAlloc|shfExecInstr, textOff, len(img.Code), 0, 0, 16, 0)
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putSh(".data", shtProgbits, shfAlloc|shfWrite, dataOff, len(img.Data), 0, 0, 16, 0)
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putSh(".symtab", shtSymtab, 0, symtabOff, 24*len(syms), secStrtab, shInfo, 8, 24)
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putSh(".strtab", shtStrtab, 0, strtabOff, len(stNames.bytes()), 0, 0, 1, 0)
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if hasRela {
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putSh(".rela.text", shtRela, 0, relaOff, 24*len(relas), secSymtab, secText, 8, 24)
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}
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putSh(".shstrtab", shtStrtab, 0, shstrOff, len(stSections.bytes()), 0, 0, 1, 0)
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// DWARF section headers.
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if dw != nil {
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putSh(".debug_abbrev", shtProgbits, 0, dw.abbrevOff, dw.abbrevSize, 0, 0, 1, 0)
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putSh(".debug_info", shtProgbits, 0, dw.infoOff, dw.infoSize, 0, 0, 1, 0)
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putSh(".debug_line", shtProgbits, 0, dw.lineOff, dw.lineSize, 0, 0, 1, 0)
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putSh(".debug_line_str", shtProgbits, 0, dw.lineStrOff, dw.lineStrSize, 0, 0, 1, 0)
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if dw.frameSize > 0 {
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putSh(".debug_frame", shtProgbits, 0, dw.frameOff, dw.frameSize, 0, 0, 8, 0)
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}
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}
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// The ELF header.
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hdr := out[:64]
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copy(hdr[0:], []byte{0x7f, 'E', 'L', 'F', elfClass64, elfDataLSB, elfVersion, 0})
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le.PutUint16(hdr[16:], etREL)
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le.PutUint16(hdr[18:], emX8664)
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le.PutUint32(hdr[20:], elfVersion)
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le.PutUint64(hdr[24:], 0) // e_entry
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le.PutUint64(hdr[32:], 0) // e_phoff
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le.PutUint64(hdr[40:], uint64(shoff)) // e_shoff
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le.PutUint32(hdr[48:], 0) // e_flags
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le.PutUint16(hdr[52:], 64) // e_ehsize
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le.PutUint16(hdr[54:], 0) // e_phentsize
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le.PutUint16(hdr[56:], 0) // e_phnum
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le.PutUint16(hdr[58:], 64) // e_shentsize
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le.PutUint16(hdr[60:], uint16(nSections))
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le.PutUint16(hdr[62:], uint16(secShstr))
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return out, nil
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}
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// objectName renders a symbol's object-file name: the identifier as written,
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// with an explicit package prefix joined by a dot.
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func objectName(pkg, name string) string {
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if pkg == "" {
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return name
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}
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return pkg + "." + name
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}
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// elfStrtab is an ELF string table under construction.
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type elfStrtab struct {
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buf []byte
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off map[string]int
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}
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func newElfStrtab() *elfStrtab {
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return &elfStrtab{buf: []byte{0}, off: map[string]int{"": 0}}
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}
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func (s *elfStrtab) add(name string) {
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if _, ok := s.off[name]; ok {
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return
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}
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s.off[name] = len(s.buf)
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s.buf = append(s.buf, name...)
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s.buf = append(s.buf, 0)
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}
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func (s *elfStrtab) at(name string) int { return s.off[name] }
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func (s *elfStrtab) bytes() []byte { return s.buf }
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