1//===-- BPFELFObjectWriter.cpp - BPF ELF Writer ---------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "MCTargetDesc/BPFMCTargetDesc.h"
10#include "llvm/BinaryFormat/ELF.h"
11#include "llvm/MC/MCELFObjectWriter.h"
12#include "llvm/MC/MCFixup.h"
13#include "llvm/MC/MCObjectWriter.h"
14#include "llvm/MC/MCValue.h"
15#include "llvm/Support/ErrorHandling.h"
16#include <cstdint>
17
18using namespace llvm;
19
20namespace {
21
22class BPFELFObjectWriter : public MCELFObjectTargetWriter {
23public:
24  BPFELFObjectWriter(uint8_t OSABI);
25  ~BPFELFObjectWriter() override = default;
26
27protected:
28  unsigned getRelocType(MCContext &Ctx, const MCValue &Target,
29                        const MCFixup &Fixup, bool IsPCRel) const override;
30};
31
32} // end anonymous namespace
33
34BPFELFObjectWriter::BPFELFObjectWriter(uint8_t OSABI)
35    : MCELFObjectTargetWriter(/*Is64Bit*/ true, OSABI, ELF::EM_BPF,
36                              /*HasRelocationAddend*/ false) {}
37
38unsigned BPFELFObjectWriter::getRelocType(MCContext &Ctx, const MCValue &Target,
39                                          const MCFixup &Fixup,
40                                          bool IsPCRel) const {
41  // determine the type of the relocation
42  switch (Fixup.getKind()) {
43  default:
44    llvm_unreachable("invalid fixup kind!");
45  case FK_SecRel_8:
46    // LD_imm64 instruction.
47    return ELF::R_BPF_64_64;
48  case FK_PCRel_4:
49    // CALL instruction.
50    return ELF::R_BPF_64_32;
51  case FK_Data_8:
52    return ELF::R_BPF_64_ABS64;
53  case FK_Data_4:
54    if (const MCSymbolRefExpr *A = Target.getSymA()) {
55      const MCSymbol &Sym = A->getSymbol();
56
57      if (Sym.isDefined()) {
58        MCSection &Section = Sym.getSection();
59        const MCSectionELF *SectionELF = dyn_cast<MCSectionELF>(&Section);
60        assert(SectionELF && "Null section for reloc symbol");
61
62        unsigned Flags = SectionELF->getFlags();
63
64        if (Sym.isTemporary()) {
65          // .BTF.ext generates FK_Data_4 relocations for
66          // insn offset by creating temporary labels.
67          // The reloc symbol should be in text section.
68          // Use a different relocation to instruct ExecutionEngine
69          // RuntimeDyld not to do relocation for it, yet still to
70          // allow lld to do proper adjustment when merging sections.
71          if ((Flags & ELF::SHF_ALLOC) && (Flags & ELF::SHF_EXECINSTR))
72            return ELF::R_BPF_64_NODYLD32;
73        } else {
74          // .BTF generates FK_Data_4 relocations for variable
75          // offset in DataSec kind.
76          // The reloc symbol should be in data section.
77          if ((Flags & ELF::SHF_ALLOC) && (Flags & ELF::SHF_WRITE))
78            return ELF::R_BPF_64_NODYLD32;
79        }
80      }
81    }
82    return ELF::R_BPF_64_ABS32;
83  }
84}
85
86std::unique_ptr<MCObjectTargetWriter>
87llvm::createBPFELFObjectWriter(uint8_t OSABI) {
88  return std::make_unique<BPFELFObjectWriter>(OSABI);
89}
90