1 //===- EhFrame.h ------------------------------------------------*- C++ -*-===// 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 #ifndef LLD_MACHO_EH_FRAME_H 10 #define LLD_MACHO_EH_FRAME_H 11 12 #include "InputSection.h" 13 #include "Relocations.h" 14 15 #include "lld/Common/LLVM.h" 16 #include "llvm/ADT/ArrayRef.h" 17 #include "llvm/ADT/PointerUnion.h" 18 #include "llvm/ADT/SmallVector.h" 19 20 /* 21 * NOTE: The main bulk of the EH frame parsing logic is in InputFiles.cpp as it 22 * is closely coupled with other file parsing logic; EhFrame.h just contains a 23 * few helpers. 24 */ 25 26 /* 27 * === The EH frame format === 28 * 29 * EH frames can either be Common Information Entries (CIEs) or Frame 30 * Description Entries (FDEs). CIEs contain information that is common amongst 31 * several FDEs. Each FDE contains a pointer to its CIE. Thus all the EH frame 32 * entries together form a forest of two-level trees, with CIEs as the roots 33 * and FDEs as the leaves. Note that a CIE must precede the FDEs which point 34 * to it. 35 * 36 * A CIE comprises the following fields in order: 37 * 1. Length of the entry (4 or 12 bytes) 38 * 2. CIE offset (4 bytes; always 0 for CIEs) 39 * 3. CIE version (byte) 40 * 4. Null-terminated augmentation string 41 * 5-8. LEB128 values that we don't care about 42 * 9. Augmentation data, to be interpreted using the aug string 43 * 10. DWARF instructions (ignored by LLD) 44 * 45 * An FDE comprises of the following: 46 * 1. Length of the entry (4 or 12 bytes) 47 * 2. CIE offset (4 bytes pcrel offset that points backwards to this FDE's CIE) 48 * 3. Function address (pointer-sized pcrel offset) 49 * 4. (std::optional) Augmentation data length 50 * 5. (std::optional) LSDA address (pointer-sized pcrel offset) 51 * 6. DWARF instructions (ignored by LLD) 52 */ 53 namespace lld::macho { 54 55 class EhReader { 56 public: 57 EhReader(const ObjFile *file, ArrayRef<uint8_t> data, size_t dataOff) 58 : file(file), data(data), dataOff(dataOff) {} 59 size_t size() const { return data.size(); } 60 // Read and validate the length field. 61 uint64_t readLength(size_t *off) const; 62 // Skip the length field without doing validation. 63 void skipValidLength(size_t *off) const; 64 uint8_t readByte(size_t *off) const; 65 uint32_t readU32(size_t *off) const; 66 uint64_t readPointer(size_t *off, uint8_t size) const; 67 StringRef readString(size_t *off) const; 68 void skipLeb128(size_t *off) const; 69 void failOn(size_t errOff, const Twine &msg) const; 70 71 private: 72 const ObjFile *file; 73 ArrayRef<uint8_t> data; 74 // The offset of the data array within its section. Used only for error 75 // reporting. 76 const size_t dataOff; 77 }; 78 79 // The EH frame format, when emitted by llvm-mc, consists of a number of 80 // "abs-ified" relocations, i.e. relocations that are implicitly encoded as 81 // pcrel offsets in the section data. The offsets refer to the locations of 82 // symbols in the input object file. When we ingest these EH frames, we convert 83 // these implicit relocations into explicit Relocs. 84 // 85 // These pcrel relocations are semantically similar to X86_64_RELOC_SIGNED_4. 86 // However, we need this operation to be cross-platform, and ARM does not have a 87 // similar relocation that is applicable. We therefore use the more verbose (but 88 // more generic) subtractor relocation to encode these pcrel values. ld64 89 // appears to do something similar -- its `-r` output contains these explicit 90 // subtractor relocations. 91 class EhRelocator { 92 public: 93 EhRelocator(InputSection *isec) : isec(isec) {} 94 95 // For the next two methods, let `PC` denote `isec address + off`. 96 // Create relocs writing the value of target - PC to PC. 97 void makePcRel(uint64_t off, 98 llvm::PointerUnion<Symbol *, InputSection *> target, 99 uint8_t length); 100 // Create relocs writing the value of PC - target to PC. 101 void makeNegativePcRel(uint64_t off, 102 llvm::PointerUnion<Symbol *, InputSection *> target, 103 uint8_t length); 104 // Insert the new relocations into isec->relocs. 105 void commit(); 106 107 private: 108 InputSection *isec; 109 // Insert new relocs here so that we don't invalidate iterators into the 110 // existing relocs vector. 111 SmallVector<Reloc, 6> newRelocs; 112 }; 113 114 } // namespace lld::macho 115 116 #endif 117