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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-exception6//7//===----------------------------------------------------------------------===//8 9#ifndef LLD_MACHO_EH_FRAME_H10#define LLD_MACHO_EH_FRAME_H11 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 it22 * is closely coupled with other file parsing logic; EhFrame.h just contains a23 * few helpers.24 */25 26/*27 * === The EH frame format ===28 *29 * EH frames can either be Common Information Entries (CIEs) or Frame30 * Description Entries (FDEs). CIEs contain information that is common amongst31 * several FDEs. Each FDE contains a pointer to its CIE. Thus all the EH frame32 * entries together form a forest of two-level trees, with CIEs as the roots33 * and FDEs as the leaves. Note that a CIE must precede the FDEs which point34 * 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 string41 * 5-8. LEB128 values that we don't care about42 * 9. Augmentation data, to be interpreted using the aug string43 * 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 length50 * 5. (std::optional) LSDA address (pointer-sized pcrel offset)51 * 6. DWARF instructions (ignored by LLD)52 */53namespace lld::macho {54 55class EhReader {56public: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 71private:72 const ObjFile *file;73 ArrayRef<uint8_t> data;74 // The offset of the data array within its section. Used only for error75 // reporting.76 const size_t dataOff;77};78 79// The EH frame format, when emitted by llvm-mc, consists of a number of80// "abs-ified" relocations, i.e. relocations that are implicitly encoded as81// pcrel offsets in the section data. The offsets refer to the locations of82// symbols in the input object file. When we ingest these EH frames, we convert83// 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 a87// similar relocation that is applicable. We therefore use the more verbose (but88// more generic) subtractor relocation to encode these pcrel values. ld6489// appears to do something similar -- its `-r` output contains these explicit90// subtractor relocations.91class EhRelocator {92public: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 107private:108 InputSection *isec;109 // Insert new relocs here so that we don't invalidate iterators into the110 // existing relocs vector.111 SmallVector<Reloc, 6> newRelocs;112};113 114} // namespace lld::macho115 116#endif117