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//===- EhFrame.h ------------------------------------------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "InputSection.h"
#include "Relocations.h"
#include "lld/Common/LLVM.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/SmallVector.h"
* NOTE: The main bulk of the EH frame parsing logic is in InputFiles.cpp as it
* is closely coupled with other file parsing logic; EhFrame.h just contains a
* few helpers.
* === The EH frame format ===
* EH frames can either be Common Information Entries (CIEs) or Frame
* Description Entries (FDEs). CIEs contain information that is common amongst
* several FDEs. Each FDE contains a pointer to its CIE. Thus all the EH frame
* entries together form a forest of two-level trees, with CIEs as the roots
* and FDEs as the leaves. Note that a CIE must precede the FDEs which point
* to it.
* A CIE comprises the following fields in order:
* 1. Length of the entry (4 or 12 bytes)
* 2. CIE offset (4 bytes; always 0 for CIEs)
* 3. CIE version (byte)
* 4. Null-terminated augmentation string
* 5-8. LEB128 values that we don't care about
* 9. Augmentation data, to be interpreted using the aug string
* 10. DWARF instructions (ignored by LLD)
* An FDE comprises of the following:
* 1. Length of the entry (4 or 12 bytes)
* 2. CIE offset (4 bytes pcrel offset that points backwards to this FDE's CIE)
* 3. Function address (pointer-sized pcrel offset)
* 4. (std::optional) Augmentation data length
* 5. (std::optional) LSDA address (pointer-sized pcrel offset)
* 6. DWARF instructions (ignored by LLD)
namespace lld::macho {
class EhReader {
EhReader(const ObjFile *file, ArrayRef<uint8_t> data, size_t dataOff)
: file(file), data(data), dataOff(dataOff) {}
size_t size() const { return data.size(); }
// Read and validate the length field.
uint64_t readLength(size_t *off) const;
// Skip the length field without doing validation.
void skipValidLength(size_t *off) const;
uint8_t readByte(size_t *off) const;
uint32_t readU32(size_t *off) const;
uint64_t readPointer(size_t *off, uint8_t size) const;
StringRef readString(size_t *off) const;
void skipLeb128(size_t *off) const;
void failOn(size_t errOff, const Twine &msg) const;
const ObjFile *file;
ArrayRef<uint8_t> data;
// The offset of the data array within its section. Used only for error
// reporting.
const size_t dataOff;
// The EH frame format, when emitted by llvm-mc, consists of a number of
// "abs-ified" relocations, i.e. relocations that are implicitly encoded as
// pcrel offsets in the section data. The offsets refer to the locations of
// symbols in the input object file. When we ingest these EH frames, we convert
// these implicit relocations into explicit Relocs.
// These pcrel relocations are semantically similar to X86_64_RELOC_SIGNED_4.
// However, we need this operation to be cross-platform, and ARM does not have a
// similar relocation that is applicable. We therefore use the more verbose (but
// more generic) subtractor relocation to encode these pcrel values. ld64
// appears to do something similar -- its `-r` output contains these explicit
// subtractor relocations.
class EhRelocator {
EhRelocator(InputSection *isec) : isec(isec) {}
// For the next two methods, let `PC` denote `isec address + off`.
// Create relocs writing the value of target - PC to PC.
void makePcRel(uint64_t off,
llvm::PointerUnion<Symbol *, InputSection *> target,
uint8_t length);
// Create relocs writing the value of PC - target to PC.
void makeNegativePcRel(uint64_t off,
llvm::PointerUnion<Symbol *, InputSection *> target,
uint8_t length);
// Insert the new relocations into isec->relocs.
void commit();
InputSection *isec;
// Insert new relocs here so that we don't invalidate iterators into the
// existing relocs vector.
SmallVector<Reloc, 6> newRelocs;
} // namespace lld::macho