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//===- EhFrame.cpp -------------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// .eh_frame section contains information on how to unwind the stack when
// an exception is thrown. The section consists of sequence of CIE and FDE
// records. The linker needs to merge CIEs and associate FDEs to CIEs.
// That means the linker has to understand the format of the section.
//
// This file contains a few utility functions to read .eh_frame contents.
//
//===----------------------------------------------------------------------===//
#include "EhFrame.h"
#include "Error.h"
#include "llvm/Object/ELF.h"
#include "llvm/Support/Dwarf.h"
#include "llvm/Support/Endian.h"
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::dwarf;
using namespace llvm::object;
using namespace llvm::support::endian;
namespace lld {
namespace elf {
// .eh_frame section is a sequence of records. Each record starts with
// a 4 byte length field. This function reads the length.
template <class ELFT> size_t readEhRecordSize(ArrayRef<uint8_t> D) {
const endianness E = ELFT::TargetEndianness;
if (D.size() < 4)
fatal("CIE/FDE too small");
// First 4 bytes of CIE/FDE is the size of the record.
// If it is 0xFFFFFFFF, the next 8 bytes contain the size instead,
// but we do not support that format yet.
uint64_t V = read32<E>(D.data());
if (V == UINT32_MAX)
fatal("CIE/FDE too large");
uint64_t Size = V + 4;
if (Size > D.size())
fatal("CIE/FIE ends past the end of the section");
return Size;
}
// Read a byte and advance D by one byte.
static uint8_t readByte(ArrayRef<uint8_t> &D) {
if (D.empty())
fatal("corrupted or unsupported CIE information");
uint8_t B = D.front();
D = D.slice(1);
return B;
}
// Skip an integer encoded in the LEB128 format.
// Actual number is not of interest because only the runtime needs it.
// But we need to be at least able to skip it so that we can read
// the field that follows a LEB128 number.
static void skipLeb128(ArrayRef<uint8_t> &D) {
while (!D.empty()) {
uint8_t Val = D.front();
D = D.slice(1);
if ((Val & 0x80) == 0)
return;
}
fatal("corrupted or unsupported CIE information");
}
template <class ELFT> static size_t getAugPSize(unsigned Enc) {
switch (Enc & 0x0f) {
case DW_EH_PE_absptr:
case DW_EH_PE_signed:
return ELFT::Is64Bits ? 8 : 4;
case DW_EH_PE_udata2:
case DW_EH_PE_sdata2:
return 2;
case DW_EH_PE_udata4:
case DW_EH_PE_sdata4:
return 4;
case DW_EH_PE_udata8:
case DW_EH_PE_sdata8:
return 8;
}
fatal("unknown FDE encoding");
}
template <class ELFT> static void skipAugP(ArrayRef<uint8_t> &D) {
uint8_t Enc = readByte(D);
if ((Enc & 0xf0) == DW_EH_PE_aligned)
fatal("DW_EH_PE_aligned encoding is not supported");
size_t Size = getAugPSize<ELFT>(Enc);
if (Size >= D.size())
fatal("corrupted CIE");
D = D.slice(Size);
}
template <class ELFT> uint8_t getFdeEncoding(ArrayRef<uint8_t> D) {
if (D.size() < 8)
fatal("CIE too small");
D = D.slice(8);
uint8_t Version = readByte(D);
if (Version != 1 && Version != 3)
fatal("FDE version 1 or 3 expected, but got " + Twine((unsigned)Version));
const unsigned char *AugEnd = std::find(D.begin(), D.end(), '\0');
if (AugEnd == D.end())
fatal("corrupted CIE");
StringRef Aug(reinterpret_cast<const char *>(D.begin()), AugEnd - D.begin());
D = D.slice(Aug.size() + 1);
// Code alignment factor should always be 1 for .eh_frame.
if (readByte(D) != 1)
fatal("CIE code alignment must be 1");
// Skip data alignment factor.
skipLeb128(D);
// Skip the return address register. In CIE version 1 this is a single
// byte. In CIE version 3 this is an unsigned LEB128.
if (Version == 1)
readByte(D);
else
skipLeb128(D);
// We only care about an 'R' value, but other records may precede an 'R'
// record. Unfortunately records are not in TLV (type-length-value) format,
// so we need to teach the linker how to skip records for each type.
for (char C : Aug) {
if (C == 'R')
return readByte(D);
if (C == 'z') {
skipLeb128(D);
continue;
}
if (C == 'P') {
skipAugP<ELFT>(D);
continue;
}
if (C == 'L') {
readByte(D);
continue;
}
fatal("unknown .eh_frame augmentation string: " + Aug);
}
return DW_EH_PE_absptr;
}
template size_t readEhRecordSize<ELF32LE>(ArrayRef<uint8_t>);
template size_t readEhRecordSize<ELF32BE>(ArrayRef<uint8_t>);
template size_t readEhRecordSize<ELF64LE>(ArrayRef<uint8_t>);
template size_t readEhRecordSize<ELF64BE>(ArrayRef<uint8_t>);
template uint8_t getFdeEncoding<ELF32LE>(ArrayRef<uint8_t>);
template uint8_t getFdeEncoding<ELF32BE>(ArrayRef<uint8_t>);
template uint8_t getFdeEncoding<ELF64LE>(ArrayRef<uint8_t>);
template uint8_t getFdeEncoding<ELF64BE>(ArrayRef<uint8_t>);
}
}
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