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//===- EhFrame.cpp -------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// .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 "Config.h"
#include "InputSection.h"
#include "Relocations.h"
#include "Target.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Strings.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/Object/ELF.h"
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::dwarf;
using namespace llvm::object;
using namespace lld;
using namespace lld::elf;
namespace {
class EhReader {
public:
EhReader(InputSectionBase *s, ArrayRef<uint8_t> d) : isec(s), d(d) {}
size_t readEhRecordSize();
uint8_t getFdeEncoding();
private:
template <class P> void failOn(const P *loc, const Twine &msg) {
fatal("corrupted .eh_frame: " + msg + "\n>>> defined in " +
isec->getObjMsg((const uint8_t *)loc - isec->data().data()));
}
uint8_t readByte();
void skipBytes(size_t count);
StringRef readString();
void skipLeb128();
void skipAugP();
InputSectionBase *isec;
ArrayRef<uint8_t> d;
};
}
size_t elf::readEhRecordSize(InputSectionBase *s, size_t off) {
return EhReader(s, s->data().slice(off)).readEhRecordSize();
}
// .eh_frame section is a sequence of records. Each record starts with
// a 4 byte length field. This function reads the length.
size_t EhReader::readEhRecordSize() {
if (d.size() < 4)
failOn(d.data(), "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(d.data());
if (v == UINT32_MAX)
failOn(d.data(), "CIE/FDE too large");
uint64_t size = v + 4;
if (size > d.size())
failOn(d.data(), "CIE/FDE ends past the end of the section");
return size;
}
// Read a byte and advance D by one byte.
uint8_t EhReader::readByte() {
if (d.empty())
failOn(d.data(), "unexpected end of CIE");
uint8_t b = d.front();
d = d.slice(1);
return b;
}
void EhReader::skipBytes(size_t count) {
if (d.size() < count)
failOn(d.data(), "CIE is too small");
d = d.slice(count);
}
// Read a null-terminated string.
StringRef EhReader::readString() {
const uint8_t *end = llvm::find(d, '\0');
if (end == d.end())
failOn(d.data(), "corrupted CIE (failed to read string)");
StringRef s = toStringRef(d.slice(0, end - d.begin()));
d = d.slice(s.size() + 1);
return s;
}
// 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.
void EhReader::skipLeb128() {
const uint8_t *errPos = d.data();
while (!d.empty()) {
uint8_t val = d.front();
d = d.slice(1);
if ((val & 0x80) == 0)
return;
}
failOn(errPos, "corrupted CIE (failed to read LEB128)");
}
static size_t getAugPSize(unsigned enc) {
switch (enc & 0x0f) {
case DW_EH_PE_absptr:
case DW_EH_PE_signed:
return config->wordsize;
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;
}
return 0;
}
void EhReader::skipAugP() {
uint8_t enc = readByte();
if ((enc & 0xf0) == DW_EH_PE_aligned)
failOn(d.data() - 1, "DW_EH_PE_aligned encoding is not supported");
size_t size = getAugPSize(enc);
if (size == 0)
failOn(d.data() - 1, "unknown FDE encoding");
if (size >= d.size())
failOn(d.data() - 1, "corrupted CIE");
d = d.slice(size);
}
uint8_t elf::getFdeEncoding(EhSectionPiece *p) {
return EhReader(p->sec, p->data()).getFdeEncoding();
}
uint8_t EhReader::getFdeEncoding() {
skipBytes(8);
int version = readByte();
if (version != 1 && version != 3)
failOn(d.data() - 1,
"FDE version 1 or 3 expected, but got " + Twine(version));
StringRef aug = readString();
// Skip code and data alignment factors.
skipLeb128();
skipLeb128();
// 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();
else
skipLeb128();
// 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();
if (c == 'z') {
skipLeb128();
continue;
}
if (c == 'P') {
skipAugP();
continue;
}
if (c == 'L') {
readByte();
continue;
}
failOn(aug.data(), "unknown .eh_frame augmentation string: " + aug);
}
return DW_EH_PE_absptr;
}
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