1 files changed, 199 insertions, 0 deletions
diff --git a/lld/ELF/EhFrame.cpp b/lld/ELF/EhFrame.cpp
new file mode 100644
index 000000000000..a9c66f29446c
--- /dev/null
+++ b/lld/ELF/EhFrame.cpp
@@ -0,0 +1,199 @@
+//===- 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;
+
+namespace lld {
+namespace 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 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 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;
+}
+
+} // namespace elf
+} // namespace lld