bolt/lib/Rewrite/BuildIDRewriter.cpp

   1 //===- bolt/Rewrite/BuildIDRewriter.cpp -----------------------------------===//
   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-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // Read and update build ID stored in ELF note section.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "bolt/Rewrite/MetadataRewriter.h"
  14 #include "bolt/Rewrite/MetadataRewriters.h"
  15 #include "llvm/Support/Errc.h"
  16
  17 using namespace llvm;
  18 using namespace bolt;
  19
  20 namespace {
  21
  22 /// The build-id is typically a stream of 20 bytes. Return these bytes in
  23 /// printable hexadecimal form.
  24 std::string getPrintableBuildID(StringRef BuildID) {
  25   std::string Str;
  26   raw_string_ostream OS(Str);
  27   for (const char &Char : BuildID)
  28     OS << format("%.2x", static_cast<unsigned char>(Char));
  29
  30   return OS.str();
  31 }
  32
  33 class BuildIDRewriter final : public MetadataRewriter {
  34
  35   /// Information about binary build ID.
  36   ErrorOr<BinarySection &> BuildIDSection{std::errc::bad_address};
  37   StringRef BuildID;
  38   std::optional<uint64_t> BuildIDOffset;
  39   std::optional<uint64_t> BuildIDSize;
  40
  41 public:
  42   BuildIDRewriter(StringRef Name, BinaryContext &BC)
  43       : MetadataRewriter(Name, BC) {}
  44
  45   Error sectionInitializer() override;
  46
  47   Error postEmitFinalizer() override;
  48 };
  49
  50 Error BuildIDRewriter::sectionInitializer() {
  51   // Typically, build ID will reside in .note.gnu.build-id section. Howerver,
  52   // a linker script can change the section name and such is the case with
  53   // the Linux kernel. Hence, we iterate over all note sections.
  54   for (BinarySection &NoteSection : BC.sections()) {
  55     if (!NoteSection.isNote())
  56       continue;
  57
  58     StringRef Buf = NoteSection.getContents();
  59     DataExtractor DE = DataExtractor(Buf, BC.AsmInfo->isLittleEndian(),
  60                                      BC.AsmInfo->getCodePointerSize());
  61     DataExtractor::Cursor Cursor(0);
  62     while (Cursor && !DE.eof(Cursor)) {
  63       const uint32_t NameSz = DE.getU32(Cursor);
  64       const uint32_t DescSz = DE.getU32(Cursor);
  65       const uint32_t Type = DE.getU32(Cursor);
  66
  67       StringRef Name =
  68           NameSz ? Buf.slice(Cursor.tell(), Cursor.tell() + NameSz) : "<empty>";
  69       Cursor.seek(alignTo(Cursor.tell() + NameSz, 4));
  70
  71       const uint64_t DescOffset = Cursor.tell();
  72       StringRef Desc =
  73           DescSz ? Buf.slice(DescOffset, DescOffset + DescSz) : "<empty>";
  74       Cursor.seek(alignTo(DescOffset + DescSz, 4));
  75
  76       if (!Cursor)
  77         return createStringError(errc::executable_format_error,
  78                                  "out of bounds while reading note section: %s",
  79                                  toString(Cursor.takeError()).c_str());
  80
  81       if (Type == ELF::NT_GNU_BUILD_ID && Name.substr(0, 3) == "GNU" &&
  82           DescSz) {
  83         BuildIDSection = NoteSection;
  84         BuildID = Desc;
  85         BC.setFileBuildID(getPrintableBuildID(Desc));
  86         BuildIDOffset = DescOffset;
  87         BuildIDSize = DescSz;
  88
  89         return Error::success();
  90       }
  91     }
  92   }
  93
  94   return Error::success();
  95 }
  96
  97 Error BuildIDRewriter::postEmitFinalizer() {
  98   if (!BuildIDSection || !BuildIDOffset)
  99     return Error::success();
 100
 101   const uint8_t LastByte = BuildID[BuildID.size() - 1];
 102   SmallVector<char, 1> Patch = {static_cast<char>(LastByte ^ 1)};
 103   BuildIDSection->addPatch(*BuildIDOffset + BuildID.size() - 1, Patch);
 104   BC.outs() << "BOLT-INFO: patched build-id (flipped last bit)\n";
 105
 106   return Error::success();
 107 }
 108 } // namespace
 109
 110 std::unique_ptr<MetadataRewriter>
 111 llvm::bolt::createBuildIDRewriter(BinaryContext &BC) {
 112   return std::make_unique<BuildIDRewriter>("build-id-rewriter", BC);
 113 }