[clangd] Fix UB in scanPreamble

[kadircet]

Summary

getMemBufferCopy triggers an UB when it receives a default constructed
StringRef. Make sure that we're always passing the null-terminated string
created in ParseInputs throughout the scanPreamble.

[hokein]

clang-tools-extra/clangd/Preamble.cpp

350–351

Looks like we have an extra cost here -- the std::string.substr will construct a new string everytime, we could save it by using llvm::StringRef(PI.Contents).substr(0, Bounds.Size).

[kadircet]

clang-tools-extra/clangd/Preamble.cpp

350–351

thanks!

clang-tools-extra/clangd/Preamble.cpp

scanPreamble(llvm::StringRef Contents, const tooling::CompileCommand &Cmd) {
   private:
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> viewImpl() const override {
       return new llvm::vfs::InMemoryFileSystem;
     }
   };
   EmptyFS FS;
   // Build and run Preprocessor over the preamble.
   ParseInputs PI;
+  // Memory buffers below expect null-terminated && non-null strings. So make
+  // sure to always use PI.Contents!
   PI.Contents = Contents.str();
   PI.TFS = &FS;
   PI.CompileCommand = Cmd;
   IgnoringDiagConsumer IgnoreDiags;
   auto CI = buildCompilerInvocation(PI, IgnoreDiags);
   if (!CI)
     return error("failed to create compiler invocation");
   CI->getDiagnosticOpts().IgnoreWarnings = true;
   auto ContentsBuffer = llvm::MemoryBuffer::getMemBuffer(PI.Contents);
   // This means we're scanning (though not preprocessing) the preamble section
   // twice. However, it's important to precisely follow the preamble bounds used
   // elsewhere.
   auto Bounds = ComputePreambleBounds(*CI->getLangOpts(), *ContentsBuffer, 0);
-  auto PreambleContents =
-      llvm::MemoryBuffer::getMemBufferCopy(Contents.substr(0, Bounds.Size));
+  auto PreambleContents = llvm::MemoryBuffer::getMemBufferCopy(
+      llvm::StringRef(PI.Contents).take_front(Bounds.Size));
   auto Clang = prepareCompilerInstance(
       std::move(CI), nullptr, std::move(PreambleContents),
       // Provide an empty FS to prevent preprocessor from performing IO. This
       // also implies missing resolved paths for includes.
       FS.view(std::nullopt), IgnoreDiags);
   if (Clang->getFrontendOpts().Inputs.empty())
     return error("compiler instance had no inputs");
   // We are only interested in main file includes.
PreamblePatch PreamblePatch::create(llvm::StringRef FileName,
   // First scan preprocessor directives in Baseline and Modified. These will be
   // used to figure out newly added directives in Modified. Scanning can fail,
   // the code just bails out and creates an empty patch in such cases, as:
   // - If scanning for Baseline fails, no knowledge of existing includes hence
   //   patch will contain all the includes in Modified. Leading to rebuild of
   //   whole preamble, which is terribly slow.
   // - If scanning for Modified fails, cannot figure out newly added ones so
   //   there's nothing to do but generate an empty patch.
-  auto BaselineScan = scanPreamble(
-      // Contents needs to be null-terminated.
-      Baseline.Preamble.getContents(), Modified.CompileCommand);
+  auto BaselineScan =
+      scanPreamble(Baseline.Preamble.getContents(), Modified.CompileCommand);
   if (!BaselineScan) {
     elog("Failed to scan baseline of {0}: {1}", FileName,
          BaselineScan.takeError());
     return PreamblePatch::unmodified(Baseline);
   }
   auto ModifiedScan = scanPreamble(Modified.Contents, Modified.CompileCommand);
   if (!ModifiedScan) {
     elog("Failed to scan modified contents of {0}: {1}", FileName,