[lld] [lld][AArch64][ELF][PAC] Support AUTH relocations and AUTH ELF marking (PR #72714)

[Fangrui Song via llvm-commits]

================
@@ -962,6 +962,44 @@ template <class ELFT> static uint32_t readAndFeatures(const InputSection &sec) {
   return featuresSet;
 }
 
+// Extract compatibility info for aarch64 pointer authentication from the
+// .note.AARCH64-PAUTH-ABI-tag section and write it to the corresponding ObjFile
+// field. See the following ABI documentation:
+// https://github.com/ARM-software/abi-aa/blob/main/pauthabielf64/pauthabielf64.rst#elf-marking
+template <class ELFT>
+static void readAArch64PauthAbiTag(const InputSection &sec, ObjFile<ELFT> &f) {
+  using Elf_Nhdr = typename ELFT::Nhdr;
+  using Elf_Note = typename ELFT::Note;
+  ArrayRef<uint8_t> data = sec.content();
+  auto reportError = [&](const Twine &msg) {
+    errorOrWarn(toString(sec.file) + ":(" + sec.name + "): " + msg);
+  };
+
+  auto *nhdr = reinterpret_cast<const Elf_Nhdr *>(data.data());
+  if (data.size() < sizeof(Elf_Nhdr) ||
+      data.size() < nhdr->getSize(sec.addralign)) {
+    reportError("section is too short");
+    return;
+  }
+
+  Elf_Note note(*nhdr);
+  if (nhdr->n_type != NT_ARM_TYPE_PAUTH_ABI_TAG)
+    reportError("invalid type field value " + Twine(nhdr->n_type) + " (" +
+                Twine(NT_ARM_TYPE_PAUTH_ABI_TAG) + " expected)");
+  if (note.getName() != "ARM")
+    reportError("invalid name field value " + note.getName() +
+                " (ARM expected)");
+
+  ArrayRef<uint8_t> desc = note.getDesc(sec.addralign);
+  if (desc.size() < 16) {
+    reportError("AArch64 PAuth compatibility info is too short "
+                "(at least 16 bytes expected)");
+    return;
+  }
+
+  f.aarch64PauthAbiTag = SmallVector<uint8_t, 0>(iterator_range(desc));
----------------
MaskRay wrote:

Change `aarch64PauthAbiTag` to an `ArrayRef<uint8_t>` to save memory

https://github.com/llvm/llvm-project/pull/72714