[lld] [llvm] [lld][AArch64][Build Attributes] Add support for converting AArch64 Build Attributes to GNU Properties (PR #131990)

[via llvm-commits]

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@@ -638,25 +699,62 @@ template <class ELFT> void ObjFile<ELFT>::parse(bool ignoreComdats) {
         }
       }
       break;
-    case EM_AARCH64:
-      // FIXME: BuildAttributes have been implemented in llvm, but not yet in
-      // lld. Remove the section so that it does not accumulate in the output
-      // file. When support is implemented we expect not to output a build
-      // attributes section in files of type ET_EXEC or ET_SHARED, but ld -r
-      // ouptut will need a single merged attributes section.
-      if (sec.sh_type == SHT_AARCH64_ATTRIBUTES)
+    case EM_AARCH64: {
+      // The specification states that if a file contains both GNU properties
+      // and AArch64 build attributes, they can be assumed to be identical.
+      // Therefore, if a file contains GNU properties, the AArch64 build
+      // attributes are ignored. If a file does not contain GNU properties, we
+      // leverage the existing GNU properties mechanism by populating the
+      // corresponding data structures, which will later be handled by
+      // Driver.cpp::readSecurityNotes. This ensures that AArch64 build
+      // attributes are represented in the linked object file as GNU properties,
+      // which are already supported by the Linux kernel and the dynamic
+      // dispatcher.
+      if (sec.sh_type == SHT_AARCH64_ATTRIBUTES) {
+        StringRef name = check(obj.getSectionName(sec, shstrtab));
+        ArrayRef<uint8_t> contents = check(obj.getSectionContents(sec));
+        AArch64AttributeParser attributes;
+        if (Error e = attributes.parse(contents, ELFT::Endianness)) {
+          InputSection isec(*this, sec, name);
+          Warn(ctx) << &isec << ": " << std::move(e);
+        } else {
+          // for functions that has to warn/err/report
+          InputSection isec(*this, sec, name);
+          KnownAArch64BuildAttrSubsections subSections =
----------------
sivan-shani wrote:

`subSections` is being used:

> static_cast<uint64_t>(subSections.pauth.tagPlatform),
> static_cast<uint64_t>(subSections.pauth.tagSchema)};
> this->andFeatures |= (subSections.fAndB.tagBTI) << 0;
> this->andFeatures |= (subSections.fAndB.tagPAC) << 1;
> this->andFeatures |= (subSections.fAndB.tagGCS) << 2;

Not quite sure what might the alternative be?

https://github.com/llvm/llvm-project/pull/131990