[llvm] [BOLT] Support instrumentation hook via DT_FINI_ARRAY (PR #67348)

[Vladislav Khmelevsky via llvm-commits]

================
@@ -1276,6 +1282,68 @@ void RewriteInstance::discoverFileObjects() {
   registerFragments();
 }
 
+Error RewriteInstance::discoverRtFiniAddress() {
+  // If FiniFunctionAddress is already set, we got if from DT_FINI. We use
+  // DT_FINI instead of DT_FINI_ARRAY if it's available.
+  if (BC->FiniFunctionAddress)
+    return Error::success();
+
+  if (!BC->FiniArrayAddress || !BC->FiniArraySize) {
+    return createStringError(
+        std::errc::not_supported,
+        "Instrumentation needs either DT_FINI or DT_FINI_ARRAY");
+  }
+
+  if (*BC->FiniArraySize < BC->AsmInfo->getCodePointerSize()) {
+    return createStringError(std::errc::not_supported,
+                             "Need at least 1 DT_FINI_ARRAY slot");
+  }
+
+  ErrorOr<BinarySection &> FiniArraySection =
+      BC->getSectionForAddress(*BC->FiniArrayAddress);
+  if (auto EC = FiniArraySection.getError())
+    return errorCodeToError(EC);
+
+  BC->FiniArraySection = &*FiniArraySection;
+
+  if (const Relocation *Reloc = FiniArraySection->getDynamicRelocationAt(0)) {
+    BC->FiniFunctionAddress = Reloc->Addend;
+    return Error::success();
+  }
+
+  if (const Relocation *Reloc = FiniArraySection->getRelocationAt(0)) {
+    BC->FiniFunctionAddress = Reloc->Value;
+    return Error::success();
+  }
+
+  return createStringError(std::errc::not_supported,
+                           "No relocation for first DT_FINI_ARRAY slot");
+}
+
+void RewriteInstance::updateRtFiniReloc() {
+  if (!BC->FiniArraySection)
----------------
yota9 wrote:

Not exactly, currently you at discoverRtFiniAddress you also set FiniFunctionAddress.  As I understand it is caused by the internal logic, which uses it to call the original function from new instrumentation fini.

I didn't notice it before to be honest, but the problem with your current change that after you set the FiniFunctionAddress you can't distinguish between the case where DT_FINI was originally in the binary and you don't have to patch DT_FINI_ARRAY and your case where you should. Correct me if I'm wrong. 

So you need to check that FiniFunctionAddress == Reloc->Value of the first dynamic slot to continue patching at least. 
My suggestion is to use FiniFunctionAddress for DT_FINI only (probably fix the comment about it) and add FiniFunctionRtAddress and check that FiniFunctionAddress != FiniFunctionRtAddress with the comment that DT_FINI is fixed during patching DYNAMIC. After it I would probably add assert to BC->FiniArrayAddress && BC->FiniArraySize just in case. And one more assert after successfully taking takeDynamicRelocationAt with BC->FiniFunctionRtAddress == Reloc->Addend; This way it seems to be less frustrating and also gives ability to rewrite comparing the addresses if would want to use not only DT_FINI and DT_FINI_ARRAY in the future (unlikely, but still).

https://github.com/llvm/llvm-project/pull/67348