[llvm] [BOLT] Add rewriting support for Linux kernel __bug_table (PR #86908)

[Maksim Panchenko via llvm-commits]

================
@@ -1223,6 +1232,52 @@ Error LinuxKernelRewriter::readBugTable() {
   return Error::success();
 }
 
+/// find_bug() uses linear search to match an address to an entry in the bug
+/// table. Hence, there is no need to sort entries when rewriting the table.
+/// When we need to erase an entry, we set its instruction address to zero.
+Error LinuxKernelRewriter::rewriteBugTable() {
+  if (!BugTableSection)
+    return Error::success();
+
+  for (BinaryFunction &BF : llvm::make_second_range(BC.getBinaryFunctions())) {
+    if (!BC.shouldEmit(BF))
+      continue;
+
+    if (!FunctionBugList.count(&BF))
+      continue;
+
+    // Bugs that will be emitted for this function.
+    DenseSet<uint32_t> EmittedIDs;
+    for (BinaryBasicBlock &BB : BF) {
+      for (MCInst &Inst : BB) {
+        if (!BC.MIB->hasAnnotation(Inst, "BugEntry"))
+          continue;
+        const uint32_t ID = BC.MIB->getAnnotationAs<uint32_t>(Inst, "BugEntry");
+        EmittedIDs.insert(ID);
+
+        // Create a relocation entry for this bug entry;
+        MCSymbol *Label =
+            BC.MIB->getOrCreateInstLabel(Inst, "__BUG_", BC.Ctx.get());
+        const uint64_t EntryOffset = (ID - 1) * BUG_TABLE_ENTRY_SIZE;
+        BugTableSection->addRelocation(EntryOffset, Label, ELF::R_X86_64_PC32,
----------------
maksfb wrote:

It's stored as a relative pointer to occupy 32 bits only. The same format is used in `readBugTable()`.

https://github.com/llvm/llvm-project/pull/86908