[lld] [lld][ELF] Add --debug-names to create merged .debug_names. (PR #86508)

[Fangrui Song via llvm-commits]

================
@@ -2711,6 +2714,606 @@ static uint32_t computeGdbHash(StringRef s) {
   return h;
 }
 
+// 4-byte alignment ensures that values in the hash lookup table and the name
+// table are aligned.
+DebugNamesBaseSection::DebugNamesBaseSection()
+    : SyntheticSection(0, SHT_PROGBITS, 4, ".debug_names") {}
+
+// Get the size of the .debug_names section header in bytes for DWARF32:
+static uint32_t getDebugNamesHeaderSize(uint32_t augmentationStringSize) {
+  return /* unit length */ 4 +
+         /* version */ 2 +
+         /* padding */ 2 +
+         /* CU count */ 4 +
+         /* TU count */ 4 +
+         /* Foreign TU count */ 4 +
+         /* Bucket Count */ 4 +
+         /* Name Count */ 4 +
+         /* Abbrev table size */ 4 +
+         /* Augmentation string size */ 4 +
+         /* Augmentation string */ augmentationStringSize;
+}
+
+void DebugNamesBaseSection::parseDebugNames(
+    InputChunk &inputChunk, OutputChunk &chunk,
+    DWARFDataExtractor &namesExtractor, DataExtractor &strExtractor,
+    function_ref<SmallVector<uint32_t, 0>(
+        const DWARFDebugNames::Header &,
+        const DWARFDebugNames::DWARFDebugNamesOffsets &)>
+        readOffsets) {
+  const LLDDWARFSection namesSec = inputChunk.section;
+  DenseMap<uint32_t, IndexEntry *> offsetMap;
+  for (const DWARFDebugNames::NameIndex &ni : *inputChunk.llvmDebugNames) {
+    NameData &nd = inputChunk.nameData.emplace_back();
+    nd.hdr = ni.getHeader();
+    if (nd.hdr.Format != DwarfFormat::DWARF32) {
+      errorOrWarn(toString(namesSec.sec) + Twine(": unsupported DWARF64"));
+      return;
+    }
+    if (nd.hdr.Version != 5) {
+      errorOrWarn(toString(namesSec.sec) + Twine(": unsupported version ") +
+                  Twine(nd.hdr.Version));
+      return;
+    }
+    const uint32_t dwarfSize =
+        dwarf::getDwarfOffsetByteSize(DwarfFormat::DWARF32);
+    const uint32_t hdrSize =
+        getDebugNamesHeaderSize(nd.hdr.AugmentationStringSize);
+    auto locs = findDebugNamesOffsets(hdrSize, nd.hdr);
+    if (locs.EntriesBase > namesExtractor.getData().size()) {
+      errorOrWarn(toString(namesSec.sec) +
+                  Twine(": index entry is out of bounds"));
+      return;
+    }
+
+    SmallVector<uint32_t, 0> entryOffsets = readOffsets(nd.hdr, locs);
+
+    // Read the entry pool.
+    offsetMap.clear();
+    nd.nameEntries.resize(nd.hdr.NameCount);
+    for (auto i : seq(nd.hdr.NameCount)) {
+      NameEntry &ne = nd.nameEntries[i];
+      uint64_t strOffset = locs.StringOffsetsBase + i * dwarfSize;
+      ne.stringOffset = strOffset;
+      uint64_t strp = namesExtractor.getRelocatedValue(dwarfSize, &strOffset);
+      StringRef name = strExtractor.getCStrRef(&strp);
+      ne.name = name.data();
+      ne.hashValue = caseFoldingDjbHash(name);
+
+      // Read a series of index entries that end with abbreviation code 0.
+      const char *errMsg = nullptr;
+      uint64_t offset = locs.EntriesBase + entryOffsets[i];
+      while (offset < namesSec.Data.size() && namesSec.Data[offset] != 0) {
+        // Read & store all entries (for the same string).
+        auto ie = makeThreadLocal<IndexEntry>();
+        ie->poolOffset = offset;
+        Error err = Error::success();
+        ie->abbrevCode =
+            static_cast<uint32_t>(namesExtractor.getULEB128(&offset, &err));
+        if (err) {
+          consumeError(std::move(err));
+          errMsg = ": invalid abbrev code in entry";
+          break;
+        }
+        auto it = ni.getAbbrevs().find_as(ie->abbrevCode);
+        if (it == ni.getAbbrevs().end()) {
+          errMsg = ": invalid abbrev code in entry";
+          break;
+        }
+
+        AttrValue attr, cuAttr = {0, 0};
+        for (DWARFDebugNames::AttributeEncoding a : it->Attributes) {
+          if (a.Index == dwarf::DW_IDX_parent) {
+            if (a.Form == dwarf::DW_FORM_ref4) {
+              attr.attrValue = namesExtractor.getU32(&offset, &err);
+              attr.attrSize = 4;
+              ie->parentOffset = locs.EntriesBase + attr.attrValue;
+            } else if (a.Form != DW_FORM_flag_present) {
+              errMsg = ": invalid form for DW_IDX_parent";
+            }
+          } else {
+            switch (a.Form) {
+            case DW_FORM_data1:
+            case DW_FORM_ref1: {
+              attr.attrValue = namesExtractor.getU8(&offset, &err);
+              attr.attrSize = 1;
+              break;
+            }
+            case DW_FORM_data2:
+            case DW_FORM_ref2: {
+              attr.attrValue = namesExtractor.getU16(&offset, &err);
+              attr.attrSize = 2;
+              break;
+            }
+            case DW_FORM_data4:
+            case DW_FORM_ref4: {
+              attr.attrValue = namesExtractor.getU32(&offset, &err);
+              attr.attrSize = 4;
+              break;
+            }
+            default:
+              errorOrWarn(toString(namesSec.sec) +
----------------
MaskRay wrote:

Here we invoke `return;`, so it should be fine?

The error recovery is indeed my argument to request for some `--noinhibit-exec` tests to prove that we don't invoke UB (checked by ubsan) at writeTo time.

https://github.com/llvm/llvm-project/pull/86508