[lld] Allow unrecognized signatures in debug sections

[CyberShadow]

Summary

An unrecognized signature (magic) at the beginning of a debug section
should not be a fatal error; it only means that the debug information
is in a format that is not supported by LLD. This can be due to it
being in CodeView versions 3 or earlier. These can occur in old import
libraries from legacy SDKs.

The test case was verified to work with MS link.exe.

[rnk]

lgtm, do you want me to land this for you?

[CyberShadow]

Thank you, and, sure!

lld/COFF/Chunks.cpp

ArrayRef<uint8_t> SectionChunk::consumeDebugMagic(ArrayRef<uint8_t> Data,
 
   // First 4 bytes are section magic.
   if (Data.size() < 4)
     fatal("the section is too short: " + SectionName);
 
   if (!SectionName.startswith(".debug$"))
     fatal("invalid section: " + SectionName);
 
-  unsigned Magic = support::endian::read32le(Data.data());
-  unsigned ExpectedMagic = SectionName == ".debug$H"
+  uint32_t Magic = support::endian::read32le(Data.data());
+  uint32_t ExpectedMagic = SectionName == ".debug$H"
                                ? DEBUG_HASHES_SECTION_MAGIC
                                : DEBUG_SECTION_MAGIC;
-  if (Magic != ExpectedMagic)
-    fatal("section: " + SectionName + " has an invalid magic: " + Twine(Magic));
+  if (Magic != ExpectedMagic) {
+    warn("ignoring section " + SectionName + " with unrecognized magic 0x" +
+         utohexstr(Magic));
+    return {};
+  }
   return Data.slice(4);
 }
 
 SectionChunk *SectionChunk::findByName(ArrayRef<SectionChunk *> Sections,
                                        StringRef Name) {
   for (SectionChunk *C : Sections)
     if (C->getSectionName() == Name)
       return C;

lld/test/COFF/allow-unknown-debug-info.test

+# RUN: yaml2obj %s > %t.obj
+# RUN: lld-link /dll /noentry /debug %t.obj 2>&1 | FileCheck %s
+
+# CHECK: ignoring section .debug$S with unrecognized magic 0x1
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_I386
+  Characteristics: []
+sections:
+  - Name:            '.debug$S'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    SectionData:     01000000
+symbols:
+  - Name:            '.debug$S'
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          4
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0