[LLDB] Warn about truncated DWARF section names on Windows

Author: Nerixyz

lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp

@@ -1036,12 +1036,23 @@ void ObjectFilePECOFF::CreateSections(SectionList &unified_section_list) {
     m_sections_up->AddSection(header_sp);
     unified_section_list.AddSection(header_sp);
 
+    std::vector<llvm::StringRef> truncated_dwarf_sections;
     const uint32_t nsects = m_sect_headers.size();
     for (uint32_t idx = 0; idx < nsects; ++idx) {
       llvm::StringRef sect_name = GetSectionName(m_sect_headers[idx]);
       ConstString const_sect_name(sect_name);
       SectionType section_type = GetSectionType(sect_name, m_sect_headers[idx]);
 
+      // Detect unknown sections matching ".debug_*"
+      // In PECOFF files, the section name in the section header can only
+      // contain 8 bytes. If a section name doesn't fit there, it can be
+      // extended in the string table. Since, officially, executable images
+      // don't have a string table, the default link.exe truncates section names
+      // to fit in the section header.
+      if (section_type == eSectionTypeOther && sect_name.size() == 8 &&
+          sect_name.starts_with(".debug_"))
+        truncated_dwarf_sections.emplace_back(sect_name);
+
       SectionSP section_sp(new Section(
           module_sp,       // Module to which this section belongs
           this,            // Object file to which this section belongs
@@ -1071,6 +1082,16 @@ void ObjectFilePECOFF::CreateSections(SectionList &unified_section_list) {
       m_sections_up->AddSection(section_sp);
       unified_section_list.AddSection(section_sp);
     }
+
+    if (!truncated_dwarf_sections.empty())
+      module_sp->ReportWarning(
+          "contains {} DWARF sections with truncated names ({}).\nWindows "
+          "executable (PECOFF) images produced by the default link.exe don't "
+          "include the required section names. A third party linker like "
+          "lld-link is required (compile with -fuse-ld=lld-link when using "
+          "Clang).",
+          truncated_dwarf_sections.size(),
+          llvm::join(truncated_dwarf_sections, ", "));
   }
 }
 

lldb/test/Shell/ObjectFile/PECOFF/lit.local.cfg

@@ -1 +1 @@
-config.suffixes = ['.yaml', '.test']
+config.suffixes = ['.yaml', '.test', '.c']

lldb/test/Shell/ObjectFile/PECOFF/truncated-dwarf.c

@@ -0,0 +1,7 @@
+// REQUIRES: target-windows
+// RUN: %build --compiler=clang-cl --force-dwarf-symbols --force-ms-link -o %t.exe -- %s
+// RUN: %lldb -f %t.exe 2>&1 | FileCheck %s
+
+int main(void) {}
+
+// CHECK: warning: {{.*}} contains 4 DWARF sections with truncated names (.debug_{{[a-z]}}, .debug_{{[a-z]}}, .debug_{{[a-z]}}, .debug_{{[a-z]}})

lldb/test/Shell/helper/build.py

@@ -173,6 +173,22 @@
     help="Specify the C/C++ standard.",
 )
 
+parser.add_argument(
+    "--force-dwarf-symbols",
+    dest="force_dwarf_symbols",
+    action="store_true",
+    default=False,
+    help="When compiling with clang-cl on Windows, use DWARF instead of CodeView",
+)
+
+parser.add_argument(
+    "--force-ms-link",
+    dest="force_ms_link",
+    action="store_true",
+    default=False,
+    help="When compiling with clang-cl on Windows, always use link.exe",
+)
+
 
 args = parser.parse_args(args=sys.argv[1:])
 
@@ -379,15 +395,20 @@ def __init__(self, toolchain_type, args):
                     )
 
         if self.mode == "link" or self.mode == "compile-and-link":
-            self.linker = (
-                self._find_linker("link")
-                if toolchain_type == "msvc"
-                else self._find_linker("lld-link", args.tools_dir)
-            )
+            if toolchain_type == "msvc" or args.force_ms_link:
+                search_paths = []
+                if toolchain_type != "msvc":
+                    search_paths.append(
+                        os.path.dirname(find_executable("cl", args.tools_dir))
+                    )
+                self.linker = self._find_linker("link", search_paths)
+            else:
+                self.linker = self._find_linker("lld-link", args.tools_dir)
             if not self.linker:
                 raise ValueError("Unable to find an appropriate linker.")
 
         self.compile_env, self.link_env = self._get_visual_studio_environment()
+        self.force_dwarf_symbols = args.force_dwarf_symbols
 
     def _find_linker(self, name, search_paths=[]):
         compiler_dir = os.path.dirname(self.compiler)
@@ -678,6 +699,8 @@ def _get_compilation_command(self, source, obj):
             args.append("/GR-")
         args.append("/Z7")
         if self.toolchain_type == "clang-cl":
+            if self.force_dwarf_symbols:
+                args.append("-gdwarf")
             args.append("-Xclang")
             args.append("-fkeep-static-consts")
             args.append("-fms-compatibility-version=19")

llvm/docs/ReleaseNotes.md

@@ -304,6 +304,8 @@ Changes to LLDB
     stop reason = SIGSEGV: sent by tkill system call (sender pid=649752, uid=2667987)
   ```
 * ELF Cores can now have their siginfo structures inspected using `thread siginfo`.
+* LLDB now detects when the names of known DWARF sections in a PECOFF file have
+  been truncated by link.exe on Windows, and advises the user how to avoid this.
 
 ### Changes to lldb-dap