[DWARF] Adjust warning condition for .dwo sections with relocations

[MaskRay]

Summary

D106624 added a .dwo warning (when there are relocations) that may fire for
non-debug sections, e.g. .rodata.dwo when there is a data symbol foo in
-fdata-sections mode. Adjust it to only warn for .debug sections.

While here, change the diagnostic to be more conventional
https://llvm.org/docs/CodingStandards.html#error-and-warning-messages and use
the relocated section name instead of the relocation section name.

This change does not handle .zdebug (support was mostly removed from LLVM) or
__debug (Mach-O, no DWO support).

[HaohaiWen]

llvm/lib/DebugInfo/DWARF/DWARFContext.cpp

1916

Does dwo file only contains dwo sections? (all dwo sections starts with .debug_). If so, we may not need to check section name.

[MaskRay]

llvm/lib/DebugInfo/DWARF/DWARFContext.cpp

1916

ELF .dwo files also contain .strtab.

What this code move tries to address is this: the control flow here doesn't exclude sections such as .text.dwo .rodata.dwo. RelSecName.startswith(".debug") is to exclude them. All of COFF, ELF, and wasm use the .debug_ prefix.

[ayermolo]

Thanks for fixing this. I didn't realize there were {.text,.rodata}.dwo sections.
Any good spec/doc I can read up on it?

[MaskRay]

In D153602#4444670, @ayermolo wrote:

Thanks for fixing this. I didn't realize there were {.text,.rodata}.dwo sections.
Any good spec/doc I can read up on it?

-ffunction-sections appends the function name to .text., so a function named dwo in C or C++ extern "C" mode will get this section.
.rodata.dwo / .data.dwo is similar.

The original code uses contains, which means all dwo prefixed functions/variables may have the problem.

[HaohaiWen]

LGTM

[ayermolo]

oh I see.... thx

llvm/lib/DebugInfo/DWARF/DWARFContext.cpp

for (const SectionRef &Section : Obj.sections()) {
                          .Case("debug_types.dwo", &TypesDWOSections)
                          .Default(nullptr)) {
         // Find debug_info and debug_types data by section rather than name as
         // there are multiple, comdat grouped, of these sections.
         DWARFSectionMap &S = (*Sections)[Section];
         S.Data = Data;
       }
 
-      if (RelocatedSection != Obj.section_end() && Name.contains(".dwo"))
-        HandleWarning(
-            createError("Unexpected relocations for dwo section " + Name));
-
       if (RelocatedSection == Obj.section_end() ||
           (RelocAction == DWARFContext::ProcessDebugRelocations::Ignore))
         continue;
 
       StringRef RelSecName;
       if (auto NameOrErr = RelocatedSection->getName())
         RelSecName = *NameOrErr;
       else
for (const SectionRef &Section : Obj.sections()) {
       // In Mach-o files, the relocations do not need to be applied if
       // there is no load offset to apply. The value read at the
       // relocation point already factors in the section address
       // (actually applying the relocations will produce wrong results
       // as the section address will be added twice).
       if (!L && isa<MachOObjectFile>(&Obj))
         continue;
 
+      if (!Section.relocations().empty() && Name.ends_with(".dwo") &&
+          RelSecName.startswith(".debug")) {
+        HandleWarning(createError("unexpected relocations for dwo section '" +
+                                  RelSecName + "'"));
+      }
+
       RelSecName = RelSecName.substr(
           RelSecName.find_first_not_of("._z")); // Skip . and _ prefixes.
 
       // TODO: Add support for relocations in other sections as needed.
       // Record relocations for the debug_info and debug_line sections.
       DWARFSectionMap *Sec = mapNameToDWARFSection(RelSecName);
       RelocAddrMap *Map = Sec ? &Sec->Relocs : nullptr;
       if (!Map) {

llvm/test/DebugInfo/X86/dwarfdump-rela-dwo.s

 # Test objective to verify warning is printed if DWO secton has relocations.
 #
 # RUN: llvm-mc -triple x86_64-unknown-linux %s -filetype=obj -o %t.o
 # RUN: llvm-dwarfdump --debug-info %t.o | FileCheck %s
 # RUN: llvm-dwarfdump --debug-info %t.o 2> %t.txt
-# RUN: cat %t.txt | FileCheck %s --check-prefix=PART2
+# RUN: FileCheck --input-file=%t.txt %s --check-prefix=PART2 --implicit-check-not=warning:
 
         .section .debug_str.dwo,"MSe",@progbits,1
 .dwo_producer:
         .asciz "Handmade DWO producer"
 .dwo_CU_5:
         .asciz "V5_dwo_compile_unit"
 
         .section  .debug_str_offsets.dwo,"e",@progbits
# And a .dwo copy for the .dwo sections.
         .byte 0x25  # DW_AT_producer
         .byte 0x0e  # DW_FORM_strp
         .byte 0x03  # DW_AT_name
         .byte 0x25  # DW_FORM_strx1
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
         .byte 0x00  # EOM(3)
 
+# PART2: warning: unexpected relocations for dwo section '.debug_abbrev.dwo'
+        .reloc ., R_X86_64_NONE, 0
+
         .section .debug_info.dwo,"e",@progbits
 # CHECK-LABEL: .debug_info.dwo
 
 # DWARF v5 split CU header.
         .long  CU_split_5_end-CU_split_5_version # Length of Unit
 CU_split_5_version:
         .short 5                # DWARF version number
         .byte 5                 # DWARF Unit Type
         .byte 8                 # Address Size (in bytes)
         .long 0 # Offset Into Abbrev. Section
         .quad 0x5a              # DWO ID
 # The split compile-unit DIE, with DW_AT_producer, DW_AT_name, DW_AT_stmt_list.
         .byte 1
         .long .dwo_producer
         .byte 1
         .byte 0 # NULL
 CU_split_5_end:
 
 # CHECK: 0x00000000: Compile Unit: length = 0x00000017, format = DWARF32, version = 0x0005, unit_type = DW_UT_split_compile, abbr_offset = 0x0000, addr_size = 0x08, DWO_id = 0x000000000000005a (next unit at 0x0000001b)
 # CHECK: 0x00000014: DW_TAG_compile_unit
 # CHECK-NEXT: DW_AT_producer  ("Handmade DWO producer")
 # CHECK-NEXT: DW_AT_name  ("V5_dwo_compile_unit")
-# PART2: warning: Unexpected relocations for dwo section rela.debug_info.dwo
+# PART2: warning: unexpected relocations for dwo section '.debug_info.dwo'
+
+## No warning, even if their names end with ".dwo".
+        .section .rodata.dwo,"a"
+        .long ext
+        .section .foo.dwo,""
+        .long .dwo_producer