[ELF] Remove support for legacy .zdebug sections

[MaskRay]

Summary

.zdebug is unlikely used any longer: gcc -gz switched from legacy
.zdebug to SHF_COMPRESSED with binutils 2.26 (2016), which has been
several years. clang 14 dropped -gz=zlib-gnu support. According to
Debian Code Search (gz=zlib-gnu), no project uses -gz=zlib-gnu.

Remove .zdebug support to (a) simplify code and (b) allow removal of llvm-mc's
--compress-debug-sections=zlib-gnu.

[peter.smith]

I'm happy with removing given that there is an official ELF replacement. Will be worth waiting a bit of time before committing to see if there are any last minute objections.

lld/ELF/InputSection.cpp

73–75

Possibly worth a warning message if a section prefix of .zebug is encountered? For example:

LLD does not support legacy .zdebug compressed debug sections produced with -gz=zlib-gnu. Please rebuild with -gz=zlib.

[MaskRay]

lld/ELF/InputSection.cpp

73–75

I have thought about this, but removing .zdebug check is desired: this avoids a section name read when creating input sections.

lld/ELF/InputSection.h

private:
 
   template <class ELFT> void copyShtGroup(uint8_t *buf);
 };
 
 static_assert(sizeof(InputSection) <= 160, "InputSection is too big");
 
 inline bool isDebugSection(const InputSectionBase &sec) {
   return (sec.flags & llvm::ELF::SHF_ALLOC) == 0 &&
-         (sec.name.startswith(".debug") || sec.name.startswith(".zdebug"));
+         sec.name.startswith(".debug");
 }
 
 // The list of all input sections.
 extern SmallVector<InputSectionBase *, 0> inputSections;
 
 // The set of TOC entries (.toc + addend) for which we should not apply
 // toc-indirect to toc-relative relaxation. const Symbol * refers to the
 // STT_SECTION symbol associated to the .toc input section.
 extern llvm::DenseSet<std::pair<const Symbol *, uint64_t>> ppc64noTocRelax;
 
 } // namespace elf
 
 std::string toString(const elf::InputSectionBase *);
 } // namespace lld
 
 #endif

lld/ELF/InputSection.cpp

InputSectionBase::InputSectionBase(InputFile *file, uint64_t flags,
 
   // The ELF spec states that a value of 0 means the section has
   // no alignment constraints.
   uint32_t v = std::max<uint32_t>(alignment, 1);
   if (!isPowerOf2_64(v))
     fatal(toString(this) + ": sh_addralign is not a power of 2");
   this->alignment = v;
 
-  // In ELF, each section can be compressed by zlib, and if compressed,
-  // section name may be mangled by appending "z" (e.g. ".zdebug_info").
-  // If that's the case, demangle section name so that we can handle a
-  // section as if it weren't compressed.
-  if ((flags & SHF_COMPRESSED) || name.startswith(".zdebug")) {
+  // If SHF_COMPRESSED is set, parse the header. The legacy .zdebug format is no
+  // longer supported.
+  if (flags & SHF_COMPRESSED) {
     if (!zlib::isAvailable())
       error(toString(file) + ": contains a compressed section, " +
             "but zlib is not available");
     invokeELFT(parseCompressedHeader);
   }
 }
 
 // Drop SHF_GROUP bit unless we are producing a re-linkable object file.
else
     return cast<OutputSection>(this);
   return sec ? sec->getParent() : nullptr;
 }
 
 // When a section is compressed, `rawData` consists with a header followed
 // by zlib-compressed data. This function parses a header to initialize
 // `uncompressedSize` member and remove the header from `rawData`.
 template <typename ELFT> void InputSectionBase::parseCompressedHeader() {
-  // Old-style header
-  if (!(flags & SHF_COMPRESSED)) {
-    assert(name.startswith(".zdebug"));
-    if (!toStringRef(rawData).startswith("ZLIB")) {
-      error(toString(this) + ": corrupted compressed section header");
-      return;
-    }
-    rawData = rawData.slice(4);
-
-    if (rawData.size() < 8) {
-      error(toString(this) + ": corrupted compressed section header");
-      return;
-    }
-
-    uncompressedSize = read64be(rawData.data());
-    rawData = rawData.slice(8);
-
-    // Restore the original section name.
-    // (e.g. ".zdebug_info" -> ".debug_info")
-    name = saver().save("." + name.substr(2));
-    return;
-  }
-
   flags &= ~(uint64_t)SHF_COMPRESSED;
 
   // New-style header
   if (rawData.size() < sizeof(typename ELFT::Chdr)) {
     error(toString(this) + ": corrupted compressed section");
     return;
   }
 

lld/docs/ReleaseNotes.rst

 Breaking changes
 ----------------
 
 * The GNU ld incompatible ``--no-define-common`` has been removed.
 * The obscure ``-dc``/``-dp`` options have been removed.
 * ``-d`` is now ignored.
 * If a prevailing COMDAT group defines STB_WEAK symbol, having a STB_GLOBAL symbol in a non-prevailing group is now rejected with a diagnostic.
   (`D120626 <https://reviews.llvm.org/D120626>`_)
+* Support for the legacy ``.zdebug`` format has been removed. Run
+  ``objcopy --decompress-debug-sections`` in case old object files use ``.zdebug``.
+  (`D126793 <https://reviews.llvm.org/D126793>`_)
 
 COFF Improvements
 -----------------
 
 * Added autodetection of MSVC toolchain, a la clang-cl.  Also added
   ``/winsysroot:`` support for explicit specification of MSVC toolchain
   location, similar to clang-cl's ``/winsysroot``. For now,
   ``/winsysroot:`` requires also passing in an explicit ``/machine:`` flag.

lld/test/ELF/compressed-debug-input.s

 # ZLIB-NEXT:   Offset:
 # ZLIB-NEXT:   Size:
 # ZLIB-NEXT:   Link:
 # ZLIB-NEXT:   Info:
 # ZLIB-NEXT:   AddressAlignment: 8
 # ZLIB-NEXT:   EntrySize: 1
 # ZLIB-NEXT: }
 
-# RUN: llvm-mc -compress-debug-sections=zlib-gnu -filetype=obj -triple=x86_64-unknown-linux %s -o %t2
-# RUN: llvm-mc -compress-debug-sections=zlib-gnu -filetype=obj -triple=powerpc64-unknown-unknown %s -o %t2-be
-# RUN: llvm-readobj --sections %t2 | FileCheck -check-prefix=GNU %s
-# RUN: llvm-readobj --sections %t2-be | FileCheck -check-prefix=GNU %s
-# GNU:      Section {
-# GNU:        Index: 2
-# GNU:        Name: .zdebug_str
-# GNU-NEXT:   Type: SHT_PROGBITS
-# GNU-NEXT:   Flags [
-# GNU-NEXT:     SHF_MERGE (0x10)
-# GNU-NEXT:     SHF_STRINGS (0x20)
-# GNU-NEXT:   ]
-# GNU-NEXT:   Address:
-# GNU-NEXT:   Offset:
-# GNU-NEXT:   Size:
-# GNU-NEXT:   Link:
-# GNU-NEXT:   Info:
-# GNU-NEXT:   AddressAlignment: 1
-# GNU-NEXT:   EntrySize: 1
-# GNU-NEXT: }
-
 # RUN: ld.lld --hash-style=sysv %t -o %t.so -shared
 # RUN: llvm-readobj --sections --section-data %t.so | FileCheck -check-prefix=DATA %s
 # RUN: ld.lld --hash-style=sysv %t-be -o %t-be.so -shared
 # RUN: llvm-readobj --sections --section-data %t-be.so | FileCheck -check-prefix=DATA %s
 
-# RUN: ld.lld --hash-style=sysv %t2 -o %t2.so -shared
-# RUN: llvm-readobj --sections --section-data %t2.so | FileCheck -check-prefix=DATA %s
-# RUN: ld.lld --hash-style=sysv %t2-be -o %t2-be.so -shared
-# RUN: llvm-readobj --sections --section-data %t2-be.so | FileCheck -check-prefix=DATA %s
-
 # DATA:      Section {
 # DATA:        Index: 6
 # DATA:        Name: .debug_str
 # DATA-NEXT:   Type: SHT_PROGBITS
 # DATA-NEXT:   Flags [
 # DATA-NEXT:     SHF_MERGE (0x10)
 # DATA-NEXT:     SHF_STRINGS (0x20)
 # DATA-NEXT:   ]

lld/test/ELF/relocatable-compressed-input.s

-# REQUIRES: x86, zlib
-
-# RUN: llvm-mc -compress-debug-sections=zlib-gnu -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
-# RUN: llvm-readobj --sections %t1 | FileCheck -check-prefix=GNU %s
-# GNU: Name: .zdebug_str
-
-# RUN: ld.lld %t1 -o %t2 -r
-# RUN: llvm-readobj --sections --section-data %t2 | FileCheck %s
-
-## Check we uncompress section and remove ".z" prefix specific for zlib-gnu compression.
-# CHECK:      Section {
-# CHECK:        Index:
-# CHECK:        Name: .debug_str
-# CHECK-NEXT:   Type: SHT_PROGBITS
-# CHECK-NEXT:   Flags [
-# CHECK-NEXT:     SHF_MERGE
-# CHECK-NEXT:     SHF_STRINGS
-# CHECK-NEXT:   ]
-# CHECK-NEXT:   Address:
-# CHECK-NEXT:   Offset:
-# CHECK-NEXT:   Size:
-# CHECK-NEXT:   Link:
-# CHECK-NEXT:   Info:
-# CHECK-NEXT:   AddressAlignment: 1
-# CHECK-NEXT:   EntrySize: 1
-# CHECK-NEXT:   SectionData (
-# CHECK-NEXT:     0000: {{.*}}  |unsigned int.cha|
-# CHECK-NEXT:     0010: {{.*}}  |r.unsigned char.|
-# CHECK-NEXT:     0020: {{.*}}  |short unsigned i|
-# CHECK-NEXT:     0030: {{.*}}  |nt.long unsigned|
-# CHECK-NEXT:     0040: {{.*}}  | int.|
-# CHECK-NEXT:   )
-# CHECK-NEXT: }
-
-.section .debug_str,"MS",@progbits,1
-.LASF2:
- .string "short unsigned int"
-.LASF3:
- .string "unsigned int"
-.LASF0:
- .string "long unsigned int"
-.LASF8:
- .string "char"
-.LASF1:
- .string "unsigned char"

lld/test/ELF/strip-debug.s

 # REQUIRES: x86, zlib
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld %t -o %t2 --strip-debug
 # RUN: llvm-readobj --sections %t2 | FileCheck %s
 # RUN: ld.lld %t -o %t2 -S
 # RUN: llvm-readobj --sections %t2 | FileCheck %s
 # RUN: ld.lld %t -o %t2 --strip-all
 # RUN: llvm-readobj --sections %t2 | FileCheck %s
 
 # CHECK-NOT: Foo
 # CHECK-NOT: Bar
 
 .section .debug_Foo,"",@progbits
-.section .zdebug_Bar,"",@progbits
-.ascii "ZLIB"
 .quad 0