[ELF] Don't consider SHF_ALLOC ".debug*" sections debug sections

[MaskRay]

Summary

Fixes PR48071

• The Rust compiler produces SHF_ALLOC .debug_gdb_scripts (which normally does not have the flag)
• .debug_gdb_scripts sections are removed from inputSections due to --strip-debug/--strip-all
• When processing --gc-sections, pieces of a SHF_MERGE section can be marked live separately

=> segfault when marking liveness of a .debug_gdb_scripts which is not split into pieces (because it is not in inputSections)

This patch circumvents the problem by not treating SHF_ALLOC ".debug*" as debug sections (to prevent --strip-debug's stripping)
(which is still useful on its own).

[MaskRay]

Fix test

[grimar]

LGTM

lld/ELF/InputSection.h

 
 #ifdef _WIN32
 static_assert(sizeof(InputSection) <= 192, "InputSection is too big");
 #else
 static_assert(sizeof(InputSection) <= 184, "InputSection is too big");
 #endif
 
 inline bool isDebugSection(const InputSectionBase &sec) {
-  return sec.name.startswith(".debug") || sec.name.startswith(".zdebug");
+  return (sec.flags & llvm::ELF::SHF_ALLOC) == 0 &&
+         (sec.name.startswith(".debug") || sec.name.startswith(".zdebug"));
 }
 
 // The list of all input sections.
 extern std::vector<InputSectionBase *> 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/test/ELF/gc-sections-strip-debug.s

+# REQUIRES: x86
+## Test that we don't strip SHF_ALLOC .debug* or crash (PR48071
+## mark liveness of a merge section which has not been split).
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld %t.o --gc-sections --strip-debug -o %t
+# RUN: llvm-readelf -S %t | FileCheck %s
+
+# CHECK: .debug_gdb_scripts
+
+.globl _start
+_start:
+  leaq .L.str(%rip), %rax
+
+.section .debug_gdb_scripts,"aMS",@progbits,1
+.L.str:
+  .asciz "Rust uses SHF_ALLOC .debug_gdb_scripts"