[ELF] Ignore sections flagged with SHF_EXCLUDE

[evgeny777]

Summary

The spec says:

This section is excluded from input to the link-edit of an executable or shared object. This flag is ignored if the SHF_ALLOC flag is also set, or if relocations exist against the section.

However both gold and ld seem to always discard such sections. The same approach was chosen for this patch.

[ruiu]

LGTM

ELF/InputFiles.cpp

void elf::ObjectFile<ELFT>::initializeSections(
   Sections.resize(Size);
   unsigned I = -1;
   const ELFFile<ELFT> &Obj = this->ELFObj;
   for (const Elf_Shdr &Sec : Obj.sections()) {
     ++I;
     if (Sections[I] == &InputSection<ELFT>::Discarded)
       continue;
 
+    if (Sec.sh_flags & SHF_EXCLUDE) {
+      Sections[I] = &InputSection<ELFT>::Discarded;
+      continue;
+    }
+
     switch (Sec.sh_type) {
     case SHT_GROUP:
       Sections[I] = &InputSection<ELFT>::Discarded;
       if (ComdatGroups.insert(getShtGroupSignature(Sec)).second)
         continue;
       for (uint32_t SecIndex : getShtGroupEntries(Sec)) {
         if (SecIndex >= Size)
           fatal(getFilename(this) + ": invalid section index in group: " +

test/ELF/exclude.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { .text : { *(.text*) } }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -section-headers %t1 | FileCheck %s
+# CHECK-NOT:      .aaa
+
+.globl _start
+_start:
+  jmp _start
+
+.section .aaa,"ae"