[ELF] Respect NonAlloc when copying flags from the previous sections

[MaskRay]

Summary

If the output section contains only symbol assignments, we copy flags
from the previous sections. Don't set SHF_ALLOC if NonAlloc is true.

We also have to change the type from SHT_NOBITS to SHT_PROGBITS.
In BFD, bfd_elf_get_default_section_type maps non-alloctable sections to SHT_PROGBITS.

Fixes PR38626

[ruiu]

ELF/LinkerScript.cpp

883

Sec->Flags can have SHF_ALLOC bit on while Sec->NonAlloc is true? If so, it sounds like a contradiction... Maybe we should fix that?

[MaskRay]

ELF/LinkerScript.cpp

883

In this function (adjustSectionBeforeSorting), Sec->Flags is consistent with Sec->NonAlloc. This is because processSectionCommands runs before and does if (Sec->NonAlloc) Sec->Flags &= ~(uint64_t)SHF_ALLOC;`.

This patch makes the inferred flags for empty output sections correct.

[ruiu]

I don't think I understand the logic. So the rule you said is that, if a section is empty, its section attribute is copied from the previous (non-empty) section. Therefore, if the previous section is SHF_ALLOC, the empty section would have SHF_ALLOC, and vice versa. But you are doing more than copying section attributes -- nullifying only SHF_ALLOC using other bits. Why?

[MaskRay]

So the rule you said is that, if a section is empty, its section attribute is copied from the previous (non-empty) section.

Yes. (SHF_ALLOC | SHF_WRITE | SHF_EXECINSTR) may be copied.

Therefore, if the previous section is SHF_ALLOC, the empty section would have SHF_ALLOC, and vice versa.

The empty OutputSection may have its NonAlloc bit set. In that case, The SHF_ALLOC bit shouldn't be copied.

But you are doing more than copying section attributes -- nullifying only SHF_ALLOC using other bits. Why?

If Sec->NonAlloc, Sec->Flags can't have the SHF_ALLOC bit set. The bit (if set originally) has been cleared by:

void LinkerScript::processSectionCommands() {
...
      if (Sec->NonAlloc)
        Sec->Flags &= ~(uint64_t)SHF_ALLOC;

[MaskRay]

Ping :)

[MaskRay]

Ping :)

[ruiu]

LGTM

ELF/LinkerScript.cpp

for (BaseCommand *&Cmd : SectionCommands) {
     if (Sec->AlignExpr)
       Sec->Alignment =
           std::max<uint32_t>(Sec->Alignment, Sec->AlignExpr().getValue());
 
     // A live output section means that some input section was added to it. It
     // might have been removed (if it was empty synthetic section), but we at
     // least know the flags.
     if (Sec->Live)
       Flags = Sec->Flags;
 
     // We do not want to keep any special flags for output section
     // in case it is empty.
     bool IsEmpty = getInputSections(Sec).empty();
     if (IsEmpty)
-      Sec->Flags = Flags & (SHF_ALLOC | SHF_WRITE | SHF_EXECINSTR);
+      Sec->Flags =
+          Flags & ((Sec->NonAlloc ? 0 : SHF_ALLOC) | SHF_WRITE | SHF_EXECINSTR);
 
     if (IsEmpty && isDiscardable(*Sec)) {
       Sec->Live = false;
       Cmd = nullptr;
     }
   }
 
   // It is common practice to use very generic linker scripts. So for any

ELF/ScriptParser.cpp

if (Tok2 != "NOLOAD" && Tok2 != "COPY" && Tok2 != "INFO" && Tok2 != "OVERLAY")
     return false;
 
   expect("(");
   if (consume("NOLOAD")) {
     Cmd->Noload = true;
   } else {
     skip(); // This is "COPY", "INFO" or "OVERLAY".
     Cmd->NonAlloc = true;
+    Cmd->Type = SHT_PROGBITS;
   }
   expect(")");
   return true;
 }
 
 // Reads an expression and/or the special directive for an output
 // section definition. Directive is one of following: "(NOLOAD)",
 // "(COPY)", "(INFO)" or "(OVERLAY)".

test/ELF/linkerscript/info-section-type.s

 # RUN: echo "SECTIONS { .bar (INFO) : { *(.foo) } };" > %t.script
 # RUN: ld.lld -o %t --script %t.script %t.o
 # RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=NONALLOC
 
 # RUN: echo "SECTIONS { .bar (OVERLAY) : { *(.foo) } };" > %t.script
 # RUN: ld.lld -o %t --script %t.script %t.o
 # RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=NONALLOC
 
+# RUN: echo "SECTIONS { .bar (INFO) : { . += 1; } };" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+# RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=NONALLOC
+
 # RUN: echo "SECTIONS { .bar 0x20000 (INFO) : { *(.foo) } };" > %t.script
 # RUN: ld.lld -o %t --script %t.script %t.o
 # RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=NONALLOC
 
 # RUN: echo "SECTIONS { .bar 0x20000 (BAR) : { *(.foo) } };" > %t.script
 # RUN: not ld.lld -o %t --script %t.script %t.o 2>&1 |\
 # RUN:   FileCheck %s --check-prefix=UNKNOWN
 # UNKNOWN: unknown section directive: BAR
 
 .section .foo,"a",@progbits
 .zero 1