[llvm-objcopy][ELF] Allow --dump-section to dump an empty non-SHT_NOBITS section

[MaskRay]

Summary

This is the ELF part of D75949.

GNU objcopy from binutils 2.35 onwards will support an empty non-SHT_NOBITS section as
well
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=e052e2ba295a65b6ea80cbc3f90495beca299c42

[jhenderson]

LGTM.

llvm/test/tools/llvm-objcopy/ELF/dump-section.test

51

Nit: the prevailing pattern for FileCheck directives in this file is currently #CHECK: (no space). I'm okay if you want to change the other lines to match the # CHECK: style, but perhaps do as a separate NFC commit. Alternatively, make this match the existing ones.

[MaskRay]

#CHECK -> # CHECK in a separate commit

llvm/test/tools/llvm-objcopy/ELF/dump-section.test

 # RUN: yaml2obj %s -o %t
 # RUN: llvm-objcopy -O binary -j .text %t %t2
 # RUN: llvm-objcopy -O binary --only-section .text %t %t3
 # RUN: llvm-objcopy --dump-section .text=%t4 %t %t5
 # RUN: llvm-objcopy --dump-section .foo=%t6 %t %t7
 # RUN: not llvm-objcopy --dump-section .bar=%t8 %t %t9 2>&1 | FileCheck %s --check-prefix=NOBITS -DINPUT=%t
+# RUN: llvm-objcopy --dump-section .empty=%t.empty %t /dev/null
 # RUN: od -t x1 %t2 | FileCheck %s --ignore-case
 # RUN: od -t x1 %t6 | FileCheck %s --ignore-case --check-prefix=NON-ALLOC
 # RUN: wc -c %t2 | FileCheck %s --check-prefix=SIZE
+# RUN: wc -c %t.empty | FileCheck %s --check-prefix=EMPTY
 # RUN: diff %t2 %t3
 # RUN: diff %t4 %t3
 
 !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_EXEC
   Machine:         EM_X86_64
 Sections:
   - Name:            .text
     Type:            SHT_PROGBITS
     Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
     AddressAlign:    0x0000000000001000
     Content:         "DEADBEEF"
   - Name:            .foo
     Type:            SHT_PROGBITS
     Flags:           [ SHF_WRITE ]
     Content:         "CAFE"
+  - Name:            .empty
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC ]
   - Name:            .bar
     Type:            SHT_NOBITS
     Flags:           [ SHF_WRITE ]
 ProgramHeaders:
 - Type: PT_LOAD
   Flags: [ PF_X, PF_R ]
   Sections:
     - Section: .text
 
 # CHECK: 0000000 de ad be ef
 
 # NON-ALLOC: 0000000 ca fe
 
 # SIZE: 4
 
 # NOBITS: error: '[[INPUT]]': cannot dump section '.bar': it has no contents
+
+# EMPTY: 0

llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp

if (Error E = Writer->finalize())
     return E;
   return Writer->write();
 }
 
 static Error dumpSectionToFile(StringRef SecName, StringRef Filename,
                                Object &Obj) {
   for (auto &Sec : Obj.sections()) {
     if (Sec.Name == SecName) {
-      if (Sec.OriginalData.empty())
+      if (Sec.Type == SHT_NOBITS)
         return createStringError(object_error::parse_failed,
                                  "cannot dump section '%s': it has no contents",
                                  SecName.str().c_str());
       Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
           FileOutputBuffer::create(Filename, Sec.OriginalData.size());
       if (!BufferOrErr)
         return BufferOrErr.takeError();
       std::unique_ptr<FileOutputBuffer> Buf = std::move(*BufferOrErr);