[llvm-readobj] Fallback to PT_NOTE if file doesn't have sections

[phosek]

Summary

This is useful when trying to read notes from stripped files and matches
the behavior of GNU readelf and eu-readelf.

[rupprecht]

I can't reproduce the test case -- it seems llvm-readelf already prints the NT_GNU_BUILD_ID when I follow the test steps.

llvm/test/tools/llvm-readobj/gnu-notes.test

6

Copy this to %t2.so so the original %t1.so is unaffected - e.g. if someone reruns one of these commands to debug a test failure, they should get the unstripped one with %t1.so even if the rest of the test case runs.

7

Can you add the corresponding FileCheck for llvm-readelf?

llvm/tools/llvm-readobj/ELFDumper.cpp

4505

Can you add this to LLVMStyle<ELFT>::printNotes too?

[phosek]

It should reproduce now.

[jhenderson]

Thanks for doing this. This was something on my radar too as an odd inconsistency.

llvm/trunk/test/tools/llvm-readobj/gnu-notes.test

7 ↗

(On Diff #215707)

Not that it matters that much, but I don't think you needed the --strict-whitespace and --match-full-lines, since the format would be the same as the unstripped case.

72–75 ↗

(On Diff #215707)

Could you make a follow-up commit to share this with the above GNU case, please? This could be achieved with something like:

# RUN: ... | FileCheck %s --check-prefixes=GNU,UNSTRIPPED -DOFFSET=0x00000238
# RUN: ... | FileCheck %s --check-prefix=GNU --implicit-check-not={{.}} -DOFFSET=0x00000040

#      GNU:Displaying notes found at file offset [[OFFSET]] with length 0x00000020:
# GNU-NEXT:  Owner                Data size   Description
# GNU-NEXT:  GNU                  0x00000010  NT_GNU_ABI_TAG (ABI version tag)
# GNU-NEXT:    OS: Linux, ABI: 2.6.32

#      UNSTRIPPED:Displaying notes found at file offset 0x00000258 with length 0x00000020:
# UNSTRIPPED-NEXT:  Owner                Data size   Description
...

The same sort of thing should work with the LLVM output. By doing this, you avoid the duplicate contents, and illustrate that the part covered by the program header should be identical.

llvm/test/tools/llvm-readobj/gnu-notes.test

 ## Test tools are able to dump different types of notes.
 
 # RUN: yaml2obj --docnum=1 %s > %t1.so
 # RUN: llvm-readelf --notes %t1.so | FileCheck %s --check-prefix=GNU --strict-whitespace --match-full-lines
 # RUN: llvm-readobj --notes %t1.so | FileCheck %s --check-prefix=LLVM
+# RUN: llvm-objcopy --strip-all %t1.so
+# RUN: llvm-readobj --notes %t1.so | FileCheck %s --check-prefix=LLVM-STRIPPED
 
 #      GNU:Displaying notes found at file offset 0x00000200 with length 0x00000020:
 # GNU-NEXT:  Owner                Data size   Description
 # GNU-NEXT:  GNU                  0x00000010  NT_GNU_ABI_TAG (ABI version tag)
 # GNU-NEXT:    OS: Linux, ABI: 2.6.32
 
 #      GNU:Displaying notes found at file offset 0x00000220 with length 0x00000020:
 # GNU-NEXT:  Owner                Data size   Description

 # LLVM-NEXT:       Owner: GNU
 # LLVM-NEXT:       Data size: 0x9
 # LLVM-NEXT:       Type: NT_GNU_GOLD_VERSION (gold version)
 # LLVM-NEXT:       Version: gold 1.11
 # LLVM-NEXT:     }
 # LLVM-NEXT:   }
 # LLVM-NEXT: ]
 
+# LLVM-STRIPPED:      Notes [
+# LLVM-STRIPPED-NEXT:   NoteSection {
+# LLVM-STRIPPED-NEXT:     Offset: 0x40
+# LLVM-STRIPPED-NEXT:     Size: 0x20
+# LLVM-STRIPPED-NEXT:     Note {
+# LLVM-STRIPPED-NEXT:       Owner: GNU
+# LLVM-STRIPPED-NEXT:       Data size: 0x10
+# LLVM-STRIPPED-NEXT:       Type: NT_GNU_BUILD_ID (unique build ID bitstring)
+# LLVM-STRIPPED-NEXT:       Build ID: 4fcb712aa6387724a9f465a32cd8c14b
+# LLVM-STRIPPED-NEXT:     }
+# LLVM-STRIPPED-NEXT:   }
+# LLVM-STRIPPED-NEXT: ]
+
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_EXEC
   Machine: EM_X86_64
 Sections:
   - Name:         .note.ABI-tag

llvm/tools/llvm-readobj/ELFDumper.cpp

auto ProcessNote = [&](const Elf_Note &Note) {
     } else if (!Descriptor.empty()) {
       OS << "   description data:";
       for (uint8_t B : Descriptor)
         OS << " " << format("%02x", B);
       OS << '\n';
     }
   };
 
-  if (Obj->getHeader()->e_type == ELF::ET_CORE) {
+  if (Obj->getHeader()->e_type == ELF::ET_CORE || Obj->sections()->empty()) {
     for (const auto &P :
          unwrapOrError(this->FileName, Obj->program_headers())) {
       if (P.p_type != PT_NOTE)
         continue;
       PrintHeader(P.p_offset, P.p_filesz);
       Error Err = Error::success();
       for (const auto &Note : Obj->notes(P, Err))
         ProcessNote(Note);