[ObjectYAML][ELF] Report incorrect offset to generate notes

All notes in the note section must be correctly aligned, including the
first. The tool should refuse to generate notes if the section offset is
incorrect in this respect.

Author: igorkudrin

llvm/lib/ObjectYAML/ELFEmitter.cpp

@@ -1796,7 +1796,7 @@ template <class ELFT>
 void ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
                                          const ELFYAML::NoteSection &Section,
                                          ContiguousBlobAccumulator &CBA) {
-  if (!Section.Notes)
+  if (!Section.Notes || Section.Notes->empty())
     return;
 
   unsigned Align;
@@ -1814,6 +1814,13 @@ void ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
     return;
   }
 
+  if (CBA.getOffset() != alignTo(CBA.getOffset(), Align)) {
+    reportError(Section.Name + ": invalid offset of a note section: 0x" +
+                Twine::utohexstr(CBA.getOffset()) + ", should be aligned to " +
+                Twine(Align));
+    return;
+  }
+
   uint64_t Offset = CBA.tell();
   for (const ELFYAML::NoteEntry &NE : *Section.Notes) {
     // Write name size.

llvm/test/tools/yaml2obj/ELF/note-section.yaml

@@ -514,3 +514,58 @@ Sections:
         Desc: 030405
       - Name: GNU
         Type: NT_GNU_BUILD_ID
+
+## Check that an incorrect offset for generating notes is reported.
+
+# RUN: not yaml2obj --docnum=19 %s 2>&1 | FileCheck %s --check-prefix=ERR_OFFSET
+# ERR_OFFSET: error: .note: invalid offset of a note section: 0x{{.*}}, should be aligned to 4
+
+--- !ELF
+FileHeader:
+  Class: ELFCLASS32
+  Data:  ELFDATA2LSB
+  Type:  ET_EXEC
+Sections:
+  - Name: .dummy
+    Type: SHT_PROGBITS
+    Size: 1
+  - Name: .note
+    Type: SHT_NOTE
+    Notes:
+      - Type: 0x1
+
+## Do not issue an error if the notes array is empty.
+
+# RUN: yaml2obj --docnum=20 %s -o - | \
+# RUN:   llvm-readobj --sections --section-data - | \
+# RUN:   FileCheck %s --check-prefix=TEST20
+
+# TEST20:      Section {
+# TEST20:      Name: .note
+# TEST20-NEXT:   Type: SHT_NOTE
+# TEST20-NEXT:   Flags [ (0x0)
+# TEST20-NEXT:   ]
+# TEST20-NEXT:   Address:
+# TEST20-NEXT:   Offset:
+# TEST20-NEXT:   Size: 0
+# TEST20-NEXT:   Link:
+# TEST20-NEXT:   Info:
+# TEST20-NEXT:   AddressAlignment: 5
+# TEST20-NEXT:   EntrySize:
+# TEST20-NEXT:   SectionData (
+# TEST20-NEXT:   )
+# TEST20-NEXT: }
+
+--- !ELF
+FileHeader:
+  Class: ELFCLASS32
+  Data:  ELFDATA2LSB
+  Type:  ET_EXEC
+Sections:
+  - Name: .dummy
+    Type: SHT_PROGBITS
+    Size: 1
+  - Name: .note
+    Type: SHT_NOTE
+    AddressAlign: 5
+    Notes: []