Add support for >= 0xff00 sections

Author: filipnavara

src/LibObjectFile/Elf/ElfObjectFile.cs

@@ -230,6 +230,8 @@ public unsafe void UpdateLayout(DiagnosticBag diagnostics)
                     var section = sections[i];
                     if (i == 0 && section.Type == ElfSectionType.Null)
                     {
+                        section.Size = VisibleSectionCount >= ElfNative.SHN_LORESERVE ? VisibleSectionCount : 0;
+                        section.Link = (SectionHeaderStringTable?.SectionIndex ?? 0) >= ElfNative.SHN_LORESERVE ? SectionHeaderStringTable : null;
                         continue;
                     }
 

src/LibObjectFile/Elf/ElfWriter{TEncoder}.cs

@@ -155,8 +155,9 @@ private unsafe void WriteSectionHeader32()
             // entries for sections
             _encoder.Encode(out hdr.e_shoff, (uint)Layout.OffsetOfSectionHeaderTable);
             _encoder.Encode(out hdr.e_shentsize, Layout.SizeOfSectionHeaderEntry);
-            _encoder.Encode(out hdr.e_shnum, (ushort)ObjectFile.VisibleSectionCount);
-            _encoder.Encode(out hdr.e_shstrndx, (ushort)(ObjectFile.SectionHeaderStringTable?.SectionIndex ?? (ushort)0));
+            _encoder.Encode(out hdr.e_shnum, ObjectFile.VisibleSectionCount >= ElfNative.SHN_LORESERVE ? (ushort)0 : (ushort)ObjectFile.VisibleSectionCount);
+            uint shstrSectionIndex = ObjectFile.SectionHeaderStringTable?.SectionIndex ?? 0u;
+            _encoder.Encode(out hdr.e_shstrndx, shstrSectionIndex >= ElfNative.SHN_LORESERVE ? (ushort)ElfNative.SHN_XINDEX : (ushort)shstrSectionIndex);
 
             Write(hdr);
         }
@@ -181,11 +182,13 @@ private unsafe void WriteSectionHeader64()
             // entries for sections
             _encoder.Encode(out hdr.e_shoff, Layout.OffsetOfSectionHeaderTable);
             _encoder.Encode(out hdr.e_shentsize, (ushort)sizeof(Elf64_Shdr));
-            _encoder.Encode(out hdr.e_shnum, (ushort)ObjectFile.VisibleSectionCount);
-            _encoder.Encode(out hdr.e_shstrndx, (ushort)(ObjectFile.SectionHeaderStringTable?.SectionIndex ?? (ushort)0));
+            _encoder.Encode(out hdr.e_shnum, ObjectFile.VisibleSectionCount >= ElfNative.SHN_LORESERVE ? (ushort)0 : (ushort)ObjectFile.VisibleSectionCount);
+            uint shstrSectionIndex = ObjectFile.SectionHeaderStringTable?.SectionIndex ?? 0u;
+            _encoder.Encode(out hdr.e_shstrndx, shstrSectionIndex >= ElfNative.SHN_LORESERVE ? (ushort)ElfNative.SHN_XINDEX : (ushort)shstrSectionIndex);
 
             Write(hdr);
         }
+
         private void CheckProgramHeaders()
         {
             if (ObjectFile.Segments.Count == 0)

src/LibObjectFile/Elf/Sections/ElfBinarySection.cs

@@ -42,7 +42,7 @@ public override ElfSectionType Type
         }
 
         public override ulong TableEntrySize => OriginalTableEntrySize;
-        
+
         /// <summary>
         /// Gets or sets the associated stream to this section.
         /// </summary>

src/LibObjectFile/Elf/Sections/ElfNullSection.cs

@@ -20,13 +20,29 @@ public override void Verify(DiagnosticBag diagnostics)
                 !Name.IsEmpty ||
                 VirtualAddress != 0 ||
                 Alignment != 0 ||
-                !Link.IsEmpty ||
                 !Info.IsEmpty ||
-                Offset != 0 ||
-                Size != 0)
+                Offset != 0)
             {
                 diagnostics.Error(DiagnosticId.ELF_ERR_InvalidNullSection, "Invalid Null section. This section should not be modified and all properties must be null");
             }
+
+            if (Size != 0 && Parent.VisibleSectionCount < ElfNative.SHN_LORESERVE)
+            {
+                diagnostics.Error(DiagnosticId.ELF_ERR_InvalidNullSection, "Invalid Null section. Size is non-zero but number of sections is lower than SHN_LORESERVE");
+            }
+            else if (Size == 0 && Parent.VisibleSectionCount >= ElfNative.SHN_LORESERVE)
+            {
+                diagnostics.Error(DiagnosticId.ELF_ERR_InvalidNullSection, "Invalid Null section. Size is zero but number of sections is higher or equal to SHN_LORESERVE");
+            }
+
+            if (!Link.IsEmpty && (Parent.SectionHeaderStringTable?.SectionIndex ?? 0) < ElfNative.SHN_LORESERVE)
+            {
+                diagnostics.Error(DiagnosticId.ELF_ERR_InvalidNullSection, "Invalid Null section. Link is non-zero but index of section header string section is lower than SHN_LORESERVE");
+            }
+            else if (Link.IsEmpty && (Parent.SectionHeaderStringTable?.SectionIndex ?? 0) >= ElfNative.SHN_LORESERVE)
+            {
+                diagnostics.Error(DiagnosticId.ELF_ERR_InvalidNullSection, "Invalid Null section. Link is non-zero but index of section header string section is higher or equal to SHN_LORESERVE");
+            }
         }
 
         public override void UpdateLayout(DiagnosticBag diagnostics)