port(MachO): unify and refactor segment writer (#1821)

On top of that, the final `__LINKEDIT` segment is no longer aligned to
16KiB and thus we won't need to artificial `STRTAB` filler section.

Issue #757

Author: marxin

libwild/src/macho.rs

@@ -54,8 +54,11 @@ const LE: Endianness = Endianness::Little;
 /// offsets right after that (1GiB).
 pub(crate) const MACHO_START_MEM_ADDRESS: u64 = 0x1_0000_0000;
 
+/// The command alignment is 8B for 64-bit platforms.
+pub(crate) const MACHO_COMMAND_ALIGNMENT: usize = 8;
+
 /// A path to the default dynamic linker.
-pub(crate) const DYLINKER_PATH: &str = "/usr/lib/dyld";
+pub(crate) const DYLINKER_PATH: &[u8] = b"/usr/lib/dyld";
 pub(crate) const DEFAULT_SEGMENT_COUNT: usize = 4;
 pub(crate) const CHAINED_FIXUP_TABLE_SIZE: u64 =
     (size_of::<ChainedFixupsHeader>() + size_of::<u32>() * (DEFAULT_SEGMENT_COUNT + 1 + 1)) as u64;
@@ -729,9 +732,7 @@ impl platform::ProgramSegmentDef for ProgramSegmentDef {
             | output_section_id::DYLD_CHAINED_FIXUPS => SegmentType::LoadCommands,
             output_section_id::TEXT | output_section_id::CSTRING => SegmentType::TextSections,
             output_section_id::DATA => SegmentType::DataSections,
-            output_section_id::CHAINED_FIXUP_TABLE | output_section_id::STRTAB => {
-                SegmentType::LinkeditSections
-            }
+            output_section_id::CHAINED_FIXUP_TABLE => SegmentType::LinkeditSections,
             _ => SegmentType::Unused,
         };
 
@@ -1167,7 +1168,8 @@ impl platform::Platform for MachO {
         sizes.increment(part_id::ENTRY_POINT, size_of::<EntryPointCommand>() as u64);
         sizes.increment(
             part_id::INTERP,
-            ((size_of::<DylinkerCommand>() + DYLINKER_PATH.len()).next_multiple_of(8)) as u64,
+            ((size_of::<DylinkerCommand>() + DYLINKER_PATH.len())
+                .next_multiple_of(MACHO_COMMAND_ALIGNMENT)) as u64,
         );
         sizes.increment(
             part_id::DYLD_CHAINED_FIXUPS,
@@ -1228,12 +1230,6 @@ impl platform::Platform for MachO {
         symbol_db: &crate::symbol_db::SymbolDb<Self>,
     ) {
         common.allocate(part_id::CHAINED_FIXUP_TABLE, CHAINED_FIXUP_TABLE_SIZE);
-        // TODO: Just a filler for now that will ensure the __LINKEDIT takes 16KiB - find a better
-        // solution.
-        common.allocate(
-            part_id::STRTAB,
-            MACHO_PAGE_ALIGNMENT.value() - CHAINED_FIXUP_TABLE_SIZE,
-        );
     }
 
     fn finalise_prelude_layout<'data>(
@@ -1292,7 +1288,6 @@ impl platform::Platform for MachO {
         builder.add_section(output_section_id::CSTRING);
         builder.add_section(output_section_id::DATA);
         // The rest (e.g. symbol table, string table).
-        builder.add_section(output_section_id::STRTAB);
         builder.add_section(output_section_id::CHAINED_FIXUP_TABLE);
 
         builder.build()
@@ -1381,11 +1376,6 @@ const SECTION_DEFINITIONS: [BuiltInSectionDetails; NUM_BUILT_IN_SECTIONS] = {
         target_segment_type: Some(SegmentType::LinkeditSections),
         ..DEFAULT_DEFS
     };
-    defs[output_section_id::STRTAB.as_usize()] = BuiltInSectionDetails {
-        kind: SectionKind::Primary(SectionName(b"STRING_TABLE")),
-        target_segment_type: Some(SegmentType::LinkeditSections),
-        ..DEFAULT_DEFS
-    };
     // Multi-part generated sections
     defs[output_section_id::SYMTAB_GLOBAL.as_usize()] = BuiltInSectionDetails {
         kind: SectionKind::Secondary(output_section_id::SYMTAB_LOCAL),