macho: keep actual file offset for zerofill sections separately

This way, we will conform to the standard practice of setting the
offset within the section header to the beginning of the file and
we will be able to track the location of the section in the file
for incremental updates.

1 files changed, 55 insertions, 19 deletions

diff --git a/src/link/MachO.zig b/src/link/MachO.zig
index 473bd077d8..9e170ca1f4 100644
--- a/src/link/MachO.zig
+++ b/src/link/MachO.zig
@@ -133,6 +133,9 @@ objc_selrefs_section_index: ?u16 = null,
 objc_classrefs_section_index: ?u16 = null,
 objc_data_section_index: ?u16 = null,
 
+bss_file_offset: ?u64 = null,
+tlv_bss_file_offset: ?u64 = null,
+
 locals: std.ArrayListUnmanaged(macho.nlist_64) = .{},
 globals: std.ArrayListUnmanaged(macho.nlist_64) = .{},
 undefs: std.ArrayListUnmanaged(macho.nlist_64) = .{},
@@ -1714,11 +1717,30 @@ fn allocateSegment(self: *MachO, index: u16, offset: u64) !void {
 
     // Allocate the sections according to their alignment at the beginning of the segment.
     var start: u64 = offset;
-    for (seg.sections.items) |*sect| {
+    for (seg.sections.items) |*sect, sect_id| {
         const alignment = try math.powi(u32, 2, sect.@"align");
         const start_aligned = mem.alignForwardGeneric(u64, start, alignment);
         const end_aligned = mem.alignForwardGeneric(u64, start_aligned + sect.size, alignment);
-        sect.offset = @intCast(u32, seg.inner.fileoff + start_aligned);
+        const file_offset = @intCast(u32, seg.inner.fileoff + start_aligned);
+
+        blk: {
+            if (index == self.data_segment_cmd_index.?) {
+                if (self.bss_section_index) |idx| {
+                    if (sect_id == idx) {
+                        self.bss_file_offset = file_offset;
+                        break :blk;
+                    }
+                }
+                if (self.tlv_bss_section_index) |idx| {
+                    if (sect_id == idx) {
+                        self.tlv_bss_file_offset = file_offset;
+                        break :blk;
+                    }
+                }
+            }
+            sect.offset = @intCast(u32, seg.inner.fileoff + start_aligned);
+        }
+
         sect.addr = seg.inner.vmaddr + start_aligned;
         start = end_aligned;
     }
@@ -1821,6 +1843,18 @@ fn writeTextBlocks(self: *MachO) !void {
         var code = try self.base.allocator.alloc(u8, sect.size);
         defer self.base.allocator.free(code);
 
+        const file_offset: u64 = blk: {
+            if (self.data_segment_cmd_index.? == match.seg) {
+                if (self.bss_section_index) |idx| {
+                    if (idx == match.sect) break :blk self.bss_file_offset.?;
+                }
+                if (self.tlv_bss_section_index) |idx| {
+                    if (idx == match.sect) break :blk self.tlv_bss_file_offset.?;
+                }
+            }
+            break :blk sect.offset;
+        };
+
         if (sect_type == macho.S_ZEROFILL or sect_type == macho.S_THREAD_LOCAL_ZEROFILL) {
             mem.set(u8, code, 0);
         } else {
@@ -1856,7 +1890,7 @@ fn writeTextBlocks(self: *MachO) !void {
             mem.set(u8, code[base_off..], 0);
         }
 
-        try self.base.file.?.pwriteAll(code, sect.offset);
+        try self.base.file.?.pwriteAll(code, file_offset);
     }
 }
 
@@ -1925,7 +1959,18 @@ pub fn writeAtom(self: *MachO, atom: *TextBlock, match: MatchingSection) !void {
     const seg = self.load_commands.items[match.seg].Segment;
     const sect = seg.sections.items[match.sect];
     const sym = self.locals.items[atom.local_sym_index];
-    const file_offset = sect.offset + sym.n_value - sect.addr;
+    const sect_offset: u64 = blk: {
+        if (self.data_segment_cmd_index.? == match.seg) {
+            if (self.bss_section_index) |idx| {
+                if (idx == match.sect) break :blk self.bss_file_offset.?;
+            }
+            if (self.tlv_bss_section_index) |idx| {
+                if (idx == match.sect) break :blk self.tlv_bss_file_offset.?;
+            }
+        }
+        break :blk sect.offset;
+    };
+    const file_offset = sect_offset + sym.n_value - sect.addr;
     try atom.resolveRelocs(self);
     log.debug("writing atom for symbol {s} at file offset 0x{x}", .{ self.getString(sym.n_strx), file_offset });
     try self.base.file.?.pwriteAll(atom.code.items, file_offset);
@@ -2873,19 +2918,6 @@ fn addLoadDylibLCs(self: *MachO) !void {
 
 fn flushZld(self: *MachO) !void {
     try self.writeTextBlocks();
-
-    // if (self.bss_section_index) |index| {
-    //     const seg = &self.load_commands.items[self.data_segment_cmd_index.?].Segment;
-    //     const sect = &seg.sections.items[index];
-    //     sect.offset = 0;
-    // }
-
-    // if (self.tlv_bss_section_index) |index| {
-    //     const seg = &self.load_commands.items[self.data_segment_cmd_index.?].Segment;
-    //     const sect = &seg.sections.items[index];
-    //     sect.offset = 0;
-    // }
-
     try self.setEntryPoint();
     try self.writeRebaseInfoTableZld();
     try self.writeBindInfoTableZld();
@@ -4203,10 +4235,12 @@ pub fn populateMissingMetadata(self: *MachO) !void {
 
         log.debug("found __thread_bss section free space 0x{x} to 0x{x}", .{ off, off + needed_size });
 
+        // We keep offset to the section in a separate variable as the actual section is usually pointing at the
+        // beginning of the file.
+        self.tlv_bss_file_offset = off;
         try data_segment.addSection(self.base.allocator, "__thread_bss", .{
             .addr = data_segment.inner.vmaddr + off - data_segment.inner.fileoff,
             .size = needed_size,
-            .offset = @intCast(u32, off),
             .@"align" = 3, // 2^3 = @sizeOf(u64)
             .flags = macho.S_THREAD_LOCAL_ZEROFILL,
         });
@@ -4229,10 +4263,12 @@ pub fn populateMissingMetadata(self: *MachO) !void {
 
         log.debug("found __bss section free space 0x{x} to 0x{x}", .{ off, off + needed_size });
 
+        // We keep offset to the section in a separate variable as the actual section is usually pointing at the
+        // beginning of the file.
+        self.bss_file_offset = off;
         try data_segment.addSection(self.base.allocator, "__bss", .{
             .addr = data_segment.inner.vmaddr + off - data_segment.inner.fileoff,
             .size = 0,
-            .offset = @intCast(u32, off),
             .@"align" = 3, // 2^3 = @sizeOf(u64)
             .flags = macho.S_ZEROFILL,
         });