From 897a5a4735d5e72c53529a9a3f3a815943568214 Mon Sep 17 00:00:00 2001
From: Jakub Konka <kubkon@jakubkonka.com>
Date: Wed, 23 Feb 2022 18:42:07 +0100
Subject: [PATCH] macho: synthesising __mh_execute_header needs to work with
 incremental

Prior to this change, the routine would assume it is called first,
before any symbol was created, thus precluding an option that in the
incremental setting, we might have already pulled a suitably defined
and exported symbol that could collide and/or be replaced by the
symbol synthesised by the linker.
---
 src/link/MachO.zig | 56 +++++++++++++++++++++++++++++-----------------
 1 file changed, 36 insertions(+), 20 deletions(-)

diff --git a/src/link/MachO.zig b/src/link/MachO.zig
index 0a093bd298..7bb8490e6f 100644
--- a/src/link/MachO.zig
+++ b/src/link/MachO.zig
@@ -3047,6 +3047,9 @@ fn createMhExecuteHeaderAtom(self: *MachO) !void {
         .seg = self.text_segment_cmd_index.?,
         .sect = self.text_section_index.?,
     };
+    const seg = self.load_commands.items[match.seg].segment;
+    const sect = seg.sections.items[match.sect];
+
     const n_strx = try self.makeString("__mh_execute_header");
     const local_sym_index = @intCast(u32, self.locals.items.len);
     var nlist = macho.nlist_64{
@@ -3054,29 +3057,42 @@ fn createMhExecuteHeaderAtom(self: *MachO) !void {
         .n_type = macho.N_SECT,
         .n_sect = @intCast(u8, self.section_ordinals.getIndex(match).? + 1),
         .n_desc = 0,
-        .n_value = 0,
+        .n_value = sect.addr,
     };
     try self.locals.append(self.base.allocator, nlist);
-
-    nlist.n_type |= macho.N_EXT;
-    const global_sym_index = @intCast(u32, self.globals.items.len);
-    try self.globals.append(self.base.allocator, nlist);
-    try self.symbol_resolver.putNoClobber(self.base.allocator, n_strx, .{
-        .where = .global,
-        .where_index = global_sym_index,
-        .local_sym_index = local_sym_index,
-        .file = null,
-    });
-
-    const atom = try self.createEmptyAtom(local_sym_index, 0, 0);
-
-    if (self.needs_prealloc) {
-        const sym = &self.locals.items[local_sym_index];
-        const vaddr = try self.allocateAtom(atom, 0, 1, match);
-        sym.n_value = vaddr;
-    } else try self.addAtomToSection(atom, match);
-
     self.mh_execute_header_index = local_sym_index;
+
+    if (self.symbol_resolver.getPtr(n_strx)) |resolv| {
+        const global = &self.globals.items[resolv.where_index];
+        if (!(global.weakDef() or !global.pext())) {
+            log.err("symbol '__mh_execute_header' defined multiple times", .{});
+            return error.MultipleSymbolDefinitions;
+        }
+        resolv.local_sym_index = local_sym_index;
+    } else {
+        const global_sym_index = @intCast(u32, self.globals.items.len);
+        nlist.n_type |= macho.N_EXT;
+        try self.globals.append(self.base.allocator, nlist);
+        try self.symbol_resolver.putNoClobber(self.base.allocator, n_strx, .{
+            .where = .global,
+            .where_index = global_sym_index,
+            .local_sym_index = local_sym_index,
+            .file = null,
+        });
+    }
+
+    // We always set the __mh_execute_header to point to the beginning of the __TEXT,__text section
+    const atom = try self.createEmptyAtom(local_sym_index, 0, 0);
+    if (self.atoms.get(match)) |last| {
+        var first = last;
+        while (first.prev) |prev| {
+            first = prev;
+        }
+        atom.next = first;
+        first.prev = atom;
+    } else {
+        try self.atoms.putNoClobber(self.base.allocator, match, atom);
+    }
 }
 
 fn resolveDyldStubBinder(self: *MachO) !void {