fuzz: fix expected section start/end symbol name on MacOS when linking libfuzzer

Not only is the section name when adding the sancov variables different.

The linker symbol ending up in the binary is also different.

Reference: https://github.com/llvm/llvm-project/blob/60105ac6bab130c2694fc7f5b7b6a5fddaaab752/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp#L1076-L1104

Author: tjog

lib/fuzzer.zig

@@ -468,27 +468,42 @@ export fn fuzzer_init(cache_dir_struct: Fuzzer.Slice) void {
     // Linkers are expected to automatically add `__start_<section>` and
     // `__stop_<section>` symbols when section names are valid C identifiers.
 
+    const ofmt = builtin.object_format;
+
+    const start_symbol_prefix: []const u8 = if (ofmt == .macho)
+        "\x01section$start$__DATA$__"
+    else
+        "__start___";
+    const end_symbol_prefix: []const u8 = if (ofmt == .macho)
+        "\x01section$end$__DATA$__"
+    else
+        "__end___";
+
+    const pc_counters_start_name = start_symbol_prefix ++ "sancov_cntrs";
     const pc_counters_start = @extern([*]u8, .{
-        .name = "__start___sancov_cntrs",
+        .name = pc_counters_start_name,
         .linkage = .weak,
-    }) orelse fatal("missing __start___sancov_cntrs symbol", .{});
+    }) orelse fatal("missing {s} symbol", .{pc_counters_start_name});
 
+    const pc_counters_end_name = end_symbol_prefix ++ "sancov_cntrs";
     const pc_counters_end = @extern([*]u8, .{
-        .name = "__stop___sancov_cntrs",
+        .name = pc_counters_end_name,
         .linkage = .weak,
-    }) orelse fatal("missing __stop___sancov_cntrs symbol", .{});
+    }) orelse fatal("missing {s} symbol", .{pc_counters_end_name});
 
     const pc_counters = pc_counters_start[0 .. pc_counters_end - pc_counters_start];
 
+    const pcs_start_name = start_symbol_prefix ++ "sancov_pcs1";
     const pcs_start = @extern([*]usize, .{
-        .name = "__start___sancov_pcs1",
+        .name = pcs_start_name,
         .linkage = .weak,
-    }) orelse fatal("missing __start___sancov_pcs1 symbol", .{});
+    }) orelse fatal("missing {s} symbol", .{pcs_start_name});
 
+    const pcs_end_name = end_symbol_prefix ++ "sancov_pcs1";
     const pcs_end = @extern([*]usize, .{
-        .name = "__stop___sancov_pcs1",
+        .name = pcs_end_name,
         .linkage = .weak,
-    }) orelse fatal("missing __stop___sancov_pcs1 symbol", .{});
+    }) orelse fatal("missing {s} symbol", .{pcs_end_name});
 
     const pcs = pcs_start[0 .. pcs_end - pcs_start];