Support -z start-stop-visibility=hidden

Fixes #826

elf/cmdline.cc

@@ -197,6 +197,8 @@ inline const char helpmsg[] = R"(
     -z nopack-relative-relocs
   -z sectionheader            Do not omit section header (default)
     -z nosectionheader        Omit section header
+  -z start_stop_visibility=[hidden,protected]
+                              Specify symbol visibility for "__start_SECNAME" and "__stop_SECNAME" symbols
   -z separate-loadable-segments
                               Separate all loadable segments onto different pages
     -z separate-code          Separate code and data onto different pages
@@ -937,9 +939,10 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       ctx.page_size = parse_number(ctx, "-z max-page-size", arg);
       if (!has_single_bit(ctx.page_size))
         Fatal(ctx) << "-z max-page-size " << arg << ": value must be a power of 2";
-    } else if (read_z_arg("start-stop-visibility")) {
-      if (arg != "hidden")
-        Fatal(ctx) << "-z start-stop-visibility: unsupported visibility: " << arg;
+    } else if (read_z_flag("start-stop-visibility=protected")) {
+      ctx.arg.z_start_stop_visibility_protected = true;
+    } else if (read_z_flag("start-stop-visibility=hidden")) {
+      ctx.arg.z_start_stop_visibility_protected = false;
     } else if (read_z_flag("noexecstack")) {
       ctx.arg.z_execstack = false;
     } else if (read_z_flag("relro")) {

elf/mold.h

@@ -1725,6 +1725,7 @@ struct Context {
     bool z_rewrite_endbr = false;
     bool z_sectionheader = true;
     bool z_shstk = false;
+    bool z_start_stop_visibility_protected = false;
     bool z_text = false;
     i64 filler = -1;
     i64 spare_dynamic_tags = 5;

elf/passes.cc

@@ -830,14 +830,20 @@ void add_synthetic_symbols(Context<E> &ctx) {
   if constexpr (is_ppc32<E>)
     ctx.extra._SDA_BASE_ = add("_SDA_BASE_");
 
+  auto add_start_stop = [&](std::string s) {
+    add(save_string(ctx, s));
+    if (ctx.arg.z_start_stop_visibility_protected)
+      get_symbol(ctx, save_string(ctx, s))->is_exported = true;
+  };
+
   for (Chunk<E> *chunk : ctx.chunks) {
     if (std::optional<std::string> name = get_start_stop_name(ctx, *chunk)) {
-      add(save_string(ctx, "__start_" + *name));
-      add(save_string(ctx, "__stop_" + *name));
+      add_start_stop("__start_" + *name);
+      add_start_stop("__stop_" + *name);
 
       if (ctx.arg.physical_image_base) {
-        add(save_string(ctx, "__phys_start_" + *name));
-        add(save_string(ctx, "__phys_stop_" + *name));
+        add_start_stop("__phys_start_" + *name);
+        add_start_stop("__phys_stop_" + *name);
       }
     }
   }

test/elf/z-start-stop-visibility.sh

@@ -1,6 +1,28 @@
 #!/bin/bash
 . $(dirname $0)/common.inc
 
-./mold -z start-stop-visibility=hidden --version > /dev/null
-! ./mold -z start-stop-visibility=protected --version 2> $t/log
-grep -q 'unsupported visibility: protected' $t/log
+cat <<EOF | $CC -o $t/a.o -c -xc -
+#include <stdio.h>
+
+__attribute__((section("hello")))
+static char msg[] = "Hello world";
+
+int main() {
+  printf("%s\n", msg);
+}
+EOF
+
+$CC -B. -o $t/exe1 $t/a.o
+readelf -W --dyn-syms $t/exe1 > $t/log1
+! grep -q __start_hello $t/log1 || false
+! grep -q __stop_hello $t/log1 || false
+
+$CC -B. -o $t/exe2 $t/a.o -Wl,-z,start-stop-visibility=hidden
+readelf -W --dyn-syms $t/exe2 > $t/log2
+! grep -q __start_hello $t/log2 || false
+! grep -q __stop_hello $t/log2 || false
+
+$CC -B. -o $t/exe3 $t/a.o -Wl,-z,start-stop-visibility=protected
+readelf -W --dyn-syms $t/exe3 > $t/log3
+grep -q __start_hello $t/log3
+grep -q __stop_hello $t/log3