Support -z rodynamic

LLVM lld supports the flag.

Fixes #1253

docs/mold.md

@@ -714,6 +714,9 @@ but as `-o magic`.
   `.text.exit` as separate sections in the final binary instead of merging
   them as `.text`.
 
+* `-z rodynamic`
+  Make the `.dynamic` section read-only.
+
 * `-z relro`, `-z norelro`:
   Some sections such as `.dynamic` have to be writable only during an
   executable or a shared library file is being loaded to memory. Once the

elf/cmdline.cc

@@ -209,6 +209,7 @@ inline const char helpmsg[] = R"(
   -z stack-size=VALUE         Set the size of the stack segment
   -z relro                    Make some sections read-only after relocation (default)
     -z norelro
+  -z rodynamic                Make the .dynamic section read-only
   -z text                     Report error if DT_TEXTREL is set
     -z notext
     -z textoff
@@ -1008,6 +1009,8 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       ctx.arg.z_sectionheader = false;
     } else if (read_z_flag("rewrite-endbr")) {
       ctx.arg.z_rewrite_endbr = true;
+    } else if (read_z_flag("rodynamic")) {
+      ctx.arg.z_rodynamic = true;
     } else if (read_flag("no-undefined")) {
       ctx.arg.z_defs = true;
     } else if (read_flag("nmagic")) {

elf/mold.h

@@ -690,13 +690,19 @@ class DynstrSection : public Chunk<E> {
 template <typename E>
 class DynamicSection : public Chunk<E> {
 public:
-  DynamicSection() {
+  DynamicSection(Context<E> &ctx) {
     this->name = ".dynamic";
-    this->is_relro = true;
     this->shdr.sh_type = SHT_DYNAMIC;
-    this->shdr.sh_flags = SHF_ALLOC | SHF_WRITE;
     this->shdr.sh_addralign = sizeof(Word<E>);
     this->shdr.sh_entsize = sizeof(ElfDyn<E>);
+
+    if (ctx.arg.z_rodynamic) {
+      this->shdr.sh_flags = SHF_ALLOC;
+      this->is_relro = false;
+    } else {
+      this->shdr.sh_flags = SHF_ALLOC | SHF_WRITE;
+      this->is_relro = true;
+    }
   }
 
   void update_shdr(Context<E> &ctx) override;
@@ -1754,6 +1760,7 @@ struct Context {
     bool z_origin = false;
     bool z_relro = true;
     bool z_rewrite_endbr = false;
+    bool z_rodynamic = false;
     bool z_sectionheader = true;
     bool z_shstk = false;
     bool z_start_stop_visibility_protected = false;

elf/output-chunks.cc

@@ -272,7 +272,7 @@ static std::vector<ElfPhdr<E>> create_phdr(Context<E> &ctx) {
 
   // Add PT_DYNAMIC
   if (ctx.dynamic && ctx.dynamic->shdr.sh_size)
-    define(PT_DYNAMIC, PF_R | PF_W, ctx.dynamic);
+    define(PT_DYNAMIC, to_phdr_flags(ctx, ctx.dynamic), ctx.dynamic);
 
   // Add PT_GNU_EH_FRAME
   if (ctx.eh_frame_hdr)
@@ -841,7 +841,7 @@ static std::vector<Word<E>> create_dynamic_section(Context<E> &ctx) {
 
   // GDB needs a DT_DEBUG entry in an executable to store a word-size
   // data for its own purpose. Its content is not important.
-  if (!ctx.arg.shared)
+  if (!ctx.arg.shared && !ctx.arg.z_rodynamic)
     define(DT_DEBUG, 0);
 
   define(DT_NULL, 0);

elf/passes.cc

@@ -157,7 +157,7 @@ void create_synthetic_sections(Context<E> &ctx) {
     ctx.eh_frame_reloc = push(new EhFrameRelocSection<E>);
 
   if (ctx.arg.shared || !ctx.dsos.empty() || ctx.arg.pie) {
-    ctx.dynamic = push(new DynamicSection<E>);
+    ctx.dynamic = push(new DynamicSection<E>(ctx));
 
     // If .dynamic exists, .dynsym and .dynstr must exist as well
     // since .dynamic refers to them.

test/elf/z-rodynamic.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+# Executables built with `-z rodynamic` crash on qemu-user for
+# PowerPC for some reason.
+[[ $MACHINE = ppc* ]] && skip
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+#include <stdio.h>
+int main() {
+  printf("Hello world\n");
+}
+EOF
+
+$CC -B. -o $t/exe1 $t/a.o
+readelf -WS $t/exe1 | grep -q '\.dynamic.* WA '
+$QEMU $t/exe1 | grep -q 'Hello world'
+
+$CC -B. -o $t/exe2 $t/a.o -Wl,-z,rodynamic
+readelf -WS $t/exe2 | grep -q '\.dynamic.* A '
+$QEMU $t/exe2 | grep -q 'Hello world'