Add hidden --use-android-relr-tags option

Pre-Android-12 (API 30 and earlier) bionic loaders predate the official
ELF gABI assignment of DT_RELR/DT_RELRSZ/DT_RELRENT and use legacy
OS-specific tag values DT_ANDROID_RELR (0x6fffe000), DT_ANDROID_RELRSZ
(0x6fffe001), DT_ANDROID_RELRENT (0x6fffe003), and section type
SHT_ANDROID_RELR (0x6fffff00). With --use-android-relr-tags, mold emits
those instead of the standard ones so binaries linked with
--pack-dyn-relocs=relr can be loaded on those older devices. The flag
is undocumented (not listed in --help) since it's only relevant to
shipping to legacy Android targets.

#1544

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rui314

src/cmdline.cc

@@ -1012,6 +1012,10 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
                read_z_flag("nopack-relative-relocs")) {
       ctx.arg.pack_dyn_relocs_relr = false;
       ctx.arg.pack_dyn_relocs_android = false;
+    } else if (read_flag("use-android-relr-tags")) {
+      ctx.arg.use_android_relr_tags = true;
+    } else if (read_flag("no-use-android-relr-tags")) {
+      ctx.arg.use_android_relr_tags = false;
     } else if (read_arg("package-metadata")) {
       ctx.arg.package_metadata = parse_package_metadata(ctx, arg);
     } else if (read_flag("stats")) {

src/elf.h

@@ -84,6 +84,7 @@ enum : u32 {
   SHT_ANDROID_REL = 0x60000001,
   SHT_ANDROID_RELA = 0x60000002,
   SHT_LLVM_ADDRSIG = 0x6fff4c03,
+  SHT_ANDROID_RELR = 0x6fffff00,
   SHT_GNU_HASH = 0x6ffffff6,
   SHT_GNU_VERDEF = 0x6ffffffd,
   SHT_GNU_VERNEED = 0x6ffffffe,
@@ -291,6 +292,9 @@ enum : u32 {
   DT_ANDROID_RELSZ = 0x60000010,
   DT_ANDROID_RELA = 0x60000011,
   DT_ANDROID_RELASZ = 0x60000012,
+  DT_ANDROID_RELR = 0x6fffe000,
+  DT_ANDROID_RELRSZ = 0x6fffe001,
+  DT_ANDROID_RELRENT = 0x6fffe003,
   DT_GNU_HASH = 0x6ffffef5,
   DT_DEPAUDIT = 0x6ffffefb,
   DT_AUDIT = 0x6ffffefc,

src/mold.h

@@ -983,9 +983,9 @@ class RelDynSection : public Chunk<E> {
 template <typename E>
 class RelrDynSection : public Chunk<E> {
 public:
-  RelrDynSection() {
+  RelrDynSection(Context<E> &ctx) {
     this->name = ".relr.dyn";
-    this->shdr.sh_type = SHT_RELR;
+    this->shdr.sh_type = ctx.arg.use_android_relr_tags ? SHT_ANDROID_RELR : SHT_RELR;
     this->shdr.sh_flags = SHF_ALLOC;
     this->shdr.sh_entsize = sizeof(Word<E>);
     this->shdr.sh_addralign = sizeof(Word<E>);
@@ -2392,6 +2392,7 @@ struct Context {
     bool suppress_warnings = false;
     bool trace = false;
     bool undefined_version = false;
+    bool use_android_relr_tags = false;
     bool warn_common = false;
     bool warn_once = false;
     bool warn_textrel = false;

src/output-chunks.cc

@@ -698,9 +698,15 @@ static std::vector<Word<E>> create_dynamic_section(Context<E> &ctx) {
   }
 
   if (ctx.relrdyn) {
-    define(DT_RELR, ctx.relrdyn->shdr.sh_addr);
-    define(DT_RELRSZ, ctx.relrdyn->shdr.sh_size);
-    define(DT_RELRENT, ctx.relrdyn->shdr.sh_entsize);
+    if (ctx.arg.use_android_relr_tags) {
+      define(DT_ANDROID_RELR, ctx.relrdyn->shdr.sh_addr);
+      define(DT_ANDROID_RELRSZ, ctx.relrdyn->shdr.sh_size);
+      define(DT_ANDROID_RELRENT, ctx.relrdyn->shdr.sh_entsize);
+    } else {
+      define(DT_RELR, ctx.relrdyn->shdr.sh_addr);
+      define(DT_RELRSZ, ctx.relrdyn->shdr.sh_size);
+      define(DT_RELRENT, ctx.relrdyn->shdr.sh_entsize);
+    }
   }
 
   if (ctx.relplt->shdr.sh_size) {

src/passes.cc

@@ -143,7 +143,7 @@ void create_synthetic_sections(Context<E> &ctx) {
   ctx.relplt = push(new RelPltSection<E>);
 
   if (ctx.arg.pack_dyn_relocs_relr)
-    ctx.relrdyn = push(new RelrDynSection<E>);
+    ctx.relrdyn = push(new RelrDynSection<E>(ctx));
 
   ctx.strtab = push(new StrtabSection<E>);
   ctx.plt = push(new PltSection<E>);

test/use-android-relr-tags.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+cat <<EOF | $CC -o $t/a.o -fPIC -c -xc -
+#include <stdio.h>
+int main() {
+  printf("Hello world\n");
+}
+EOF
+
+# Probe whether the host toolchain can link a PIE; skip if not.
+$CC -o $t/probe $t/a.o -pie 2> /dev/null || skip
+
+# Default: standard DT_RELR tags (35/36/37) and SHT_RELR (19).
+$CC -B. -o $t/exe1 $t/a.o -pie -Wl,--pack-dyn-relocs=relr
+readelf -SW $t/exe1 | grep -F .relr.dyn | grep -Ew 'RELR'
+readelf --dynamic $t/exe1 > $t/log1
+grep -Ew 'RELR|<unknown>: 24' $t/log1
+grep -Ew 'RELRSZ|<unknown>: 23' $t/log1
+grep -Ew 'RELRENT|<unknown>: 25' $t/log1
+
+# --use-android-relr-tags: legacy DT_ANDROID_RELR* (in the OS-specific
+# range) and SHT_ANDROID_RELR section type, for compatibility with
+# pre-Android-12 loaders that don't recognize the standard tags.
+$CC -B. -o $t/exe2 $t/a.o -pie \
+  -Wl,--pack-dyn-relocs=relr -Wl,--use-android-relr-tags
+readelf -SW $t/exe2 | grep -F .relr.dyn | grep -Ev '\bRELR\b'
+readelf --dynamic $t/exe2 > $t/log2
+grep -E '0x0*6fffe000\b' $t/log2
+grep -E '0x0*6fffe001\b' $t/log2
+grep -E '0x0*6fffe003\b' $t/log2
+# Make sure the standard tags are NOT emitted in this mode.
+not grep -Ew 'RELR|<unknown>: 24' $t/log2
+not grep -Ew 'RELRSZ|<unknown>: 23' $t/log2
+not grep -Ew 'RELRENT|<unknown>: 25' $t/log2
+
+# --no-use-android-relr-tags reverts to the standard tags.
+$CC -B. -o $t/exe3 $t/a.o -pie \
+  -Wl,--pack-dyn-relocs=relr \
+  -Wl,--use-android-relr-tags -Wl,--no-use-android-relr-tags
+readelf --dynamic $t/exe3 > $t/log3
+grep -Ew 'RELR|<unknown>: 24' $t/log3