[ELF] Support --package-metadata

Fixes #505

docs/mold.1

@@ -646,6 +646,14 @@ to run executables or shared libraries linked with
 .Fl -pack-dyn-relocs=relr ,
 and only ChromeOS, Android and Fuchsia support it as of now in 2022.
 .Pp
+.It -package-metadata Ns = Ns string
+Embed
+.Ar string
+to a .note.package section. This option in intended to be used by a \
+package management command such as
+.Cm rpm
+to embed metadata regarding a package to each executable file.
+.Pp
 .It Fl -perf
 Print performance statistics.
 .Pp

elf/cmdline.cc

@@ -98,6 +98,7 @@ static const char helpmsg[] = R"(
   --oformat=binary            Omit ELF, section and program headers
   --pack-dyn-relocs=[relr,none]
                               Pack dynamic relocations
+  --package-metadata=STRING   Set a given string to .note.package
   --perf                      Print performance statistics
   --pie, --pic-executable     Create a position independent executable
     --no-pie, --no-pic-executable
@@ -619,6 +620,8 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       ctx.arg.pack_dyn_relocs_relr = true;
     } else if (read_flag("pack-dyn-relocs=none")) {
       ctx.arg.pack_dyn_relocs_relr = false;
+    } else if (read_arg("package-metadata")) {
+      ctx.arg.package_metadata = arg;
     } else if (read_flag("stats")) {
       ctx.arg.stats = true;
       Counter::enabled = true;

elf/elf.h

@@ -229,6 +229,7 @@ static constexpr u32 NT_GNU_HWCAP = 2;
 static constexpr u32 NT_GNU_BUILD_ID = 3;
 static constexpr u32 NT_GNU_GOLD_VERSION = 4;
 static constexpr u32 NT_GNU_PROPERTY_TYPE_0 = 5;
+static constexpr u32 NT_FDO_PACKAGING_METADATA = 0xcafe1a7e;
 
 static constexpr u32 GNU_PROPERTY_AARCH64_FEATURE_1_AND = 0xc0000000;
 static constexpr u32 GNU_PROPERTY_X86_FEATURE_1_AND = 0xc0000002;

elf/mold.h

@@ -873,6 +873,20 @@ class BuildIdSection : public Chunk<E> {
   static constexpr i64 HEADER_SIZE = 16;
 };
 
+template <typename E>
+class NotePackageSection : public Chunk<E> {
+public:
+  NotePackageSection() {
+    this->name = ".note.package";
+    this->shdr.sh_type = SHT_NOTE;
+    this->shdr.sh_flags = SHF_ALLOC;
+    this->shdr.sh_addralign = 4;
+  }
+
+  void update_shdr(Context<E> &ctx) override;
+  void copy_buf(Context<E> &ctx) override;
+};
+
 template <typename E>
 class NotePropertySection : public Chunk<E> {
 public:
@@ -1609,6 +1623,7 @@ struct Context {
     std::string fini = "_fini";
     std::string init = "_init";
     std::string output = "a.out";
+    std::string package_metadata;
     std::string plugin;
     std::string rpaths;
     std::string soname;
@@ -1718,6 +1733,7 @@ struct Context {
   std::unique_ptr<VerneedSection<E>> verneed;
   std::unique_ptr<VerdefSection<E>> verdef;
   std::unique_ptr<BuildIdSection<E>> buildid;
+  std::unique_ptr<NotePackageSection<E>> note_package;
   std::unique_ptr<NotePropertySection<E>> note_property;
   std::unique_ptr<GdbIndexSection<E>> gdb_index;
   std::unique_ptr<ThumbToArmSection> thumb_to_arm;

elf/output-chunks.cc

@@ -2025,6 +2025,26 @@ void BuildIdSection<E>::write_buildid(Context<E> &ctx) {
   }
 }
 
+template <typename E>
+void NotePackageSection<E>::update_shdr(Context<E> &ctx) {
+  if (!ctx.arg.package_metadata.empty()) {
+    // +17 is for the header and the NUL terminator
+    this->shdr.sh_size = align_to(ctx.arg.package_metadata.size() + 17, 4);
+  }
+}
+
+template <typename E>
+void NotePackageSection<E>::copy_buf(Context<E> &ctx) {
+  u32 *buf = (u32 *)(ctx.buf + this->shdr.sh_offset);
+  memset(buf, 0, this->shdr.sh_size);
+
+  buf[0] = 4;                                      // Name size
+  buf[1] = this->shdr.sh_size - 16;                // Content size
+  buf[2] = NT_FDO_PACKAGING_METADATA;              // Type
+  memcpy(buf + 3, "FDO", 4);                       // Name
+  write_string(buf + 4, ctx.arg.package_metadata); // Content
+}
+
 template <typename E>
 void NotePropertySection<E>::update_shdr(Context<E> &ctx) {
   features = -1;
@@ -2557,6 +2577,7 @@ void RelocSection<E>::copy_buf(Context<E> &ctx) {
   template class VerneedSection<E>;                                     \
   template class VerdefSection<E>;                                      \
   template class BuildIdSection<E>;                                     \
+  template class NotePackageSection<E>;                                 \
   template class NotePropertySection<E>;                                \
   template class GdbIndexSection<E>;                                    \
   template class GabiCompressedSection<E>;                              \

elf/passes.cc

@@ -82,6 +82,7 @@ void create_synthetic_sections(Context<E> &ctx) {
 
   ctx.versym = push(new VersymSection<E>);
   ctx.verneed = push(new VerneedSection<E>);
+  ctx.note_package = push(new NotePackageSection<E>);
   ctx.note_property = push(new NotePropertySection<E>);
 
   if constexpr (std::is_same_v<E, ARM32>) {

test/elf/package-metadata.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+export LC_ALL=C
+set -e
+CC="${CC:-cc}"
+CXX="${CXX:-c++}"
+GCC="${GCC:-gcc}"
+GXX="${GXX:-g++}"
+OBJDUMP="${OBJDUMP:-objdump}"
+MACHINE="${MACHINE:-$(uname -m)}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+t=out/test/elf/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+#include <stdio.h>
+int main() {
+  printf("Hello world\n");
+}
+EOF
+
+$CC -B. -o $t/exe $t/a.o -Wl,-package-metadata='{"foo":"bar"}'
+readelf -x .note.package $t/exe | fgrep -q '{"foo":"bar"}'
+
+echo OK