ld: Initial DT_RELR support

Add a -z pack-relative-relocs option to enable DT_RELR and create a
relr.dyn section for DT_RELR.  DT_RELR is implemented with the linker
relaxation infrastructure, but it doesn't require the --relax option
enabled.  -z pack-relative-relocs implies -z combreloc.  -z nocombreloc
implies -z nopack-relative-relocs.

-z pack-relative-relocs is chosen over the similar option in lld,
--pack-dyn-relocs=relr, to implement a glibc binary lockout mechanism
with a special glibc version symbol, to avoid random crashes of DT_RELR
binaries with the existing glibc binaries.

bfd/

	* elf-bfd.h (elf_link_hash_table): Add srelrdyn.
	* elflink.c (_bfd_elf_link_create_dynamic_sections): Create a
	.relr.dyn section for DT_RELR.

include/

	* bfdlink.h (bfd_link_info): Add enable_dt_relr.

ld/

	* News: Mention -z pack-relative-relocs and
	-z nopack-relative-relocs.
	* ld.texi: Document -z pack-relative-relocs and
	-z nopack-relative-relocs.
	* ldelf.c (ldelf_after_parse): Disable DT_RELR if not building
	PIE nor shared library.  Add 3 spare dynamic tags for DT_RELR,
	DT_RELRSZ and DT_RELRENT.
	* ldlang.c (lang_relax_sections): Also enable relaxation if
	DT_RELR is enabled.
	* emulparams/elf32_x86_64.sh: Source dt-relr.sh.
	* emulparams/elf_i386.sh: Likewise.
	* emulparams/elf_x86_64.sh: Likewise.
	* emulparams/dt-relr.sh: New file.
	* scripttempl/elf.sc: Support .relr.dyn.

bfd/elf-bfd.h

@@ -707,6 +707,7 @@ struct elf_link_hash_table
   asection *irelplt;
   asection *irelifunc;
   asection *dynsym;
+  asection *srelrdyn;
 };
 
 /* Returns TRUE if the hash table is a struct elf_link_hash_table.  */

bfd/elflink.c

@@ -359,6 +359,17 @@ _bfd_elf_link_create_dynamic_sections (bfd *abfd, struct bfd_link_info *info)
 	elf_section_data (s)->this_hdr.sh_entsize = 4;
     }
 
+  if (info->enable_dt_relr)
+    {
+      s = bfd_make_section_anyway_with_flags (abfd, ".relr.dyn",
+					      (bed->dynamic_sec_flags
+					       | SEC_READONLY));
+      if (s == NULL
+	  || !bfd_set_section_alignment (s, bed->s->log_file_align))
+	return false;
+      elf_hash_table (info)->srelrdyn = s;
+    }
+
   /* Let the backend create the rest of the sections.  This lets the
      backend set the right flags.  The backend will normally create
      the .got and .plt sections.  */

include/bfdlink.h

@@ -413,6 +413,10 @@ struct bfd_link_info
   /* TRUE if PT_GNU_RELRO segment should be created.  */
   unsigned int relro: 1;
 
+  /* TRUE if DT_RELR should be enabled for compact relative
+     relocations.  */
+  unsigned int enable_dt_relr: 1;
+
   /* TRUE if separate code segment should be created.  */
   unsigned int separate_code: 1;
 

ld/NEWS

@@ -1,5 +1,8 @@
 -*- text -*-
 
+* Add -z pack-relative-relocs/-z no pack-relative-relocs to x86 ELF
+  linker to pack relative relocations in the DT_RELR section.
+
 * Add support for the LoongArch architecture.
 
 * Add -z indirect-extern-access/-z noindirect-extern-access to x86 ELF

ld/emulparams/dt-relr.sh

@@ -0,0 +1,18 @@
+HAVE_DT_RELR=yes
+PARSE_AND_LIST_OPTIONS_PACK_RELATIVE_RELOCS='
+  fprintf (file, _("\
+  -z pack-relative-relocs     Pack relative relocations\n"));
+  fprintf (file, _("\
+  -z nopack-relative-relocs   Do not pack relative relocations (default)\n"));
+'
+
+PARSE_AND_LIST_ARGS_CASE_Z_PACK_RELATIVE_RELOCS='
+      else if (strcmp (optarg, "pack-relative-relocs") == 0)
+	link_info.enable_dt_relr = true;
+      else if (strcmp (optarg, "nopack-relative-relocs") == 0)
+	link_info.enable_dt_relr = false;
+'
+
+
+PARSE_AND_LIST_OPTIONS="$PARSE_AND_LIST_OPTIONS $PARSE_AND_LIST_OPTIONS_PACK_RELATIVE_RELOCS"
+PARSE_AND_LIST_ARGS_CASE_Z="$PARSE_AND_LIST_ARGS_CASE_Z $PARSE_AND_LIST_ARGS_CASE_Z_PACK_RELATIVE_RELOCS"

ld/emulparams/elf32_x86_64.sh

@@ -7,6 +7,7 @@ source_sh ${srcdir}/emulparams/cet.sh
 source_sh ${srcdir}/emulparams/x86-report-relative.sh
 source_sh ${srcdir}/emulparams/x86-64-level.sh
 source_sh ${srcdir}/emulparams/static.sh
+source_sh ${srcdir}/emulparams/dt-relr.sh
 SCRIPT_NAME=elf
 ELFSIZE=32
 OUTPUT_FORMAT="elf32-x86-64"

ld/emulparams/elf_i386.sh

@@ -6,6 +6,7 @@ source_sh ${srcdir}/emulparams/cet.sh
 source_sh ${srcdir}/emulparams/x86-report-relative.sh
 source_sh ${srcdir}/emulparams/x86-64-level.sh
 source_sh ${srcdir}/emulparams/static.sh
+source_sh ${srcdir}/emulparams/dt-relr.sh
 SCRIPT_NAME=elf
 OUTPUT_FORMAT="elf32-i386"
 NO_RELA_RELOCS=yes

ld/emulparams/elf_x86_64.sh

@@ -8,6 +8,7 @@ source_sh ${srcdir}/emulparams/x86-report-relative.sh
 source_sh ${srcdir}/emulparams/x86-64-level.sh
 source_sh ${srcdir}/emulparams/x86-64-lam.sh
 source_sh ${srcdir}/emulparams/static.sh
+source_sh ${srcdir}/emulparams/dt-relr.sh
 SCRIPT_NAME=elf
 ELFSIZE=64
 OUTPUT_FORMAT="elf64-x86-64"

ld/ld.texi

@@ -1430,6 +1430,15 @@ called.
 @item origin
 Specify that the object requires @samp{$ORIGIN} handling in paths.
 
+@item pack-relative-relocs
+@itemx nopack-relative-relocs
+Generate compact relative relocation in position-independent executable
+and shared library.  It adds @code{DT_RELR}, @code{DT_RELRSZ} and
+@code{DT_RELRENT} entries to the dynamic section.  It is ignored when
+building position-dependent executable and relocatable output.
+@option{nopack-relative-relocs} is the default, which disables compact
+relative relocation.  Supported for i386 and x86-64.
+
 @item relro
 @itemx norelro
 Create an ELF @code{PT_GNU_RELRO} segment header in the object.  This

ld/ldelf.c

@@ -71,6 +71,15 @@ ldelf_after_parse (void)
 	einfo (_("%P: warning: -z dynamic-undefined-weak ignored\n"));
       link_info.dynamic_undefined_weak = 0;
     }
+
+  /* Disable DT_RELR if not building PIE nor shared library.  */
+  if (!bfd_link_pic (&link_info))
+    link_info.enable_dt_relr = 0;
+
+  /* Add 3 spare tags for DT_RELR, DT_RELRSZ and DT_RELRENT.  */
+  if (link_info.enable_dt_relr)
+    link_info.spare_dynamic_tags += 3;
+
   after_parse_default ();
   if (link_info.commonpagesize > link_info.maxpagesize)
     einfo (_("%F%P: common page size (0x%v) > maximum page size (0x%v)\n"),

ld/ldlang.c

@@ -7709,7 +7709,8 @@ lang_find_relro_sections (void)
 void
 lang_relax_sections (bool need_layout)
 {
-  if (RELAXATION_ENABLED)
+  /* NB: Also enable relaxation to layout sections for DT_RELR.  */
+  if (RELAXATION_ENABLED || link_info.enable_dt_relr)
     {
       /* We may need more than one relaxation pass.  */
       int i = link_info.relax_pass;

ld/scripttempl/elf.sc

@@ -10,6 +10,7 @@
 #		empty.
 #	HAVE_NOINIT - Include a .noinit output section in the script.
 #	HAVE_PERSISTENT - Include a .persistent output section in the script.
+#	HAVE_DT_RELR - Include a .relr.dyn output section in the script.
 #	SMALL_DATA_CTOR - .ctors contains small data.
 #	SMALL_DATA_DTOR - .dtors contains small data.
 #	DATA_ADDR - if end-of-text-plus-one-page isn't right for data start
@@ -520,6 +521,9 @@ emit_dyn()
     fi
   fi
   rm -f ldscripts/dyntmp.$$
+  if test -n "${COMBRELOC}" && test -n "${HAVE_DT_RELR}"; then
+    echo "  .relr.dyn : { *(.relr.dyn) }"
+  fi
 }
 
 test -n "${NON_ALLOC_DYN}${SEPARATE_CODE}" || emit_dyn