ld: Add glibc dependency for DT_RELR

When DT_RELR is enabled, to avoid random run-time crash with older glibc
binaries without DT_RELR support, add a GLIBC_ABI_DT_RELR symbol version,
which is provided by glibc with DT_RELR support, dependency on the shared
C library if it provides a GLIBC_2.XX symbol version.

bfd/

	* elflink.c (elf_link_add_dt_relr_dependency): New function.
	(bfd_elf_size_dynamic_sections): Call
	elf_link_add_dt_relr_dependency if DT_RELR is enabled.

ld/

	* ld.texi: Mention GLIBC_ABI_DT_RELR in -z pack-relative-relocs
	entry.
	* testsuite/ld-elf/dt-relr-glibc-1.c: New file.
	* testsuite/ld-elf/dt-relr-glibc-1a.rd: Likewise.
	* testsuite/ld-elf/dt-relr-glibc-1b.rd: Likewise.
	* testsuite/ld-elf/dt-relr.exp: Likewise.

bfd/elflink.c

@@ -2213,6 +2213,85 @@ _bfd_elf_export_symbol (struct elf_link_hash_entry *h, void *data)
   return true;
 }
 
+/* Return true if GLIBC_ABI_DT_RELR is added to the list of version
+   dependencies successfully.  GLIBC_ABI_DT_RELR will be put into the
+   .gnu.version_r section.  */
+
+static bool
+elf_link_add_dt_relr_dependency (struct elf_find_verdep_info *rinfo)
+{
+  bfd *glibc_bfd = NULL;
+  Elf_Internal_Verneed *t;
+  Elf_Internal_Vernaux *a;
+  size_t amt;
+  const char *relr = "GLIBC_ABI_DT_RELR";
+
+  /* See if we already know about GLIBC_PRIVATE_DT_RELR.  */
+  for (t = elf_tdata (rinfo->info->output_bfd)->verref;
+       t != NULL;
+       t = t->vn_nextref)
+    {
+      const char *soname = bfd_elf_get_dt_soname (t->vn_bfd);
+      /* Skip the shared library if it isn't libc.so.  */
+      if (!soname || !startswith (soname, "libc.so."))
+	continue;
+
+      for (a = t->vn_auxptr; a != NULL; a = a->vna_nextptr)
+	{
+	  /* Return if GLIBC_PRIVATE_DT_RELR dependency has been
+	     added.  */
+	  if (a->vna_nodename == relr
+	      || strcmp (a->vna_nodename, relr) == 0)
+	    return true;
+
+	  /* Check if libc.so provides GLIBC_2.XX version.  */
+	  if (!glibc_bfd && startswith (a->vna_nodename, "GLIBC_2."))
+	    glibc_bfd = t->vn_bfd;
+	}
+
+      break;
+    }
+
+  /* Skip if it isn't linked against glibc.  */
+  if (glibc_bfd == NULL)
+    return true;
+
+  /* This is a new version.  Add it to tree we are building.  */
+  if (t == NULL)
+    {
+      amt = sizeof *t;
+      t = (Elf_Internal_Verneed *) bfd_zalloc (rinfo->info->output_bfd,
+					       amt);
+      if (t == NULL)
+	{
+	  rinfo->failed = true;
+	  return false;
+	}
+
+      t->vn_bfd = glibc_bfd;
+      t->vn_nextref = elf_tdata (rinfo->info->output_bfd)->verref;
+      elf_tdata (rinfo->info->output_bfd)->verref = t;
+    }
+
+  amt = sizeof *a;
+  a = (Elf_Internal_Vernaux *) bfd_zalloc (rinfo->info->output_bfd, amt);
+  if (a == NULL)
+    {
+      rinfo->failed = true;
+      return false;
+    }
+
+  a->vna_nodename = relr;
+  a->vna_flags = 0;
+  a->vna_nextptr = t->vn_auxptr;
+  a->vna_other = rinfo->vers + 1;
+  ++rinfo->vers;
+
+  t->vn_auxptr = a;
+
+  return true;
+}
+
 /* Look through the symbols which are defined in other shared
    libraries and referenced here.  Update the list of version
    dependencies.  This will be put into the .gnu.version_r section.
@@ -6940,6 +7019,13 @@ bfd_elf_size_dynamic_sections (bfd *output_bfd,
       if (sinfo.failed)
 	return false;
 
+      if (info->enable_dt_relr)
+	{
+	  elf_link_add_dt_relr_dependency (&sinfo);
+	  if (sinfo.failed)
+	    return false;
+	}
+
       if (elf_tdata (output_bfd)->verref == NULL)
 	s->flags |= SEC_EXCLUDE;
       else

ld/ld.texi

@@ -1437,7 +1437,9 @@ 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.
+relative relocation.  When linked against the GNU C Library, a
+GLIBC_ABI_DT_RELR symbol version dependency on the shared C Library is
+added to the output.  Supported for i386 and x86-64.
 
 @item relro
 @itemx norelro

ld/testsuite/ld-elf/dt-relr-glibc-1.c

@@ -0,0 +1,11 @@
+#define REL(n) \
+  static int data##n; \
+  void *p##n = &data##n;
+
+REL(1)
+REL(2)
+REL(3)
+REL(4)
+REL(5)
+REL(6)
+REL(7)

ld/testsuite/ld-elf/dt-relr-glibc-1a.rd

@@ -0,0 +1,4 @@
+#failif
+#...
+  0x[a-f0-9]+:   Name: GLIBC_ABI_DT_RELR  Flags: none  Version: [0-9]+
+#...

ld/testsuite/ld-elf/dt-relr-glibc-1b.rd

@@ -0,0 +1,7 @@
+#...
+Version needs section '.gnu.version_r' contains 1 entry:
+ Addr: 0x[0-9a-f]+ +Offset: 0x[0-9a-f]+ +Link: +[0-9]+ +\(.dynstr\)
+ +0+: Version: 1 +File: libc\.so\.6(|\.1) +Cnt: +[0-9]+
+#...
+  0x[a-f0-9]+:   Name: GLIBC_ABI_DT_RELR  Flags: none  Version: [0-9]+
+#pass

ld/testsuite/ld-elf/dt-relr.exp

@@ -0,0 +1,44 @@
+# Expect script for DT_RELR.
+#   Copyright (C) 2022 Free Software Foundation, Inc.
+#
+# This file is part of the GNU Binutils.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
+# MA 02110-1301, USA.
+#
+
+# Linux tests.
+if { ![istarget "*-*-linux*"] } {
+    return
+}
+
+run_cc_link_tests [list \
+    [list \
+	"Build dt-relr-glibc-1a.so" \
+	"-shared $NO_DT_RELR_CC_LDFLAGS" \
+	"-fPIC" \
+	{ dt-relr-glibc-1.c } \
+	{{readelf {--version-info} dt-relr-glibc-1a.rd}} \
+	"glibc-relr-1a.so" \
+    ] \
+    [list \
+	"Build dt-relr-glibc-1b.so" \
+	"-shared $DT_RELR_CC_LDFLAGS" \
+	"-fPIC" \
+	{ dt-relr-glibc-1.c } \
+	{{readelf {-W --version-info} dt-relr-glibc-1b.rd}} \
+	"dt-relr-glibc-1b.so" \
+    ] \
+]