Re: [PATCH 10/10] ld: Add glibc dependency for DT_RELR

["H.J. Lu" <hjl.tools@gmail.com>]

On Fri, Jan 7, 2022 at 8:43 PM Fangrui Song <i@maskray.me> wrote:
>
> On 2022-01-07, H.J. Lu via Binutils wrote:
> >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                           | 86 +++++++++++++++++++++++++
> > ld/ld.texi                              |  4 +-
> > ld/testsuite/ld-elf/dt-relr-glibc-1.c   | 11 ++++
> > ld/testsuite/ld-elf/dt-relr-glibc-1a.rd |  4 ++
> > ld/testsuite/ld-elf/dt-relr-glibc-1b.rd |  7 ++
> > ld/testsuite/ld-elf/dt-relr.exp         | 44 +++++++++++++
> > 6 files changed, 155 insertions(+), 1 deletion(-)
> > create mode 100644 ld/testsuite/ld-elf/dt-relr-glibc-1.c
> > create mode 100644 ld/testsuite/ld-elf/dt-relr-glibc-1a.rd
> > create mode 100644 ld/testsuite/ld-elf/dt-relr-glibc-1b.rd
> > create mode 100644 ld/testsuite/ld-elf/dt-relr.exp
> >
> >diff --git a/bfd/elflink.c b/bfd/elflink.c
> >index 31b13f5df7a..05ac1cb7a63 100644
> >--- a/bfd/elflink.c
> >+++ b/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;
>
> Having the dependency on ld.so is better.
> (The linker knows the --dynamic-linker value.)
> libc.so does not need to know whether DT_RELR is used.
>
> It's also easier on glibc side.
> Defining  __glibc_abi_dt_relr@@GLIBC_ABI_DT_RELR in elf/Versions is
> simpler than defining it in libc.so and updating every libc.abilist.

I chose libc.so instead of ld.so since shared libraries may
not be linked against ld.so.  The glibc change can be simple:

https://patchwork.sourceware.org/project/glibc/patch/20211123223053.3334224-1-hjl.tools@gmail.com/

Please replace GLIBC_ABI_VERSION_GEN2 with
GLIBC_ABI_DT_RELR.

or like

https://gitlab.com/x86-glibc/glibc/-/commit/3bc53baa5304855b5105dabd7726b23d454605fb

I can go either way.




> >+      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
> >diff --git a/ld/ld.texi b/ld/ld.texi
> >index 457089ec06a..a16657ddb45 100644
> >--- a/ld/ld.texi
> >+++ b/ld/ld.texi
> >@@ -1439,7 +1439,9 @@ and shared library.  It adds @code{DT_RELR}, @code{DT_RELRSZ} and
> > building position-dependent executable and relocatable output.  This
> > option also implies @option{combreloc} and @option{--relax}.
> > @option{nopack-relative-relocs} is the default, which disables
> >-compact relative relocation.  Supported for i386 and x86-64.
> >+compact 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
> >diff --git a/ld/testsuite/ld-elf/dt-relr-glibc-1.c b/ld/testsuite/ld-elf/dt-relr-glibc-1.c
> >new file mode 100644
> >index 00000000000..beacffe29e7
> >--- /dev/null
> >+++ b/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)
> >diff --git a/ld/testsuite/ld-elf/dt-relr-glibc-1a.rd b/ld/testsuite/ld-elf/dt-relr-glibc-1a.rd
> >new file mode 100644
> >index 00000000000..51bda5d70a1
> >--- /dev/null
> >+++ b/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]+
> >+#...
> >diff --git a/ld/testsuite/ld-elf/dt-relr-glibc-1b.rd b/ld/testsuite/ld-elf/dt-relr-glibc-1b.rd
> >new file mode 100644
> >index 00000000000..6556a6d939e
> >--- /dev/null
> >+++ b/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
> >diff --git a/ld/testsuite/ld-elf/dt-relr.exp b/ld/testsuite/ld-elf/dt-relr.exp
> >new file mode 100644
> >index 00000000000..51d21e400ab
> >--- /dev/null
> >+++ b/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" \
> >+    ] \
> >+]
> >--
> >2.33.1
> >



--
H.J.