Add GLIBC_ABI_DT_RELR for DT_RELR support

The EI_ABIVERSION field of the ELF header in executables and shared
libraries can be bumped to indicate the minimum ABI requirement on the
dynamic linker.  However, EI_ABIVERSION in executables isn't checked by
the Linux kernel ELF loader nor the existing dynamic linker.  Executables
will crash mysteriously if the dynamic linker doesn't support the ABI
features required by the EI_ABIVERSION field.  The dynamic linker should
be changed to check EI_ABIVERSION in executables.

Add a glibc version, GLIBC_ABI_DT_RELR, to indicate DT_RELR support so
that the existing dynamic linkers will issue an error on executables with
GLIBC_ABI_DT_RELR dependency.  When there is a DT_VERNEED entry with
libc.so on DT_NEEDED, issue an error if there is a DT_RELR entry without
GLIBC_ABI_DT_RELR dependency.

Support __placeholder_only_for_empty_version_map as the placeholder symbol
used only for empty version map to generate GLIBC_ABI_DT_RELR without any
symbols.

3 files changed, 50 insertions, 4 deletions

diff --git a/elf/Makefile b/elf/Makefile
index e4907d5..073807b 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -1145,8 +1145,12 @@ $(eval $(call include_dsosort_tests,dso-sort-tests-1.def))
 $(eval $(call include_dsosort_tests,dso-sort-tests-2.def))
 endif
 
-check-abi: $(objpfx)check-abi-ld.out
-tests-special += $(objpfx)check-abi-ld.out
+check-abi: $(objpfx)check-abi-ld.out \
+	   $(objpfx)check-abi-version-libc.out
+tests-special += \
+  $(objpfx)check-abi-ld.out \
+  $(objpfx)check-abi-version-libc.out \
+# tests-special
 update-abi: update-abi-ld
 update-all-abi: update-all-abi-ld
 
@@ -2779,3 +2783,9 @@ $(objpfx)tst-p_align3: $(objpfx)tst-p_alignmod3.so
 $(objpfx)tst-p_align3.out: tst-p_align3.sh $(objpfx)tst-p_align3
 	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'; \
 	$(evaluate-test)
+
+$(objpfx)check-abi-version-libc.out: $(common-objpfx)libc.so
+	LC_ALL=C $(READELF) -V -W $< \
+		| sed -ne '/.gnu.version_d/, /.gnu.version_r/ p' \
+		| grep GLIBC_ABI_DT_RELR > $@; \
+	$(evaluate-test)
diff --git a/elf/Versions b/elf/Versions
index 8bed855..a9ff278 100644
--- a/elf/Versions
+++ b/elf/Versions
@@ -23,6 +23,11 @@ libc {
   GLIBC_2.35 {
     _dl_find_object;
   }
+  GLIBC_ABI_DT_RELR {
+    # This symbol is used only for empty version map and will be removed
+    # by scripts/versions.awk.
+    __placeholder_only_for_empty_version_map;
+  }
   GLIBC_PRIVATE {
     # functions used in other libraries
     __libc_early_init;
diff --git a/elf/dl-version.c b/elf/dl-version.c
index b47bd91..cda0889 100644
--- a/elf/dl-version.c
+++ b/elf/dl-version.c
@@ -214,12 +214,19 @@ _dl_check_map_versions (struct link_map *map, int verbose, int trace_mode)
 	      while (1)
 		{
 		  /* Match the symbol.  */
+		  const char *string = strtab + aux->vna_name;
 		  result |= match_symbol (DSO_FILENAME (map->l_name),
 					  map->l_ns, aux->vna_hash,
-					  strtab + aux->vna_name,
-					  needed->l_real, verbose,
+					  string, needed->l_real, verbose,
 					  aux->vna_flags & VER_FLG_WEAK);
 
+		  /* 0xfd0e42: _dl_elf_hash ("GLIBC_ABI_DT_RELR").  */
+		  if (aux->vna_hash == 0xfd0e42
+		      && __glibc_likely (strcmp (string,
+						 "GLIBC_ABI_DT_RELR")
+					 == 0))
+		    map->l_dt_relr_ref = 1;
+
 		  /* Compare the version index.  */
 		  if ((unsigned int) (aux->vna_other & 0x7fff) > ndx_high)
 		    ndx_high = aux->vna_other & 0x7fff;
@@ -352,6 +359,30 @@ _dl_check_map_versions (struct link_map *map, int verbose, int trace_mode)
 	}
     }
 
+  /* When there is a DT_VERNEED entry with libc.so on DT_NEEDED, issue
+     an error if there is a DT_RELR entry without GLIBC_ABI_DT_RELR
+     dependency.  */
+  if (dyn != NULL
+      && map->l_info[DT_NEEDED] != NULL
+      && map->l_info[DT_RELR] != NULL
+      && __glibc_unlikely (!map->l_dt_relr_ref))
+    {
+      const char *strtab = (const void *) D_PTR (map, l_info[DT_STRTAB]);
+      const ElfW(Dyn) *d;
+      for (d = map->l_ld; d->d_tag != DT_NULL; ++d)
+	if (d->d_tag == DT_NEEDED)
+	  {
+	    const char *name = strtab + d->d_un.d_val;
+	    if (strncmp (name, "libc.so.", 8) == 0)
+	      {
+		_dl_exception_create
+		  (&exception, DSO_FILENAME (map->l_name),
+		   N_("DT_RELR without GLIBC_ABI_DT_RELR dependency"));
+		goto call_error;
+	      }
+	  }
+    }
+
   return result;
 }