Re: [PATCH v11 2/7] Add GLIBC_ABI_DT_RELR for DT_RELR support

[Fangrui Song <maskray@google.com>]

On 2022-04-22, H.J. Lu wrote:
>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.
>---
> elf/Makefile         | 14 ++++++++++++--
> elf/Versions         |  5 +++++
> elf/dl-version.c     | 35 +++++++++++++++++++++++++++++++++--
> include/link.h       |  2 ++
> scripts/abilist.awk  |  2 ++
> scripts/versions.awk |  7 ++++++-
> 6 files changed, 60 insertions(+), 5 deletions(-)
>
>diff --git a/elf/Makefile b/elf/Makefile
>index 8ed6c3b0b1..f288f866f2 100644
>--- a/elf/Makefile
>+++ b/elf/Makefile
>@@ -1143,8 +1143,12 @@ $(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
>
>@@ -2773,3 +2777,9 @@ $(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 8bed855d8c..a9ff278de7 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 b47bd91727..cda0889209 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;
> }
>
>diff --git a/include/link.h b/include/link.h
>index 03db14c7b0..0ac82d7c77 100644
>--- a/include/link.h
>+++ b/include/link.h
>@@ -177,6 +177,8 @@ struct link_map
> 	lt_library,		/* Library needed by main executable.  */
> 	lt_loaded		/* Extra run-time loaded shared object.  */
>       } l_type:2;
>+    unsigned int l_dt_relr_ref:1; /* Nonzero if GLIBC_ABI_DT_RELR is
>+				     referenced.  */
>     unsigned int l_relocated:1;	/* Nonzero if object's relocations done.  */
>     unsigned int l_init_called:1; /* Nonzero if DT_INIT function called.  */
>     unsigned int l_global:1;	/* Nonzero if object in _dl_global_scope.  */
>diff --git a/scripts/abilist.awk b/scripts/abilist.awk
>index 24a34ccbed..6cc7af6ac8 100644
>--- a/scripts/abilist.awk
>+++ b/scripts/abilist.awk
>@@ -55,6 +55,8 @@ $2 == "g" || $2 == "w" && (NF == 7 || NF == 8) {
>   # caused STV_HIDDEN symbols to appear in .dynsym, though that is useless.
>   if (NF > 7 && $7 == ".hidden") next;
>
>+  if (version ~ /^GLIBC_ABI_/ && !include_abi_version) next;
>+
>   if (version == "GLIBC_PRIVATE" && !include_private) next;
>
>   desc = "";
>diff --git a/scripts/versions.awk b/scripts/versions.awk
>index 357ad1355e..d70b07bd1a 100644
>--- a/scripts/versions.awk
>+++ b/scripts/versions.awk
>@@ -185,8 +185,13 @@ END {
> 	closeversion(oldver, veryoldver);
> 	veryoldver = oldver;
>       }
>-      printf("%s {\n  global:\n", $2) > outfile;
>       oldver = $2;
>+      # Skip the placeholder symbol used only for empty version map.
>+      if ($3 == "__placeholder_only_for_empty_version_map;") {
>+	printf("%s {\n", $2) > outfile;
>+	continue;
>+      }
>+      printf("%s {\n  global:\n", $2) > outfile;
>     }
>     printf("   ") > outfile;
>     for (n = 3; n <= NF; ++n) {
>-- 
>2.35.1

Still LG

Reviewed-by: Fangrui Song <maskray@google.com>