[PATCH v3 3/5] Add GLIBC_ABI_DT_RELR for DT_RELR support
Fangrui Song
maskray@google.com
Tue Mar 1 19:13:17 GMT 2022
On 2022-03-01, H.J. Lu wrote:
>On Mon, Feb 28, 2022 at 8:22 PM Fangrui Song <maskray@google.com> wrote:
>>
>> On 2022-02-07, 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. Issue an error if there is a DT_RELR entry
>> >without GLIBC_ABI_DT_RELR dependency nor GLIBC_PRIVATE definition.
>> >
>> >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 | 18 ++++++++++++++++--
>> > elf/Versions | 5 +++++
>> > elf/dl-version.c | 33 +++++++++++++++++++++++++++++++--
>> > elf/libc-abi-version.exp | 1 +
>> > include/link.h | 6 ++++++
>> > scripts/abilist.awk | 2 ++
>> > scripts/versions.awk | 7 ++++++-
>> > 7 files changed, 67 insertions(+), 5 deletions(-)
>> > create mode 100644 elf/libc-abi-version.exp
>> >
>> >diff --git a/elf/Makefile b/elf/Makefile
>> >index 71b08c75dd..a6515e8a21 100644
>> >--- a/elf/Makefile
>> >+++ b/elf/Makefile
>> >@@ -48,6 +48,10 @@ routines = \
>> > rtld_static_init \
>> > # routines
>> >
>> >+ifeq ($(have-dt-relr),yes)
>> >+check-abi-version-libc = $(objpfx)check-abi-version-libc.out
>> >+endif
>> >+
>> > # The core dynamic linking functions are in libc for the static and
>> > # profiled libraries.
>> > dl-routines = \
>> >@@ -1106,8 +1110,8 @@ $(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 $(check-abi-version-libc)
>> >+tests-special += $(objpfx)check-abi-ld.out $(check-abi-version-libc)
>> > update-abi: update-abi-ld
>> > update-all-abi: update-all-abi-ld
>> >
>> >@@ -2747,3 +2751,13 @@ $(objpfx)check-tst-relr-pie.out: $(objpfx)tst-relr-pie
>> > | sed -ne '/required from libc.so/,$$ p' \
>> > | grep GLIBC_ABI_DT_RELR > $@; \
>> > $(evaluate-test)
>> >+
>> >+$(objpfx)check-abi-version-libc.out: libc-abi-version.exp \
>> >+ $(objpfx)libc.symlist-abi-version
>> >+ cmp $^ > $@; \
>> >+ $(evaluate-test)
>> >+
>> >+$(objpfx)libc.symlist-abi-version: $(common-objpfx)libc.so
>> >+ LC_ALL=C $(NM) -D $< | grep " GLIBC_ABI_" \
>> >+ | sed "s/^0\+/00000000/" > $@T
>> >+ mv -f $@T $@
>>
>> This checks the SHN_ABS symbol.
>> ld.lld does not add the SHN_ABS symbol and does not plan to add it as,
>> Perhaps just check the placeholder symbol.
>
>Does lld support version dependency without a symbol?
>I want to avoid adding the unused symbols to libc.so.
Yes. My https://reviews.llvm.org/D120701 adds GLIBC_ABI_DT_RELR verneed
without a symbol.
It just doesn't synthesize the SHN_ABS symbol (name is the version name) which seems not useful.
>> GNU nm before binutils 2.35 does not display @ or @@ in -D mode.
>
>I will check it.
Thanks!
Perhaps $(READELF) -V $< | grep GLIBC_ABI_DT_RELR
will be quite good.
>> >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..720ec596a5 100644
>> >--- a/elf/dl-version.c
>> >+++ b/elf/dl-version.c
>> >@@ -214,12 +214,20 @@ _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);
>> >
>> >+ if (map->l_abi_version == lav_none
>> >+ /* 0xfd0e42: _dl_elf_hash ("GLIBC_ABI_DT_RELR"). */
>> >+ && aux->vna_hash == 0xfd0e42
>> >+ && __glibc_likely (strcmp (string,
>> >+ "GLIBC_ABI_DT_RELR")
>> >+ == 0))
>> >+ map->l_abi_version = lav_dt_relr_ref;
>> >+
>> > /* Compare the version index. */
>> > if ((unsigned int) (aux->vna_other & 0x7fff) > ndx_high)
>> > ndx_high = aux->vna_other & 0x7fff;
>> >@@ -253,6 +261,16 @@ _dl_check_map_versions (struct link_map *map, int verbose, int trace_mode)
>> > ent = (ElfW(Verdef) *) (map->l_addr + def->d_un.d_ptr);
>> > while (1)
>> > {
>> >+ /* 0x0963cf85: _dl_elf_hash ("GLIBC_PRIVATE"). */
>> >+ if (ent->vd_hash == 0x0963cf85)
>> >+ {
>> >+ ElfW(Verdaux) *aux = (ElfW(Verdaux) *) ((char *) ent
>> >+ + ent->vd_aux);
>> >+ if (__glibc_likely (strcmp ("GLIBC_PRIVATE",
>> >+ strtab + aux->vda_name) == 0))
>> >+ map->l_abi_version = lav_private_def;
>> >+ }
>> >+
>> > if ((unsigned int) (ent->vd_ndx & 0x7fff) > ndx_high)
>> > ndx_high = ent->vd_ndx & 0x7fff;
>> >
>> >@@ -352,6 +370,17 @@ _dl_check_map_versions (struct link_map *map, int verbose, int trace_mode)
>> > }
>> > }
>> >
>> >+ /* Issue an error if there is a DT_RELR entry without GLIBC_ABI_DT_RELR
>> >+ dependency nor GLIBC_PRIVATE definition. */
>> >+ if (map->l_info[DT_RELR] != NULL
>> >+ && __glibc_unlikely (map->l_abi_version == lav_none))
>> >+ {
>> >+ _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/elf/libc-abi-version.exp b/elf/libc-abi-version.exp
>> >new file mode 100644
>> >index 0000000000..455088dc6b
>> >--- /dev/null
>> >+++ b/elf/libc-abi-version.exp
>> >@@ -0,0 +1 @@
>> >+00000000 A GLIBC_ABI_DT_RELR
>> >diff --git a/include/link.h b/include/link.h
>> >index bef2820b40..ce9e3d5214 100644
>> >--- a/include/link.h
>> >+++ b/include/link.h
>> >@@ -177,6 +177,12 @@ struct link_map
>> > lt_library, /* Library needed by main executable. */
>> > lt_loaded /* Extra run-time loaded shared object. */
>> > } l_type:2;
>> >+ enum /* ABI dependency of this object. */
>> >+ {
>> >+ lav_none, /* No ABI dependency. */
>> >+ lav_dt_relr_ref, /* Need GLIBC_ABI_DT_RELR. */
>> >+ lav_private_def /* Define GLIBC_PRIVATE. */
>> >+ } l_abi_version:2;
>> > 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) {
>> >--
>> >2.34.1
>> >
>>
>> Looks like a Serpent OS contributor has noticed this patch series
>> https://github.com/llvm/llvm-project/issues/53775#issuecomment-1046361237 .
>> I will try implementing -z pack-relative-relocs for ld.lld
>
>
>
>--
>H.J.
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