Can DT_RELR catch up glibc 2.35?
Fangrui Song
maskray@google.com
Thu Nov 18 00:30:25 GMT 2021
On 2021-11-17, H.J. Lu wrote:
>On Wed, Nov 17, 2021 at 4:46 AM Adhemerval Zanella
><adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> On 16/11/2021 21:26, H.J. Lu wrote:
>> > On Tue, Nov 16, 2021 at 1:07 PM Adhemerval Zanella
>> > <adhemerval.zanella@linaro.org> wrote:
>> >>
>> >>
>> >>
>> >> On 12/11/2021 04:47, Fangrui Song wrote:
>> >>> I am glad that https://sourceware.org/pipermail/libc-alpha/2021-October/132029.html
>> >>> ("[PATCH v2] elf: Support DT_RELR relative relocation format [BZ #27924]") gets
>> >>> some traction and many folks acknowledge the size benefit.
>> >>> (On my Arch Linux, I measured 8% decrease for my /usr/bin.)
>> >>
>> >> I brought this to the weekly glibc call two weeks ago and if I recall correctly
>> >> the *main* issue is we need a proper generic ABI definition published to move this
>> >> forward on glibc side (H.J.Lu was adamant about).
>> >>
>> >> From my part, current status where we have multiple system that already support
>> >> it (android, chromeos, freebsd) and with a toolchain that supports build/check
>> >> glibc on at least 4 different ABIs (lld 13 on x86 and arm) is good enough.
>> >>
>> >> We lack of proper testing while using bfd might a drawback, since we lack a way
>> >> to generate binaries without linker support.
>> >>
>> >>>
>> >>> There are two potential issues.
>> >>>
>> >>> 1. Lack of "Time travel compatibility" detector
>> >>> 2. Some folks feel that unable to test with scripts/build-many-glibcs.py is a problem.
>> >>> (ld.lld --pack-dyn-relocs=relr (since July 2018) is the only linker implementation
>> >>> and scripts/build-many-glibcs.py doesn't have an lld configuration)
>> >>>
>> >>> Let me address them for you.
>> >>>
>> >>> ---
>> >>>
>> >>> 1.
>> >>>
>> >>> "Time travel compatibility" means running a new object on an old system.
>> >>> A new object using DT_RELR doesn't have the R_*_RELATIVE part in
>> >>> .rel.dyn/.rela.dyn and is destined to crash.
>> >>>
>> >>> If the GNU ld implementation (which may take a while) adopts an
>> >>> undefined versioned .dynsym symbol (e.g. _dl_have_relr
>> >>> https://sourceware.org/pipermail/binutils/2021-October/118347.html),
>> >>> we can guarantee old ld.so will report an error.
>> >>> The undefined symbol needs to be versioned because ld -shared (default
>> >>> to --allow-shlib-undefined) does not error on unversioned symbols. Say
>> >>> GNU ld adopts something like _dl_have_relr@GLIBC_2.40 . Now it is funny as GNU
>> >>> ld needs to know the glibc version "GLIBC_2.40", not just the stem
>> >>> glibc-flavored symbol name "_dl_have_relr".
>> >>
>> >> This might be troublesome to backport, since it would require to use a higher
>> >> version than the baseline one. I am not sure if distro will be willing or plan
>> >> to backport such feature though.
>> >>
>> >>>
>> >>> There are non-Linux OSes which don't like a "_dl_have_relr" symbol name.
>> >>> GNU ld would have to provide options in two flavors, one with
>> >>> _dl_have_relr@GLIBC_2.40, one without. Among glibc systems, there are
>> >>> plenty of distros there which don't rigidly require a friendly
>> >>> diagnostic for "time traverl compatibility", e.g. I pretty sure many
>> >>> Gentoo Linux folks doing aggressive optimizations know that their
>> >>> executables don't run on old systems.
>> >>
>> >> I think even other Linux libc, such as musl, won't be willing to support
>> >> tying the DT_RELR to a loader/libc symbol existing (musl even less because
>> >> it explicit does not support symbol versioning).
>> >>
>> >>>
>> >>> An alternative to _dl_have_relr is EI_ABIVERSION. That is probably even
>> >>> less appealing because bumping the version locks out many ELF consumers.
>> >>> https://maskray.me/blog/2021-10-31-relative-relocations-and-relr#ei_abiversion
>> >>> In addition, I noticed that Debian ld.so 2.32 just seems to ignore EI_ABIVERSION.
>> >>
>> >> The problem with EI_ABIVERSION is a limitation of glibc, which only checks
>> >> EI_ABIVERSION on open_verify() and this is not called on default process
>> >> execution, where kernel will be one responsible to load both the binary
>> >> and the interpreter:
>> >>
>> >> ---
>> >> $ cat test.c
>> >> #include <stdio.h>
>> >>
>> >> int main ()
>> >> {
>> >> return 0;
>> >> }
>> >> $ gdb ./test
>> >> [...]
>> >> (gdb) starti
>> >> [...]
>> >> process 1420253
>> >> Mapped address spaces:
>> >>
>> >> Start Addr End Addr Size Offset objfile
>> >> 0x555555554000 0x555555555000 0x1000 0x0 /tmp/test/test
>> >> 0x555555555000 0x555555556000 0x1000 0x1000 /tmp/test/test
>> >> 0x555555556000 0x555555557000 0x1000 0x2000 /tmp/test/test
>> >> 0x555555557000 0x555555559000 0x2000 0x2000 /tmp/test/test
>> >> 0x7ffff7fc2000 0x7ffff7fc6000 0x4000 0x0 [vvar]
>> >> 0x7ffff7fc6000 0x7ffff7fc8000 0x2000 0x0 [vdso]
>> >> 0x7ffff7fc8000 0x7ffff7fc9000 0x1000 0x0 /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
>> >> 0x7ffff7fc9000 0x7ffff7ff1000 0x28000 0x1000 /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
>> >> 0x7ffff7ff1000 0x7ffff7ffb000 0xa000 0x29000 /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
>> >> 0x7ffff7ffb000 0x7ffff7fff000 0x4000 0x32000 /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
>> >> 0x7ffffffde000 0x7ffffffff000 0x21000 0x0 [stack]
>> >> 0xffffffffff600000 0xffffffffff601000 0x1000 0x0 [vsyscall]
>> >> ---
>> >>
>> >> However, the test is correctly executed on any load library and/or if the
>> >> executable is executed by issuing the loader directly:
>> >>
>> >> ---
>> >> $ readelf -h test
>> >> ELF Header:
>> >> Magic: 7f 45 4c 46 02 01 01 00 *04* 00 00 00 00 00 00 00
>> >> [...]
>> >> $ /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 ./test
>> >> ./test: error while loading shared libraries: ./test: ELF file ABI version invalid
>> >> ---
>> >>
>> >> I think this is an bug, since it basically defeats the EI_ABIVERSION check
>> >> and makes programs executed by issuing the loader with a different semantic
>> >> than the one executed through execve syscall.
>> >>
>> >> Afaik kernel does not pass such information on auxv vector (we might ask
>> >> for a AT_EHDR eventually) so a potential fix will cost us some extra
>> >> syscalls on every program execution (to read and check the ELF Header with
>> >> similar test done on open_verify()).
>> >>
>> >> However it does *not* help on older glibc which will still accept old binaries.
>> >>
>> >>>
>> >>> % r2 -wqc 'wx 22 @ 8' a; readelf -Wh a | grep ABI; ./a
>> >>> OS/ABI: UNIX - GNU
>> >>> ABI Version: 34
>> >>> hello
>> >>>
>> >>
>> >> I am not really sure if the 'time travel compatibility' is really an issue,
>> >> although I saw reports where users try to use chromeos library on glibc that
>> >> fails in some strange ways (most likely due DT_RELR). If user is deploying
>> >> a *opt-in* feature that requires proper dynamic loader support, I would
>> >> expect it know the environment he is targeting.
>> >>
>> >> So I think the best course of action for this issue is indeed fix EI_ABIVERSION
>> >> and make DT_RELR a new 'libc-abis' entry. We might backport the EI_ABIVERSION
>> >> fix to some older releases, and distros that want to use DT_RELR should do also.
>> >
>> > Given that EI_ABIVERSION doesn't really work, should we revisit my
>> > GNU_PROPERTY_1_GLIBC_2_NEEDED proposal:
>> >
>> > https://sourceware.org/pipermail/binutils/2021-October/118292.html
>>
>> The GNU_PROPERTY_1_GLIBC_2_NEEDED still does not really help much if the idea
>> is to backport DT_RELR to older version and it still adds logic on the static
>> linker about glibc symbol version. I would like that static linker know as
>> little as possible about glibc version, EI_ABIVERSION is way simpler and
>> already express ABI extensions.
>>
>> I still think for DT_RELR instead of inventing another GNU extension, we might
>> fix EI_ABIVERSION and use it properly. Checking with kernel, I think it should
>> be simple: the elf header is located at the AT_PHDR - sizeof (ElfW(Ehdr)), so we
>> can refactor the tests at open_verify and use on rtld.c for the case execve()
>> is called for the executable.
>
>The scheme should work for older systems without changes. Can we add
>GLIBC_PRIVATE_DT_RELR? Linker adds GLIBC_PRIVATE_DT_RELR
>version dependency when DT_RELR is generated
For CCed folks who may be puzzled about the context,
I have a write-up
https://maskray.me/blog/2021-10-31-relative-relocations-and-relr#time-travel-compatibility
which provides my reply to HJ's question as well.
A synthesized versioned undefined dynamic symbol can indeed catch "time
travel compatibility", but the mechanism would be the first time ld adds an option variant
for a particular libc implementation (glibc) locking out all other
implementations: --pack-dyn-relocs=relr-glibc or -z relr-glibc.
Sigh, it is really not pretty.
We know many other libc implementations don't want to synthesize such a
symbol.
"If user is deploying a *opt-in* feature that requires proper dynamic
loader support, I would expect they know the environment they are targeting."
May I suggest that: if a glibc distro really worries that users deploy
ld.lld --pack-dyn-relocs=relr on their new system and back port that to
old systems, just remove DT_RELR support from your local glibc? Since
ld.lld --pack-dyn-relocs=relr doesn't work on your system with glibc
2.35, people wouldn't complain about not working on older versions.
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