Can DT_RELR catch up glibc 2.35?

Wed Nov 17 12:46:12 GMT 2021

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.