Re: Can DT_RELR catch up glibc 2.35?

["H.J. Lu" <hjl.tools@gmail.com>]

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

-- 
H.J.