[llvm-dev] llvm-strip creates unloadable shared objects on linux-armv7hf

Thu Oct 31 05:04:25 PDT 2019

Everyone,

Just to close this loop, the patch was accepted into master today:
https://github.com/llvm/llvm-project/commit/fb4a55010ee9bd03720609c8542f770775576fc8

Thanks for everyone guiding me!

Tobias

On Fri, Oct 18, 2019 at 8:41 PM Tobias Hieta <tobias at plexapp.com> wrote:
>
> Jordan,
>
> I have sent the patch via Phabricator: https://reviews.llvm.org/D69188
>
> Let me know if I got it right.
>
> -- Tobias
>
> On Thu, Oct 17, 2019 at 7:12 PM Jordan Rupprecht <rupprecht at google.com> wrote:
> >
> > Tobias,
> > I don't have much experience with ARM, but from your report and Peter's explanation of why LLD does it, I agree we should be consistent with LLD and keep the section.
> >
> > From my skimming of the LLD sources, it looks like we keep arm attributes section based on the section type being SHT_ARM_ATTRIBUTES, not on the name being ".ARM.attributes". Other than that, this change seems good to accept.
> >
> > Feel free to send a patch! (btw, my review handle is rupprecht, not rupprect).
> >
> > On Thu, Oct 17, 2019 at 5:29 AM Peter Smith via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hello Tobias,
> >>
> >> I think that looks reasonable to me, I think it will be down to the
> >> llvm-objcopy team whether they want to make .ARM.attributes a special
> >> case or not. The best way to find out is to submit a patch, citing the
> >> problems with old versions of libc, I'd expect that you'll need to add
> >> a test case for the patch to be accepted. To do that it is probably
> >> best to look at the existing tests for llvm-strip and try and copy
> >> them. The test could be as simple as generating a binary with a
> >> section of type SHT_ARM_ATTRIBUTES and checking that strip didn't
> >> remove it. These tests sometimes use yaml2obj to generate an ELF file
> >> without needing a compiler and linker. Running the tests should be as
> >> simple as ninja check-llvm or make check-llvm depending on whether you
> >> used ninja or make when building llvm. If you want to run just one
> >> test then you can use bin/llvm-lit -v -a /path/to/test.s (from your
> >> build directory).
> >>
> >> The instructions on how to contribute are in
> >> https://llvm.org/docs/DeveloperPolicy.html the people that I know have
> >> been active in llvm-objdump are MaskRay (Fangrui Song), rupprect
> >> (Jordan Rupprecht), grimar (George Rimar). If you include these people
> >> on the reviewers then I'm sure they'll be able to add anyone else that
> >> they think would be interested.
> >>
> >> Hope this helps
> >>
> >> Peter
> >>
> >> On Thu, 17 Oct 2019 at 12:53, Tobias Hieta <tobias at plexapp.com> wrote:
> >> >
> >> > Hello Peter,
> >> >
> >> > I was able to fix this issue with this simple patch against llvm-objcopy:
> >> >
> >> > diff --git a/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
> >> > b/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
> >> > index dd6a7d7e14b..c0dfd3a9838 100644
> >> > --- a/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
> >> > +++ b/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
> >> > @@ -503,6 +503,8 @@ static Error replaceAndRemoveSections(const
> >> > CopyConfig &Config, Object &Obj) {
> >> > return false;
> >> > if (StringRef(Sec.Name).startswith(".gnu.warning"))
> >> > return false;
> >> > + if (StringRef(Sec.Name).startswith(".ARM.attributes"))
> >> > + return false;
> >> > if (Sec.ParentSegment != nullptr)
> >> > return false;
> >> > return (Sec.Flags & SHF_ALLOC) == 0;
> >> >
> >> > Is this a good way of fixing the issue? Happy to read up on how to
> >> > submit patches if you think this is the way to go.
> >> >
> >> > -- Tobias
> >> >
> >> >
> >> > On Thu, Oct 17, 2019 at 12:27 PM Peter Smith <peter.smith at linaro.org> wrote:
> >> > >
> >> > > It is possible that it was present in older versions of glibc, with
> >> > > the limited time I've got for archaeology I found
> >> > > https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1712938
> >> > > My change in LLD was https://reviews.llvm.org/D27718 although it
> >> > > doesn't help much.
> >> > >
> >> > > You may be able to follow the bread crumb trail from the launchpad
> >> > > bug. Good luck!
> >> > >
> >> > > Peter
> >> > >
> >> > > On Thu, 17 Oct 2019 at 10:59, Tobias Hieta <tobias at plexapp.com> wrote:
> >> > > >
> >> > > > Peter,
> >> > > >
> >> > > > Thanks for your detailed response. I am not 100% sure on what libc we
> >> > > > are using here, it's a netgear NAS device. We publish Plex for
> >> > > > multiple NAS vendors devices and their toolchain always seems to lag
> >> > > > behind a bit, which is why we invested in building our own toolchain
> >> > > > based on LLVM tools.
> >> > > >
> >> > > > Does seem to be glibc though:
> >> > > > ii libc-bin 2.19-18+deb8u10.netgear1 armel GNU C Library: Binaries
> >> > > >
> >> > > > Maybe it's just because it's older why this happens?
> >> > > >
> >> > > > Thanks,
> >> > > > Tobias
> >> > > >
> >> > > > On Thu, Oct 17, 2019 at 11:25 AM Peter Smith <peter.smith at linaro.org> wrote:
> >> > > > >
> >> > > > > Hello Tobias,
> >> > > > >
> >> > > > > Does your system happen to be using eglibc? I ran into this problem on
> >> > > > > an Ubuntu 14.04 system and it was down to the eglibc dynamic loader
> >> > > > > using the .ARM.attributes section when performing dlopen. Strictly
> >> > > > > speaking the .ARM.attributes section is only defined for relocatable
> >> > > > > objects, the ABI says that their presence in executables or dynamic
> >> > > > > loaders is neither required or forbidden, so it is somewhat risky for
> >> > > > > any portable program to depend on their presence.
> >> > > > >
> >> > > > > Since eglibc was merged back into glibc this dependency on
> >> > > > > .ARM.attributes went away. For LLD we took the position that it was
> >> > > > > worth keeping the .ARM.attributes to placate eglibc, as this was more
> >> > > > > likely to be encountered 2 years ago. I've not got a strong position
> >> > > > > on llvm-strip, in theory it should be strippable from executable and
> >> > > > > shared libraries, but there may be a case that eglibc is important
> >> > > > > enough to not strip by default.
> >> > > > >
> >> > > > > Peter
> >> > > > >
> >> > > > > On Thu, 17 Oct 2019 at 10:16, Tobias Hieta via llvm-dev
> >> > > > > <llvm-dev at lists.llvm.org> wrote:
> >> > > > > >
> >> > > > > > Hello Rui,
> >> > > > > >
> >> > > > > > Thanks for your reply. I tried with the keep-section argument and that
> >> > > > > > made the shared library work.
> >> > > > > >
> >> > > > > > Should these sections be kept around by default maybe?
> >> > > > > >
> >> > > > > > -- Tobias
> >> > > > > >
> >> > > > > > On Thu, Oct 17, 2019 at 11:06 AM Rui Ueyama <ruiu at google.com> wrote:
> >> > > > > > >
> >> > > > > > > One thing I noticed is that llvm-strip seemed to remove a .ARM.attributes section. Can you try --keep-section=.ARM.attributes to tell to the command to keep the section?
> >> > > > > > >
> >> > > > > > > On Thu, Oct 17, 2019 at 5:37 PM Tobias Hieta via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >> > > > > > >>
> >> > > > > > >> Hello,
> >> > > > > > >>
> >> > > > > > >> Recently we tried to streamline our toolchain by removing some GNU
> >> > > > > > >> tools with LLVM tools to avoid having multiple copies of strip, nm, ar
> >> > > > > > >> and similar tools. Today we ran into a really strange issue where
> >> > > > > > >> shared objects where not loadable.
> >> > > > > > >>
> >> > > > > > >> We where able to track this down to llvm-strip with default arguments
> >> > > > > > >> creating a shared object that doesn't load correctly. It works if we
> >> > > > > > >> switch from not passing args to llvm-strip to pass -strip-all-gnu
> >> > > > > > >>
> >> > > > > > >> I built bzip2 and libbz2.so to show this case here:
> >> > > > > > >> https://1drv.ms/u/s!Amjta5FRkBbbkiz14aHTRJe03LlL?e=2mGvTN
> >> > > > > > >>
> >> > > > > > >> This is the error we got by using the stripped shared object with
> >> > > > > > >> llvm-strip -strip-all:
> >> > > > > > >>
> >> > > > > > >> admin at Netgear-RN212:~/b$ LD_PRELOAD=./libbz2.so.all ./bzip2
> >> > > > > > >> ERROR: ld.so: object './libbz2.so.all' from LD_PRELOAD cannot be
> >> > > > > > >> preloaded (cannot open shared object file): ignored.
> >> > > > > > >> ./bzip2: error while loading shared libraries: libbz2.so: cannot open
> >> > > > > > >> shared object file: No such file or directory
> >> > > > > > >>
> >> > > > > > >> And with strip-all-gnu it works:
> >> > > > > > >> admin at Netgear-RN212:~/b$ LD_PRELOAD=./libbz2.so.all_gnu ./bzip2
> >> > > > > > >> bzip2: I won't write compressed data to a terminal.
> >> > > > > > >> bzip2: For help, type: `bzip2 --help'.
> >> > > > > > >>
> >> > > > > > >> This only seems to happen on Linux-armv7hf - we haven't seen this on
> >> > > > > > >> any of the intel platforms.
> >> > > > > > >>
> >> > > > > > >> Thanks,
> >> > > > > > >> Tobias
> >> > > > > > >> _______________________________________________
> >> > > > > > >> LLVM Developers mailing list
> >> > > > > > >> llvm-dev at lists.llvm.org
> >> > > > > > >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> > > > > > _______________________________________________
> >> > > > > > LLVM Developers mailing list
> >> > > > > > llvm-dev at lists.llvm.org
> >> > > > > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> _______________________________________________
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