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

Fri Oct 18 11:41:39 PDT 2019

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
>> > > > > > _______________________________________________
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