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

Thu Oct 17 10:12:24 PDT 2019

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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