Re: [PATCH] x86: work around clang IAS bug referencing __force_order

[Sedat Dilek]

On Sat, Aug 15, 2020 at 10:23 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
>
> On Sat, Aug 15, 2020 at 5:28 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> >
> > On Sat, Aug 15, 2020 at 2:27 AM Nick Desaulniers
> > <ndesaulniers@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Aug 14, 2020 at 3:57 PM Nick Desaulniers
> > > <ndesaulniers@xxxxxxxxxx> wrote:
> > > >
> > > > On Fri, Aug 14, 2020 at 2:19 PM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> > > > >
> > > > > On Fri, Aug 14, 2020 at 7:29 PM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> > > > > >
> > > > > > Thanks for the proposal.
> > > > > >
> > > > > > I have adapted it to fit my patchset against Linux v5.8.
> > > > > >
> > > > > > Both Debian's GCC-10 and a snapshot version of LLVM toolchain
> > > > > > v11.0.0-rc1+ seems to be OK.
> > > > > >
> > > > >
> > > > > Yupp, OK.
> > > > >
> > > > > I was able to boot FreeDOS 1.2 VM in VirtualBox GUI.
> > > >
> > > > Hi Sedat,
> > > > Apologies, but it's not clear to me precisely which patch you tested.
> > > > Can you please confirm whether you tested:
> > > > 1. Arnd's patch that started this thread.
> > > > 2. My proposed diff adding -fno-addrsig to CFLAGS_powernow-k6.o.
> > > > 3. My proposed diff removing __force_order from the kernel.
> > > >
> > > > I'm hoping you were referring to testing 3., but it's not clear to me.
> > >
> > > Ah, sorry, I missed your comment on github:
> > > https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674282107
> > >
> > > Ok, I will look at more disassembly next week and hopefully have a
> > > patch ready, with your tested by tag.
> > >
> >
> > Sorry for not being precise - I tested with solution (3.).
> > Later I added the diff I used as mentioned in your above comment.
> >
> > See [1]:
> >
> > > In a 2nd run building with a selfmade clang-11 and LLVM "bin"utils is fine, too.
> >
> > I cannot say much to older versions of GCC and/or LLVM/Clang if
> > removing "__force_order" works fine.
> >
> > Another (4.) solution:
> > Sami tried successfully by adding "__weak" declaration with
> > CONFIG_LKDTM=m (see [2]).
> > I am OK if this works, too.
> >
> > Please, see my attachments.
> >
> > - Sedat -
> >
> > [1] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674340760
> > [2] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674182703
>
> Unfortunately, the diff from Sami does not work together with Arvind's
> patchset...
>
> x86/boot: Remove run-time relocations from compressed kernel
>
> ...which got included in <tip.git#x86/boot> recently.
>
> I see the following:
>
>   ld.lld-11 -m elf_x86_64  -pie  --no-dynamic-linker -T
> arch/x86/boot/compressed/vmlinux.lds
> arch/x86/boot/compressed/kernel_info.o
> arch/x86/boot/compressed/head_64.o arch/x86/boot/compressed/misc.o
> arch/x86/boot/compressed/string.o arch/x86/boot/compressed/cmdline.o
> arch/x86/boot/compressed/error.o arch/x86/boot/compressed/piggy.o
> arch/x86/boot/compressed/cpuflags.o
> arch/x86/boot/compressed/early_serial_console.o
> arch/x86/boot/compressed/kaslr.o arch/x86/boot/compressed/kaslr_64.o
> arch/x86/boot/compressed/mem_encrypt.o
> arch/x86/boot/compressed/pgtable_64.o arch/x86/boot/compressed/acpi.o
> arch/x86/boot/compressed/efi_thunk_64.o
> drivers/firmware/efi/libstub/lib.a -o arch/x86/boot/compressed/vmlinux
> ld.lld-11: error: Unexpected GOT entries detected!
> ld.lld-11: error: Unexpected run-time relocations detected!
> ld.lld-11: error: Unexpected GOT entries detected!
> ld.lld-11: error: Unexpected run-time relocations detected!
> make[5]: *** [arch/x86/boot/compressed/Makefile:91:
> arch/x86/boot/compressed/vmlinux] Error 1
>
> When you need further informations, please let me know.
>
> - Sedat -
>
> [1] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674182703

When I revert...

commit df6d4f9db79c1a5d6f48b59db35ccd1e9ff9adfc
"x86/boot/compressed: Don't declare __force_order in kaslr_64.c"

...I can build, boot on bare metal and start FreeDOS VM in VirtualBox.

For more details see [2].

- Sedat -

[1] https://git.kernel.org/linus/df6d4f9db79c1a5d6f48b59db35ccd1e9ff9adfc
[2] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674378085