Re: [PATCH] x86/boot: allow a relocatable kernel to be linked with lld

[Nick Desaulniers]

On Sun, May 17, 2020 at 1:25 PM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> On Sun, May 17, 2020 at 12:44:29PM -0700, Fangrui Song wrote:
> > On 2020-05-16, Dmitry Golovin wrote:
> > >15.05.2020, 21:50, "Borislav Petkov" <bp@xxxxxxxxx>:
> > >>
> > >> I need more info here about which segment is read-only?
> > >>
> > >> Is this something LLD does by default or what's happening?
> > >>
> > >
> > >Probably should have quoted the original error message:
> > >
> > >    ld.lld: error: can't create dynamic relocation R_386_32 against
> > >    symbol: _bss in readonly segment; recompile object files with -fPIC
> > >    or pass '-Wl,-z,notext' to allow text relocations in the output
> >
> > Do we know where do these R_386_32 come from?
> >
> > When linking in -shared mode, the linker assumes the image is a shared
> > object and has undetermined image base at runtime. An absolute
> > relocation needs a text relocation (a relocation against a readonly
> > segment).
> >
> > When neither -z notext nor -z text is specified, GNU ld is in an
> > indefinite state where it will enable text relocations (DT_TEXTREL
> > DF_TEXTREL) on demand. It is not considered a good practice for
> > userspace applications to do this.
> >
> > Of course the kernel is different....... I know little about the kernel,
> > but if there is a way to make the sections containing R_386_32
> > relocations writable (SHF_WRITE), that will be a better solution to me.
> > In LLD, -z notext is like making every section SHF_WRITE.
>
> The assembly files head_32.S and head_64.S in arch/x86/boot/compressed
> create bogus relocations in .head.text and .text.
>
> This is the source of the common warning when using the bfd linker, for
> eg on 64-bit:
>   LD      arch/x86/boot/compressed/vmlinux
>   ld: arch/x86/boot/compressed/head_64.o: warning: relocation in read-only section `.head.text'
>   ld: warning: creating a DT_TEXTREL in object
>
> These relocations are "bogus", i.e. they're unwanted and the kernel
> won't actually boot if anything were to perform those relocations before
> running it. They're also the cause of the 64-bit kernel requiring linker
> support for the -z noreloc-overflow option to link it as PIE.
>
> From arch/x86/boot/compressed/Makefile:
>
> # To build 64-bit compressed kernel as PIE, we disable relocation
> # overflow check to avoid relocation overflow error with a new linker
> # command-line option, -z noreloc-overflow.
> KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z
> noreloc-overflow" \
>         && echo "-z noreloc-overflow -pie --no-dynamic-linker")
>
> The relocations come from code like
>         leal    gdt(%ebp), %eax
> which should really be
>         leal    (gdt-startup_32)(%ebp), %eax
> to be technically correct.
>
> I've played around with fixing the head code to avoid creating the
> relocations in the first place, but never got around to submitting
> anything: if there is interest in this, I can polish it up and send it
> around.

We'd be happy to help test and review. :)
-- 
Thanks,
~Nick Desaulniers