Re: [PATCH v2] scripts: Do not strip .rela.dyn section

[Alexandre Ghiti]

Hi Ard,

On 03/04/2025 17:11, Ard Biesheuvel wrote:
> On Thu, 3 Apr 2025 at 16:42, Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> > riscv uses the .rela.dyn section to relocate the kernel at runtime but
> > that section is stripped from vmlinux. That prevents kexec to
> > successfully load vmlinux since it does not contain the relocations info
> > needed.
> Maybe explain that .rela.dyn contains runtime relocations, which are
> only emitted if they are actually needed - as opposed to the static
> relocations that are not emitted as SHF_ALLOC sections, and are not
> considered to be part of the runtime image in the first place.


Ok I'll do.


> It
> would be nice if we could use --remove-relocations= here, which only
> removes static relocations, but unfortunately, llvm-objcopy does not
> support this.
>
> Also, I wonder if this should apply to all of .rel.dyn, .rela.dyn and
> .relr.dyn, as they all carry runtime relocations.


Ok, I'll add them to the next version.


> Finally, I'd be curious to know why RISC-V relies on --emit-relocs in
> the first place? Is the relocs check really needed? If not, it would
> be a nice opportunity to get rid of Makefile.postlink entirely.


So I had to check and it happens that this was an issue with the 
toolchain, I should check if that still happens with newer ones.

commit 559d1e45a16dcf1542e430ea3dce9ab625be98d0
Author: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
Date:   Wed Mar 29 06:53:29 2023 +0200

    riscv: Use --emit-relocs in order to move .rela.dyn in init

    To circumvent an issue where placing the relocations inside the init
    sections produces empty relocations, use --emit-relocs. But to avoid
    carrying those relocations in vmlinux, use an intermediate
    vmlinux.relocs file which is a copy of vmlinux *before* stripping its
    relocations.

    Suggested-by: Björn Töpel <bjorn@xxxxxxxxxx>
    Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
    Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
    Link: 
https://lore.kernel.org/r/20230329045329.64565-7-alexghiti@xxxxxxxxxxxx
    Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>


> In any case, for this change, or a variation along the lines of what I
> wrote above,
>
> Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>


Thanks,

Alex


> > Fixes: 71d815bf5dfd ("kbuild: Strip runtime const RELA sections correctly")
> > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > ---
> >   scripts/Makefile.lib | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> > index cad20f0e66ee..0a1f1e67a0ed 100644
> > --- a/scripts/Makefile.lib
> > +++ b/scripts/Makefile.lib
> > @@ -377,7 +377,7 @@ quiet_cmd_objcopy = OBJCOPY $@
> >   cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@
> >
> >   quiet_cmd_strip_relocs = RSTRIP  $@
> > -cmd_strip_relocs = $(OBJCOPY) --remove-section='.rel*' $@
> > +cmd_strip_relocs = $(OBJCOPY) --remove-section='.rel*' --remove-section=!.rela.dyn $@
> >
> >   # Gzip
> >   # ---------------------------------------------------------------------------
> > --
> > 2.39.2
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