Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

[Nathan Chancellor]

On 6/7/2021 1:54 PM, 'Nick Desaulniers' via Clang Built Linux wrote:
> On Mon, Jun 7, 2021 at 2:46 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
>
> Thanks, the below diff resolves the linker error reported in
> https://github.com/ClangBuiltLinux/linux/issues/1388
>
> Both readelf implementations seem happy with the results, too.
>
> Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>
> Nathan,
> Can you please test the below diff and see if that resolves your boot
> issue reported in:
> https://github.com/ClangBuiltLinux/linux/issues/1384

Unfortunately, it does not appear to resolve that issue.

$ git log -2 --decorate=no --oneline
eea6a9d6d277 Peter's fix
614124bea77e Linux 5.13-rc5

$ strings /mnt/c/Users/natec/Linux/kernel-investigation | grep microsoft
5.13.0-rc5-microsoft-standard-WSL2-00001-geea6a9d6d277 
(nathan@archlinux-ax161) #3 SMP Tue Jun 8 09:46:19 MST 2021

My VM still never makes it to userspace.

> > ---
> >   tools/objtool/elf.c | 25 ++++++++++++++++++++++++-
> >   1 file changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> > index 743c2e9d0f56..41bca1d13d8e 100644
> > --- a/tools/objtool/elf.c
> > +++ b/tools/objtool/elf.c
> > @@ -717,7 +717,7 @@ static int elf_add_string(struct elf *elf, struct section *strtab, char *str)
> >
> >   struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
> >   {
> > -       struct section *symtab;
> > +       struct section *symtab, *symtab_shndx;
> >          struct symbol *sym;
> >          Elf_Data *data;
> >          Elf_Scn *s;
> > @@ -769,6 +769,29 @@ struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
> >          symtab->len += data->d_size;
> >          symtab->changed = true;
> >
> > +       symtab_shndx = find_section_by_name(elf, ".symtab_shndx");
> > +       if (symtab_shndx) {
> > +               s = elf_getscn(elf->elf, symtab_shndx->idx);
> > +               if (!s) {
> > +                       WARN_ELF("elf_getscn");
> > +                       return NULL;
> > +               }
> > +
> > +               data = elf_newdata(s);
> > +               if (!data) {
> > +                       WARN_ELF("elf_newdata");
> > +                       return NULL;
> > +               }
> > +
> > +               data->d_buf = &sym->sym.st_size; /* conveniently 0 */
> > +               data->d_size = sizeof(Elf32_Word);
> > +               data->d_align = 4;
> > +               data->d_type = ELF_T_WORD;
> > +
> > +               symtab_shndx->len += 4;
> > +               symtab_shndx->changed = true;
> > +       }
> > +
> >          sym->sec = find_section_by_index(elf, 0);
> >
> >          elf_add_symbol(elf, sym);
>
>
> The only thing that's still different is that the `file` command still
> prints "no section header."
>
> $ find . -name \*.lto.o | xargs file | rev | cut -d , -f 1 | rev |
> sort | uniq -c
>        1  no section header
>     8377  not stripped
>        1  too many section headers (33683)
>        1  too many section headers (50758)
> $ file --version
> file-5.39
>
> That's drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o, fs/xfs/xfs.lto.o,
> drivers/gpu/drm/i915/i915.lto.o, respectively.  I'm not sure that's a
> problem, yet, and whether 9bc0bb50727c8ac69fbb33fb937431cf3518ff37 is
> even related yet; those might just be huge drivers and figured it was
> reporting somewhere in case it ever comes up again.  CONFIG_LTO
> implies -ffunction-sections -fdata-sections, and
> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION explicitly sets those, too.