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

[Fāng-ruì Sòng]

On 2021-06-07, Peter Zijlstra wrote:
> On Mon, Jun 07, 2021 at 11:22:11AM +0200, Peter Zijlstra wrote:
> > On Mon, Jun 07, 2021 at 09:56:48AM +0200, Peter Zijlstra wrote:
> > > On Sat, Jun 05, 2021 at 06:58:39PM -0700, Fāng-ruì Sòng wrote:
> > > > On Sat, Jun 5, 2021 at 3:39 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > > > I think you've absolutely nailed it; but would you have more information
> > > > > or a code reference to what you're speaking about? My complete ELF
> > > > > and libelf knowledge is very limited and as demonstrated here, I'm not
> > > > > at all sure how all that extended index stuff is supposed to work.
> > > >
> > > > The section index field of an Elf{32,64}_Sym (st_shndx) is 16-bit, so
> > > > it cannot represent a section index greater than 0xffff.
> > > > ELF actually reserves values in 0xff00~0xff00 for other purposes, so
> > > > st_shndx cannot represent a section whose index is greater or equal to
> > > > 0xff00.
> > >
> > > Right, that's about as far as I got, but never could find details on how
> > > the extension worked in detail, and I clearly muddled it :/
> >
> > OK, so I'm all confused again...
> >
> > So a .symtab entry has:
> >
> > 	st_name  -- strtab offset for the name string
> > 	st_value -- where this symbol lives
> > 	st_size  -- size of symbol in bytes
> > 	st_shndx -- section index to interpret the @st_value above
> > 	st_info  -- type+bind
> > 	st_other -- visibility
> >
> > The thing is, we're adding UNDEF symbols, for the linker to resolve.
> > UNDEF has:
> >
> > 	st_value := 0
> > 	st_size  := 0
> > 	st_shndx := 0
> > 	st_info  := GLOBAL + NOTYPE
> > 	st_other := 0
> >
> > Per that, sh_shndx isn't >= SHN_LORESERVE, and I figured we all good.
> >
> >
> > Is the problem that .symtab_shndx is expected to contain the exact same
> > number of entries as .symtab? And I'm adding to .symtab and not to
> > .symtab_shndx, hence getting them out of sync?

Yes. http://www.sco.com/developers/gabi/latest/ch4.sheader.html says
"Each value corresponds one to one with a symbol table entry and appear in the same order as those entries."

> > Let me try adding 0s to .symtab_shndx. See if that makes readelf
> > happier.
>
> That does indeed seem to do the trick. Bit daft if you ask me, anybody
> reading that file ought to have a handy bucket of 0s available, but
> whatever.

Does the representation use the section index directly? (sym->sym.st_shndx)
This can be fragile when the number of sections changes..., e.g. elf_add_section

So in llvm-objcopy's representation, the section index is represented as
the section object.

struct Symbol {
  ...
  SectionBase *DefinedIn = nullptr;
  ...
};

In the writer stage, sections are assigned 32-bit indexes and the writer
knows that an SHN_XINDEX for a symbol is needed if the index is >= 0xff00.

> ---
> 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);