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

[Peter Zijlstra]

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?

Let me try adding 0s to .symtab_shndx. See if that makes readelf
happier.