linux-kernel - Re: objtool "no non-local symbols" error with tip of tree LLVM

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Date:   Wed, 18 May 2022 10:14:56 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com
Subject: Re: objtool "no non-local symbols" error with tip of tree LLVM

On Wed, May 18, 2022 at 09:17:27AM -0700, Josh Poimboeuf wrote:
> On Wed, May 18, 2022 at 07:30:06AM +0200, Peter Zijlstra wrote:
> > On Tue, May 17, 2022 at 06:24:29PM -0700, Josh Poimboeuf wrote:
> > > On Tue, May 17, 2022 at 05:42:04PM +0200, Peter Zijlstra wrote:
> > > > +	for (;;) {
> > > > +		symtab_data = elf_getdata(s, symtab_data);
> > > > +		if (t)
> > > > +			shndx_data = elf_getdata(t, shndx_data);
> > > >  
> > > > +		if (!symtab_data) {
> > > > +			if (!idx) {
> > > > +				void *buf;
> > > 
> > > I'm confused by whatever this is doing, how is !symtab_data possible,
> > > i.e. why would symtab not have data?
> > 
> > Elf_Data *elf_getdata(Elf_Scn *scn, Elf_Data *data);
> > 
> > is an iterator, if @data is null it will return the first element, which
> > you then feed into @data the next time to get the next element, once it
> > returns NULL, you've found the end.
> > 
> > In our specific case, we iterate the data sections, if idx fits inside
> > the current section, we good, otherwise we lower idx by however many did
> > fit and try the next.
> 
> Ok, I think I see.  But why are there multiple data blocks to begin
> with?  It's because of a previous call to elf_newdata() right?
> 
> If so then I don't see how it would "fit" in an existing data block,
> since each block should already be full.  Or... is the hole the one you
> just made, by moving the old symbol out?
> 
> If so, the function seems weirdly generalized for the two distinct cases
> and the loop seems unnecessary.  When adding a symbol at the end, just
> use elf_newdata().  When adding a symbol in the middle, the hole should
> be in the first data block.

Then I went for a bike ride and realized that if adding enough section
symbols to a file which didn't have very many non-locals, the hole might
occur in a later data block.

So yeah, this looks fine :-)

Another idea I had was to forego elf_newdata() entirely in favor of just
rewriting the original data block every time.  But this is also fine.

-- 
Josh