linux-kernel - Re: [PATCH] objtool: Fix symbol creation

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Date:   Thu, 19 May 2022 00:10:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.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: [PATCH] objtool: Fix symbol creation

On Wed, May 18, 2022 at 10:36:04AM -0700, Josh Poimboeuf wrote:
> On Wed, May 18, 2022 at 09:41:52AM +0200, Peter Zijlstra wrote:
> > +static int elf_update_symbol(struct elf *elf, struct section *symtab,
> > +			     struct section *symtab_shndx, struct symbol *sym)
> >  {
> > -	Elf_Data *data, *shndx_data = NULL;
> > -	Elf32_Word first_non_local;
> > -	struct symbol *sym;
> > -	Elf_Scn *s;
> > -
> > -	first_non_local = symtab->sh.sh_info;
> > -
> > -	sym = find_symbol_by_index(elf, first_non_local);
> > -	if (!sym) {
> > -		WARN("no non-local symbols !?");
> > -		return first_non_local;
> > -	}
> > +	Elf_Data *symtab_data = NULL, *shndx_data = NULL;
> > +	Elf64_Xword entsize = symtab->sh.sh_entsize;
> > +	Elf32_Word shndx = sym->sec->idx;
> 
> So if it's a global UNDEF symbol then I think 'sym->sec' can be NULL and
> this blows up?

Oh indeed, sym->sec ? sym->sec->idx : SHN_UNDEF it is.

> > +	for (;;) {
> > +		/* get next data descriptor for the relevant sections */
> > +		symtab_data = elf_getdata(s, symtab_data);
> > +		if (t)
> > +			shndx_data = elf_getdata(t, shndx_data);
> > +
> > +		/* end-of-list */
> > +		if (!symtab_data) {
> > +			/* if @idx == 0, it's the next contiguous entry, create it */
> > +			if (!idx) {
> > +				void *buf;
> 
> Could just do the "index out of range warning" here to reduce the
> indentation level.

Sure.

> > +	/* setup extended section index magic and write the symbol */
> > +	if (shndx >= SHN_UNDEF && shndx < SHN_LORESERVE) {
> 
> > +		sym->sym.st_shndx = shndx;
> > +		if (!shndx_data)
> > +			shndx = 0;
> 
> I think this '0' is SHN_UNDEF?
> 
> Also shouldn't 'sym->sym.st_shndx' get the same value?

This is when there isn't an extended section index. Specifically
gelf_update_symshndx() requires that when @shndx_data == NULL, @shndx
must be 0 too.