Re: [PATCH] recordmcount: support >64k sections

[Sami Tolvanen <samitolvanen@google.com>]

Hi Matt,

On Thu, Apr 23, 2020 at 02:47:34PM -0700, Matt Helsley wrote:
> > +static unsigned int get_shnum(Elf_Ehdr const *ehdr, Elf_Shdr const *shdr0)
> 
> I noticed this returns an unsigned int ...
> 
> > +	Elf_Shdr *const shdr0 = (Elf_Shdr *)(old_shoff + (void *)ehdr);
> > +	unsigned const old_shnum = get_shnum(ehdr, shdr0);
> 
> While this is not explicitly called out as an unsigned int. Perhaps we
> could just make this and new_shnum explicit unsigned ints and then...

> > +	if (!ehdr->e_shnum || new_shnum >= SHN_LORESERVE) {
> > +		ehdr->e_shnum = 0;
> > +		shdr0->sh_size = w(new_shnum);
> > +	} else
> > +		ehdr->e_shnum = w2(2 + w2(ehdr->e_shnum));
> 
> If we make the unsigned int change proposed above can we reuse new_shnum
> here like so:
> 		ehdr->e_shnum = w2(new_shnum);
> 
> So this if/else is doing the inverse of get_shnum(). I think the code
> could be cleaned up a little and prepare for moving to objtool by
> putting it in a helper function.

Sure, sounds good to me.

> > +	for (relhdr = shdr0, k = nhdr; k; --k, ++relhdr) {
> > +		if (relhdr->sh_type == SHT_SYMTAB)
> > +			symtab = (void *)ehdr + relhdr->sh_offset;
> > +		else if (relhdr->sh_type == SHT_SYMTAB_SHNDX)
> > +			symtab_shndx = (void *)ehdr + relhdr->sh_offset;
> > +
> > +		if (symtab && symtab_shndx)
> > +			break;
> > +	}
> 
> Could you break this out into a helper function? find_symtab() maybe? Again, I think
> that helper would go away with conversion to objtool.

Agreed, this wouldn't be needed with libelf. I'll send v2 shortly.
Thanks for the review!

Sami