Re: module: preserve Elf information for livepatch modules

[Jessica Yu]

+++ Rusty Russell [11/01/16 11:55 +1030]:
> Hi Jessica,
>
>        Nice patch series.  Minor issues below, but they're really
> just nits which can be patched afterwards if you're getting sick of
> rebasing :)

Thanks Rusty!

> > +#ifdef CONFIG_LIVEPATCH
> > +/*
> > + * copy_module_elf - preserve Elf information about a module
> > + *
> > + * Copy relevant Elf information from the load_info struct.
> > + * Note: not all fields from the original load_info are
> > + * copied into mod->info.
>
> This makes me nervous, to have a struct which is half-initialized.
>
> Better would be to have a separate type for this, eg:
>
> struct livepatch_modinfo {
>        Elf_Ehdr hdr;
>        Elf_Shdr *sechdrs;
>        char *secstrings;
>        /* FIXME: Which of these do you need? */
>        struct {
>        	unsigned int sym, str, mod, vers, info, pcpu;
>        } index;
> };
>
> This also avoids an extra allocation as hdr is no longer a ptr.

Sure, sounds good.

> > +	/*
> > +	 * Update symtab's sh_addr to point to a valid
> > +	 * symbol table, as the temporary symtab in module
> > +	 * init memory will be freed
> > +	 */
> > +	mod->info->sechdrs[mod->info->index.sym].sh_addr = (unsigned long)mod->core_symtab;
>
> This comment is a bit misleading: it's actually pointing into the
> temporary module copy, which will be discarded.  The init section is
> slightly different...

Ah, perhaps I'm misunderstanding something..

Since copy_module_elf() is called after move_module(), my
understanding is that all the section sh_addr's should be pointing
to either module core memory or module init memory (instead of the
initial temporary copy of the module in info->hdr). Since the symbol
table section is marked with INIT_OFFSET_MASK, it will reside in
module init memory (and freed near the end of do_init_module()),
which is why I am updating the sh_addr here to point to core_symtab
instead. For livepatch modules, the core_symtab will be a complete
copy of the symbol table instead of the slimmed down version. Please
correct me if my understanding is incorrect.

Thanks for the patch review,
Jessica