Re: [PATCH v2 3/8] livepatch: Add klp-convert tool

[Joao Moreira]

On 3/20/19 4:36 PM, Miroslav Benes wrote:
> > > [ ... snip ... ]
> > > +
> > > +/* Checks if sympos is valid, otherwise prints valid sympos list */
> > > +static bool valid_sympos(struct sympos *sp)
> > > +{
> > > +	struct symbol_entry *e;
> > > +	int counter = 0;
> > > +
> > > +	list_for_each_entry(e, &symbols, list) {
> > > +		if ((strcmp(e->symbol_name, sp->symbol_name) == 0) &&
> > > +		    (strcmp(e->object_name, sp->object_name) == 0)) {
> > > +			if (counter == sp->pos)
> > > +				return true;
> > > +			counter++;
> > > +		}
> > > +	}
> > > +
> > > +	WARN("Provided KLP_SYMPOS does not match a symbol: %s.%s,%d",
> > > +			sp->object_name, sp->symbol_name, sp->pos);
> > > +	print_valid_module_relocs(sp->symbol_name);
> > > +
> > > +	return false;
> > > +}
> >
> > I believe there is an off-by-one error condition either here, or in
> > livepatch kernel core sympos disambiguator code (in which case, external
> > tools like kpatch-build would also need to be adjusted).
> >
> > The scenarios that I've observed using klp-convert and kpatch-build:
> >
> >    sympos == 0, uniquely named symbol
> >    sympos == 1, non-unique symbol name, first instance
> >    sympos == 2, non-unique symbol name, second instance
> >    ...
>
> Yes, that is exactly how we defined it back then.
>   
> > When I built a klp-convert selftest, I made sure that the target module
> > contained multiple symbols of the same name across two .o files.
> > However, when I tried to use a KLP_MODULE_RELOC annotation in the
> > livepatch to resolve to the second (ie, last) symbol, using a sympos=2,
> > klp-convert kept telling me that the only valid KLP_SYMPOS were 0 and 1.
> >
> > The following code adjusts klp-convert to match the sympos logic, as I
> > understand it in livepatch/core.c and kpatch-build;
> >
> >    [squash] livepatch/klp-convert: fix klp-convert off-by-one sympos
> >    https://github.com/torvalds/linux/commit/1ed8e5baa98f7920bbbaa562278b3ed44552e01f
> >
> > This change also adds a check that sympos == 0 is in fact a uniquely
> > named symbol.

Nice catch, thanks for it. All looks good to me.

> Looks good to me. Maybe we can even make it simpler and use similar
> approach as in klp_find_callback() and klp_find_object_symbol() from
> kernel/livepatch/core.c. On the other hand, given that we want to print
> useful output in all cases, the code might happen to be more complicated
> and definitely ugly.

I don't have an opinion here.

>   
> > > [ ... snip ... ]
> > > +
> > > +int main(int argc, const char **argv)
> > > +{
> > > +	const char *klp_in_module, *klp_out_module, *symbols_list;
> > > +	struct rela *rela, *tmprela;
> > > +	struct section *sec, *aux;
> > > +	struct sympos sp;
> > > +	struct elf *klp_elf;
> > > +
> > > +	if (argc != 4) {
> > > +		WARN("Usage: %s <Symbols.list> <input.ko> <out.ko>", argv[0]);
> > > +		return -1;
> > > +	}
> > > +
> > > +	symbols_list = argv[1];
> > > +	klp_in_module = argv[2];
> > > +	klp_out_module = argv[3];
> > > +
> > > +	klp_elf = elf_open(klp_in_module);
> > > +	if (!klp_elf) {
> > > +		WARN("Unable to read elf file %s\n", klp_in_module);
> > > +		return -1;
> > > +	}
> > > +
> > > +	if (!load_syms_lists(symbols_list))
> > > +		return -1;
> > > +
> > > +	if (!load_usr_symbols(klp_elf)) {
> > > +		WARN("Unable to load user-provided sympos");
> > > +		return -1;
> > > +	}
> > > +
> > > +	list_for_each_entry_safe(sec, aux, &klp_elf->sections, list) {
> > > +		if (!is_rela_section(sec))
> > > +			continue;
> > > +
> > > +		list_for_each_entry_safe(rela, tmprela, &sec->relas, list) {
> > > +			if (!must_convert(rela->sym))
> > > +				continue;
> > > +
> > > +			if (!find_missing_position(rela->sym, &sp)) {
> > > +				WARN("Unable to find missing symbol");
> >
> > A really minor suggestion, but I found it useful to tell the user
> > exactly which symbol was missing:
> >
> >    [squash] livepatch/klp-convert: add missing symbol name to warning
> >    https://github.com/torvalds/linux/commit/7a41a8f3d9b9e11f0c75914cb925af1486c1ecc6
>
> I think that Joao has exactly the same hunk staged somewhere
> already. You are right. The message is not much informative.

Yes, someone had the same idea for the klp-convert which we use inside 
SUSE. But it did not make it to this patch-set.

Nice that Joe added it here :)

> Miroslav