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

Miroslav Benes
2019-03-20T19:37:02Z

> > [ ... 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.

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.

> > [ ... 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.

Miroslav