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

Joao Moreira
2019-03-26T20:14:07Z

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
>