Hello Ard,
On 13/03/18 18:32, Alexander Sverdlin wrote:
>>>>> u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val)
>>>>> {
>>>>> struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core
>>>>> :
>>>>> &mod->arch.init;
>>>>> + struct plt_entries *plt;
>>>>> + int idx;
>>>>>
>>>>> - struct plt_entries *plt = (struct plt_entries
>>>>> *)pltsec->plt->sh_addr;
>>> ^^^^^^^^^^^ (*)
>>>
>>>>> - int idx = 0;
>>>>> + /* cache the address, ELF header is available only during module
>>>>> load */
>>>>> + if (!pltsec->plt_ent)
>>>>> + pltsec->plt_ent = (struct plt_entries
>>>>> *)pltsec->plt->sh_addr;
>>>>> + plt = pltsec->plt_ent;
>>>>> +
>>>> Where is plt_ent ever used?
>>> Above is exactly the place it's used.
>>> I need to cache it because after the module load is finished the ELF header
>>> is freed,
>>> pltsec->plt pointer (*) is not valid any more.
>>> With the above modification it's possible to call the function during the
>>> whole life
>>> time of the module.
>>>
>> Right, ok. That's a problem.
>>
>> This means that you are relying on get_module_plt() being called at
>> least once at module load time, which is not guaranteed.
> This is indeed guaranteed. For FTRACE use case. If it's being called from
> FTRACE in
> run time, this would mean there were long calls in this module section, which
> in
> turn means, get_module_plt() was called at least once for this module and this
> section.
>
> This doesn't hold in general, though.
>
> In any case, if you insist, I can try to rework the whole stuff implementing
> module_finalize().
now when I have a new implementation via module_finalize(), I must admit it's
not possible to
do it sanely this way.
module_finalize() can only add entries at the end of PLT, which means, they
will be different
from the entries module loader/relocator has created before, which means,
FTRACE will not
be able to replace these entries with NOPs.
As I don't want to do O(N) search on every dynamic ftrace operation, seems this
is not an
option.
Either v4 has to be accepted, or I cannot propose a solution for upstream
FTRACE+MODULES_PLT
combination.
--
Best regards,
Alexander Sverdlin.
