Re: [PATCH v4 2/2] ARM: ftrace: Add MODULE_PLTS support

[Ard Biesheuvel]

On 13 March 2018 at 17:32, Alexander Sverdlin
<alexander.sverdlin@xxxxxxxxx> wrote:
> Hi Ard!
>
> On 13/03/18 18:18, Ard Biesheuvel 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().
>

I think it would be much better to use the module_finalize() hook for
this, given that it is only called once already, and all the data you
need is still available.

Note that ARM already has such a function, so you'll need to add a
call there, i.e.,

if (IS_ENABLED(CONFIG_ARM_MODULE_PLTS))
    module_plt_alloc_fixed();

or something like that. The CONFIG_FTRACE dependency can be kept local
to module-plts.c