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Subject: Re: [PATCH v4 2/2] ARM: ftrace: Add MODULE_PLTS support
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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From:   Alexander Sverdlin <alexander.sverdlin@nokia.com>
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Date:   Tue, 13 Mar 2018 19:28:15 +0100
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Hi!

On 13/03/18 18:39, 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.

No problem, but some kind of (*) block would still be required, because
get_module_plt() has to work after module_finalize() as well as *before* it.
So before module_finalize() we would have to dereference pltsec->plt->sh_addr conditionally.
 
> 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
> 

-- 
Best regards,
Alexander Sverdlin.