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

From: Ard Biesheuvel
Date: Tue Mar 13 2018 - 13:18:52 EST


On 13 March 2018 at 17:13, Alexander Sverdlin
<alexander.sverdlin@xxxxxxxxx> wrote:
> Hello Ard,
>
> On 13/03/18 17:12, 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.

Instead, you should set this member (and perhaps the entire prealloc
routine) in a module_finalize() implementation.

>>> + if (!pltsec->plt_count)
>>> + prealloc_fixed(pltsec, plt);
>
> I'll prepare v5 based on your other comments.
>
> --
> Best regards,
> Alexander Sverdlin.