Messages in this thread | | | Subject | Re: [PATCH v4 2/2] ARM: ftrace: Add MODULE_PLTS support | From | Alexander Sverdlin <> | Date | Tue, 20 Mar 2018 13:28:45 +0100 |
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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.
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