Re: [PATCH] arm64/module-plts: Consider the special case where plt_max_entries is 0

[Will Deacon]

On Tue, Jul 14, 2020 at 08:48:11AM +0000, 彭浩(Richard) wrote:
> On Thu, Jul 09, 2020 at 07:18:01AM +0000,Peng Hao(Richard) wrote:
> > On Thu, 9 Jul 2020 at 09:50, Peng Hao(Richard) <richard.peng@xxxxxxxx> wrote:
> > >> >Apparently, you are hitting a R_AARCH64_JUMP26 or R_AARCH64_CALL26
> > >> >relocation that operates on a b or bl instruction that is more than
> > >> >128 megabytes away from its target.
> > >> >
> > >> My understanding is that a module that calls functions that are not part of the module will use PLT.
> > >> Plt_max_entries =0 May occur if a module does not depend on other module functions.
> > >>
> > >
> > >A PLT slot is allocated for each b or bl instruction that refers to a
> > >symbol that lives in a different section, either of the same module
> > > (e.g., bl in .init calling into .text), of another module, or of the
> > >core kernel.
> > >
> > >I don't see how you end up with plt_max_entries in this case, though.
> > if a module does not depend on other module functions, PLT entries in the module is equal to 0.
> 
> >This brings me back to my earlier question: if there are no PLT entries in
> >the module, then count_plts() will not find any R_AARCH64_JUMP26 or
> >R_AARCH64_CALL26 relocations that require PLTs and will therefore return 0.
> >The absence of these relocations means that module_emit_plt_entry() will not
> >be called by apply_relocate_add(), and so your patch should have no effect.
> 1.The module in question is the calling function from core kernel.( Ib_core.ko triggered the warning multiple times).
> 2. There are multiple threads loading IB_core.ko
> [   73.388931]  ###cpu=33, name=ib_core, core_plts=0, init_plts=0  
> [   73.402102]  #### cpu=33,pid=2297,name=ib_core, module_emit_plt_entry:plt_num_entries=1, plt_max_entries=0 (warning)
> [   73.439391]  ###cpu=24, name=ib_core, core_plts=0, init_plts=0  
> [   73.448617]  ###cpu=4, name=ib_core, core_plts=0, init_plts=0  
> [   73.547535]  ###cpu=221, name=ib_core, core_plts=0, init_plts=0  
> [   75.198075]  #### cpu=24,pid=2336,name=ib_core, module_emit_plt_entry:plt_num_entries=1, plt_max_entries=0 (warning)
> [   75.489496]  #### cpu=4,pid=2344,name=ib_core, module_emit_plt_entry:plt_num_entries=1, plt_max_entries=0(warning)
> 
> I don't understand why count_plts returns 0 when CONFIG_RANDOMIZE_BASE=n for R_AARCH64_JUMP26 and R_AARCH64_CALL26.
> 
> 3. Set CONFIG_ARM64_MODULE_PLTS=y and restart the server several times without triggering this warning.

Can you provide a means for us to reproduce this failure with an upstream
kernel, please? I really can't tell what's going on from the report. If I
can reproduce the problem locally, then I'm happy to take a look.

Thanks,

Will