Re: [PATCH v4 1/2] module: add elf_check_module_arch for module specific elf arch checks

[Jessica Yu]

+++ Michael Ellerman [16/06/21 12:37 +1000]:
> Jessica Yu <jeyu@xxxxxxxxxx> writes:
> > +++ Nicholas Piggin [15/06/21 12:05 +1000]:
> > > Excerpts from Jessica Yu's message of June 14, 2021 10:06 pm:
> > > > +++ Nicholas Piggin [11/06/21 19:39 +1000]:
> > > > > The elf_check_arch() function is used to test usermode binaries, but
> > > > > kernel modules may have more specific requirements. powerpc would like
> > > > > to test for ABI version compatibility.
> > > > >
> > > > > Add an arch-overridable function elf_check_module_arch() that defaults
> > > > > to elf_check_arch() and use it in elf_validity_check().
> > > > >
> > > > > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > > > > [np: split patch, added changelog]
> > > > > Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> > > > > ---
> > > > > include/linux/moduleloader.h | 5 +++++
> > > > > kernel/module.c              | 2 +-
> > > > > 2 files changed, 6 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
> > > > > index 9e09d11ffe5b..fdc042a84562 100644
> > > > > --- a/include/linux/moduleloader.h
> > > > > +++ b/include/linux/moduleloader.h
> > > > > @@ -13,6 +13,11 @@
> > > > >  * must be implemented by each architecture.
> > > > >  */
> > > > >
> > > > > +// Allow arch to optionally do additional checking of module ELF header
> > > > > +#ifndef elf_check_module_arch
> > > > > +#define elf_check_module_arch elf_check_arch
> > > > > +#endif
> > > >
> > > > Hi Nicholas,
> > > >
> > > > Why not make elf_check_module_arch() consistent with the other
> > > > arch-specific functions? Please see module_frob_arch_sections(),
> > > > module_{init,exit}_section(), etc in moduleloader.h. That is, they are
> > > > all __weak functions that are overridable by arches. We can maybe make
> > > > elf_check_module_arch() a weak symbol, available for arches to
> > > > override if they want to perform additional elf checks. Then we don't
> > > > have to have this one-off #define.
>
> > > Like this? I like it. Good idea.
> >
> > Yeah! Also, maybe we can alternatively make elf_check_module_arch() a
> > separate check entirely so that the powerpc implementation doesn't
> > have to include that extra elf_check_arch() call. Something like this maybe?
>
> My thinking for making elf_check_module_arch() the only hook was that
> conceivably you might not want/need to call elf_check_arch() from
> elf_check_module_arch().
>
> So having a single module specific hook allows arch code to decide
> how to implement the check, which may or may not involve calling
> elf_check_arch(), but that becomes an arch implementation detail.

Thanks for the feedback! Yeah, that's fair too. Well, I ended up doing
it this way mostly to create less churn/change of behavior, since in
its current state elf_check_arch() is already being called for each
arch. Additionally I wanted to save the powerpc implementation of
elf_check_module_arch() an extra elf_check_arch() call. In any case I
have a slight preference for having a second hook to allow arches add
any extra checks in addition to elf_check_arch(). Thanks!