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

[Jessica Yu]

+++ 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?

diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 9e09d11ffe5b..2f9ebd593b4f 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h
@@ -39,6 +39,9 @@ bool module_init_section(const char *name);
  */
 bool module_exit_section(const char *name);

+/* Arch may override to do additional checking of ELF header architecture */
+int elf_check_module_arch(Elf_Ehdr *hdr);
+
 /*
  * Apply the given relocation to the (simplified) ELF.  Return -error
  * or 0.
diff --git a/kernel/module.c b/kernel/module.c
index fdd6047728df..9963a979ed54 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2923,6 +2923,11 @@ static int validate_section_offset(struct load_info *info, Elf_Shdr *shdr)
        return 0;
 }

+int __weak elf_check_module_arch(Elf_Ehdr *hdr)
+{
+       return 1;
+}
+
 /*
  * Sanity checks against invalid binaries, wrong arch, weird elf version.
  *
@@ -2941,6 +2946,7 @@ static int elf_validity_check(struct load_info *info)
        if (memcmp(info->hdr->e_ident, ELFMAG, SELFMAG) != 0
            || info->hdr->e_type != ET_REL
            || !elf_check_arch(info->hdr)
+           || !elf_check_module_arch(info->hdr)
            || info->hdr->e_shentsize != sizeof(Elf_Shdr))
                return -ENOEXEC;