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Date:   Wed, 16 Jun 2021 11:18:44 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v4 1/2] module: add elf_check_module_arch for module
 specific elf arch checks
To:     Jessica Yu <jeyu@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michal =?iso-8859-1?q?Such=E1nek?= <msuchanek@suse.de>
References: <20210611093959.821525-1-npiggin@gmail.com>
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        <YMdGWjBOmcstBwOl@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de>
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Excerpts from Jessica Yu's message of June 15, 2021 10:17 pm:
> +++ 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@ellerman.id.au>
>>>>[np: split patch, added changelog]
>>>>Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>>>>---
>>>> 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 heade=
r
>>>>+#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.
>=20
> 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 may=
be?

Yeah we can do that. Would you be okay if it goes via powerpc tree? If=20
yes, then we should get your Ack (or SOB because it seems to be entirely=20
your patch now :D)

Thanks,
Nick

>=20
> 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);
>=20
> +/* Arch may override to do additional checking of ELF header architectur=
e */
> +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_inf=
o *info, Elf_Shdr *shdr)
>          return 0;
>   }
>=20
> +int __weak elf_check_module_arch(Elf_Ehdr *hdr)
> +{
> +       return 1;
> +}
> +
>   /*
>    * Sanity checks against invalid binaries, wrong arch, weird elf versio=
n.
>    *
> @@ -2941,6 +2946,7 @@ static int elf_validity_check(struct load_info *inf=
o)
>          if (memcmp(info->hdr->e_ident, ELFMAG, SELFMAG) !=3D 0
>              || info->hdr->e_type !=3D ET_REL
>              || !elf_check_arch(info->hdr)
> +           || !elf_check_module_arch(info->hdr)
>              || info->hdr->e_shentsize !=3D sizeof(Elf_Shdr))
>                  return -ENOEXEC;
>  =20
>=20
>=20