linux-kernel - Re: [PATCH v4 1/2] module: add elf_check_module

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Message-ID: <YMoBgsVPMkzEAQxV@linux.fritz.box>
Date:   Wed, 16 Jun 2021 15:49:54 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Michal Suchánek <msuchanek@...e.de>
Subject: Re: [PATCH v4 1/2] module: add elf_check_module_arch for module
 specific elf arch checks

+++ Michael Ellerman [16/06/21 12:37 +1000]:
>Jessica Yu <jeyu@...nel.org> 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@...erman.id.au>
>>>>>[np: split patch, added changelog]
>>>>>Signed-off-by: Nicholas Piggin <npiggin@...il.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 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!