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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Jessica Yu <jeyu@kernel.org>, Nicholas Piggin <npiggin@gmail.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Michal =?utf-8?Q?Such=C3=A1nek?= <msuchanek@suse.de>
Subject: Re: [PATCH v4 1/2] module: add elf_check_module_arch for module
 specific elf arch checks
In-Reply-To: <YMiaZOqhHck9iy0n@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de>
References: <20210611093959.821525-1-npiggin@gmail.com>
 <20210611093959.821525-2-npiggin@gmail.com>
 <YMdGWjBOmcstBwOl@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de>
 <1623722110.amu32mwaqs.astroid@bobo.none>
 <YMiaZOqhHck9iy0n@p200300cbcf109700df096d564fe976c3.dip0.t-ipconnect.de>
Date:   Wed, 16 Jun 2021 12:37:12 +1000
Message-ID: <87v96esffr.fsf@mpe.ellerman.id.au>
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Jessica Yu <jeyu@kernel.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@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 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.

It's also one arch hook instead of two (although elf_check_arch()
already exists).

But I don't feel that strongly either way, whatever you prefer.

cheers