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 [off-list ref] writes:

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+++ Nicholas Piggin [15/06/21 12:05 +1000]:

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Excerpts from Jessica Yu's message of June 14, 2021 10:06 pm:

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+++ Nicholas Piggin [11/06/21 19:39 +1000]:

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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.

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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!