linux-kernel - Re: [PATCH 0/3] modpost: work around unaligned data access

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Message-Id: <20241225162656.26005-1-glaubitz@physik.fu-berlin.de>
Date: Wed, 25 Dec 2024 17:26:56 +0100
From: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To: masahiroy@...nel.org
Cc: gregkh@...e.de,
	linux-arm-kernel@...ts.infradead.org,
	linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	nathan@...nel.org,
	nicolas@...sle.eu,
	rusty@...tcorp.com.au,
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Subject: Re: [PATCH 0/3] modpost: work around unaligned data access

Hi Masahiro,

> The latest binutils stopped aligning section data in relocatable ELF.
> It saves small number of bytes that were previously inserted between
> sections. However, modpost crashes due to unaligned access:
>   https://sourceware.org/bugzilla/show_bug.cgi?id=32435
>   https://sourceware.org/bugzilla/show_bug.cgi?id=32493
>
> Similar to kernel space, unaligned data access in userspace can be
> problematic on some architectures.
>
> Simple example on ARM:
>
>  $ CC=arm-linux-gnueabihf-gcc
>  $ echo 'int foo(int *p) { return *p + *(p + 1); }' | ${CC} -O2 -x c - -c -o foo.o
>  $ echo 'int foo(int *p); int main(void) { char str[16] = "helloworld"; return foo((int *)(str + 1)); }' | ${CC} -x c - -c -o main.o
>  $ ${CC} -static -o unalign-test main.o foo.o
>  $ qemu-armhf unalign-test
>  qemu: uncaught target signal 7 (Bus error) - core dumped
>  Bus error (core dumped)
>
> In this case, the 'ldrd' instruction causes a bus error due to an
> unaligned access. If the -O2 flag is omitted, this issue does not occur.
>
> To avoid potential unaligned access, use the get_unaligned() approach
> as seen in the kernel space.

I have just applied this series against Linus' tree and I can confirm that it fixes the
unalignment access anymore on sparc64. There is no more "Bus error" and the build succeeds.

Tested-by: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>

Thanks,
Adrian

--
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