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

[Masahiro Yamada]

On Thu, Dec 26, 2024 at 1:27 AM John Paul Adrian Glaubitz
<glaubitz@xxxxxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxxxxx>


Thanks for the compile test.

Loadable modules (*.ko files) are relocatable ELF.
So, there is no alignment in *.ko files any more
if the latest binutils is used.

Just in case, I did run-tests for arm and arm64.
Even if there is no alignment in *.ko files,
I confirmed that the kernel can load modules.

If you have time, please run-test and
double-check loadable modules are working.



-- 
Best Regards
Masahiro Yamada