Subject: Re: [PATCH v4 15/19] lib/crc32: make crc32c() go directly to lib

From: Eric Biggers <[email-masked]>
Date: Sun, 19 Oct 2025 09:23:46 -0700

On Sun, Oct 19, 2025 at 11:10:25AM +0300, Askar Safin wrote:
> On Sun, Oct 19, 2025 at 9:09 AM Askar Safin <[email-masked]> wrote:
> >
> > Eric Biggers <[email-masked]>:
> > > Now that the lower level __crc32c_le() library function is optimized for
> >
> > This patch (i. e. 38a9a5121c3b ("lib/crc32: make crc32c() go directly to lib"))
> > solves actual bug I found in practice. So, please, backport it
> > to stable kernels.
> 
> Oops. I just noticed that this patch removes module "libcrc32c".
> And this breaks build for Debian kernel v6.12.48.
> Previously I tested minimal build using "make localmodconfig".
> Now I tried full build of Debian kernel using "dpkg-buildpackage".
> And it failed, because some of Debian files reference "libcrc32c",
> which is not available.
> 
> So, please, don't backport this patch to stable kernels.
> I'm sorry.

Right, this commit simplified the CRC library design by removing the
libcrc32c module.  initramfs build scripts that hard-coded the addition
of libcrc32c.ko into the ramdisk (which I don't think was ever necessary
in the first place, though it did used to be useful to hard-code some of
the *other* CRC modules like crc32c-intel) had to be updated to remove
it.  It looks like Debian did indeed do that, and they updated it in
https://salsa.debian.org/kernel-team/linux/-/commit/6c242c647f84bfdbdc22a6a758fa59da4e941a10#1251f9400a85485d275e1709758350aa098709a8

As for your original problem, I'd glad to see that the simplified design
is preventing problems.  There's an issue with backporting this commit
alone, though.  This was patch 15 of a 19-patch series for a good
reason: the CRC-32C implementation in lib/ wasn't architecture-optimized
until after patches 1-14 of this series.  Backporting this commit alone
would make crc32c() no longer utilize architecture-optimized code.

Now, it already didn't do so reliably (and this patch series fixed
that).  However, backporting this commit alone would make it never do
so.  So it would regress performance in some cases.

Since the errors you're actually getting are:

    [   19.619731] Invalid ELF header magic: != ELF
    modprobe: can't load module libcrc32c (kernel/lib/libcrc32c.ko.xz): unknown symbol in module, or unknown parameter

I do have to wonder if this is actually a busybox bug or
misconfiguration, where it's passing a compressed module to the kernel
without decompressing it?  And removing the module just hid the problem.

- Eric