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

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

[Askar Safin]

Eric Biggers <ebiggers@kernel.org>:
> 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.

I did bisect.

It is possible to apply this patch on top of v6.12.48 without conflicts.

The bug actually prevents me for using my system (more details below).

Here is steps to reproduce bug I noticed.

Build kernel so:

$ cat /tmp/mini
CONFIG_64BIT=y
CONFIG_PRINTK=y
CONFIG_SERIAL_8250=y
CONFIG_TTY=y
CONFIG_SERIAL_8250_CONSOLE=y
CONFIG_BLK_DEV_INITRD=y
CONFIG_RD_GZIP=y
CONFIG_BINFMT_ELF=y
CONFIG_BINFMT_SCRIPT=y
CONFIG_PROC_FS=y
CONFIG_SYSFS=y
CONFIG_DEVTMPFS=y
CONFIG_MODULES=y
CONFIG_BTRFS_FS=m
CONFIG_MODULE_COMPRESS=y
CONFIG_MODULE_COMPRESS_XZ=y
CONFIG_MODULE_COMPRESS_ALL=y
CONFIG_MODULE_DECOMPRESS=y
CONFIG_PRINTK_TIME=y
$ make allnoconfig KCONFIG_ALLCONFIG=/tmp/mini
$ make

Then create initramfs, which contains statically built busybox
(I used busybox v1.37.0 (Debian 1:1.37.0-6+b3)) and modules we just created.

Then run Qemu using command line similar to this:

qemu-system-x86_64 -kernel arch/x86/boot/bzImage -initrd i.gz -append 'console=ttyS0 panic=1 rdinit=/bin/busybox sh' -m 256 -no-reboot -enable-kvm -serial stdio -display none

Then in busybox shell type this:

# mkdir /proc
# busybox mount -t proc proc /proc
# modprobe btrfs

On buggy kernels I get this output:

# modprobe btrfs
[   19.614228] raid6: skipped pq benchmark and selected sse2x4
[   19.614638] raid6: using intx1 recovery algorithm
[   19.616569] xor: measuring software checksum speed
[   19.616937]    prefetch64-sse  : 42616 MB/sec
[   19.617270]    generic_sse     : 41320 MB/sec
[   19.617531] xor: using function: prefetch64-sse (42616 MB/sec)
[   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

The bug is reproducible on all kernels from v6.12 until this commit.
And it is not reproducible on all kernels, which contain this commit.
I found this using bisect.

This bug actually breaks my workflow. I have btrfs as root filesystem.
Initramfs, generated by Debian, doesn't suit my needs. So I'm going
to create my own initramfs from scratch. (Note that I use Debian Trixie,
which has v6.12.48 kernel.) During testing this initramfs in Qemu
I noticed that command "modprobe btrfs" fails with error given above.
(I not yet tried to test this initramfs on real hardware.)

So, this bug actually breaks my workflow.

So, please backport this patch (i. e. 38a9a5121c3b ("lib/crc32: make crc32c() go directly to lib"))
to stable kernels.

I tested that this patch can be applied without conflicts on top of v6.12.48,
and this patch indeed fixes the bug for v6.12.48.

If you want, I can give more info.

It is possible that this is in fact bug in busybox, not in Linux.
But still I think that backporting this patch is good idea.

This busybox thread my be related:
https://lists.busybox.net/pipermail/busybox/2023-May/090309.html

-- 
Askar Safin

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

[Askar Safin]

On Sun, Oct 19, 2025 at 9:09 AM Askar Safin <safinaskar@gmail.com> wrote:
>
> Eric Biggers <ebiggers@kernel.org>:
> > 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.



-- 
Askar Safin

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

[Eric Biggers]

On Sun, Oct 19, 2025 at 11:10:25AM +0300, Askar Safin wrote:
> On Sun, Oct 19, 2025 at 9:09 AM Askar Safin <safinaskar@gmail.com> wrote:
> >
> > Eric Biggers <ebiggers@kernel.org>:
> > > 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