Where are the 5.8.0.43 sources ?

symdeb; Hello

Source code for default packages are in the repository:
Use the command apt source <package> (don't use sudo with it) to download the source of a package.

-hope this helps-

Interestingly

apt-get build-dep linux linux-image-$(uname -r)
E: Could not open lock file /var/lib/dpkg/lock-frontend - open (13: Permission denied)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), are you root?

then.

sudo apt-get build-dep linux linux-image-$(uname -r)
[sudo] password for user:
Reading package lists... Done
Picking 'linux-signed' as source package instead of 'linux-image-5.8.0-43-lowlatency'
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

I found somewhere to apt-get source linux-image-unsigned-$(uname -r)
that can download and does not need sudo.
Is that the appropriate one for low latency ?

https://packages.ubuntu.com/search?keywords=linux-source&searchon=names&suite=groovy&section=all

so you will install the source with:
apt-get source linux-source

that does not seem to be the whole kernel.

apt-get source linux-source
Reading package lists... Done
Picking 'linux-meta' as source package instead of 'linux-source'
NOTICE: 'linux-meta' packaging is maintained in the 'Git' version control system at:
git://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux-meta/+git/groovy
Please use:
git clone git://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux-meta/+git/groovy
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 24.9 kB of source archives.
Get:1 http://archive.ubuntu.com/ubuntu groovy-updates/main linux-meta 5.8.0.43.48 (dsc) [9,724 B]
Get:2 http://archive.ubuntu.com/ubuntu groovy-updates/main linux-meta 5.8.0.43.48 (tar) [15.2 kB]
Fetched 24.9 kB in 1s (22.4 kB/s)
dpkg-source: info: extracting linux-meta in linux-meta-5.8.0.43.48
dpkg-source: info: unpacking linux-meta_5.8.0.43.48.tar.xz

There seems to be a confusion between the "linux" source package (containing the real source) and "linux-meta" which is a supporting package that only cares for installing the newer kernel packages, but does not contain any real software.

And

apt source
and
apt-get-source
do not need admin rights (e.g. sudo), but
apt build-dep
and
apt-get buid-dep
need sudo.

I am now using
sudo apt-get build-dep linux-image-$(uname -r)
apt source linux linux-image-$(uname -r)
This gets the signed version.
Assume that's ok for low latency build ?

It seems to be even more complicated.

The source for the unsigned kernel and the modules and header files is in the source package "linux"
The source for the signed kernels in in the source package "linux-signed"

Maybe you have to build both - the unsigned first and the signed one only later.