vmlinux BTF as a module (was Re: [PATCH bpf-next v4 0/3] Allow mmap of /sys/kernel/btf/vmlinux)

[Alan Maguire <alan.maguire@oracle.com>]

> Hi Alan,
> 
> Thanks for taking a look at this. I've been following your related effort
> to allow /sys/kernel/btf/vmlinux as a module in support of small systems
> with kernel-size constraints, and wondered how this series might affect
> that work? Such support would be well-received in the embedded space when
> it happens, so am keen to understand.
> 
> Thanks,
> Tony

hi Tony

I had something nearly working a few months back but there are a bunch
of complications that made it a bit trickier than I'd first anticipated.
One challenge for example is that we want /sys/kernel/btf to behave just
as it would if vmlinux BTF was not a module. My original hope was to
just have the vmlinux BTF module forceload early, but the request module
approach won't work since the vmlinux_btf.ko module would have to be
part of the initrd image. A question for you on this - I presume that's
what you want to avoid, right? So I'm assuming that we need to extract
the .BTF section out of the vmlinu[xz] binary and out of initrd into a
later-loading vmlinux_btf.ko module for small-footprint systems. Is that
correct?

The reason I ask is having a later-loading vmlinux_btf.ko is a bit of a
pain since we need to walk the set of kernel modules and load their BTF,
relocate it and do kfunc registration. If we can simplify things via a
shared module dependency on vmlinux_btf.ko that would be great, but I'd
like to better understand the constraints from the small system
perspective first. Thanks!

Alan