On Wed, Feb 25, 2026 at 06:32:40AM -0800, Greg KH wrote:
> On Wed, Feb 25, 2026 at 10:35:06AM +0800, Shung-Hsi Yu wrote:
> > Cc BPF mailing list and maintainers. Plus other in the referenced
> > thread.
> >
> > Hi Harshit,
> >
> > On Tue, Feb 24, 2026 at 09:18:04AM -0300, Ricardo B. Marlière wrote:
> > > On Tue Feb 24, 2026 at 4:53 AM -03, Harshit Mogalapalli wrote:
> > > > On 24/02/26 13:08, Shung-Hsi Yu wrote:
> > > >> This patchset backport the corresponding BPF selftests for commit
> > > >> d946f3c98328 ("bpf: Check skb->transport_header is set in
> > > >> bpf_skb_check_mtu"), which has already been included since 6.12.63.
> > > >>
> > > >> The BPF selftest added in commit 6cc73f35406c ("selftests/bpf: Test
> > > >> bpf_skb_check_mtu(BPF_MTU_CHK_SEGS) when transport_header is not set")
> > > >> additionally depends on network namespace support for BPF selftests
> > > >> added by Bastien, otherwise the MTU in root networking namespace will
> > > >> be
> > > >> set to 10, causing other BPF selftests to fail. Credit goes to Ricardo
> > > >> Marlière for figuring out the dependency.
> > > >
> > > > Note:
> > > > I have recently learnt that ideally we are supposed to run upstream
> > > > latest kselftests on stable kernels as well. If a feature is not
> > > > supported the kselftests are meant to be skipped.
> >
> > Thanks you for the bringing this up! This was also mentioned by Daniel
> > (Borkmann), but my experience aligns with Ricardo's.
> >
> > > That is not true for BPF, from my (limited) experience.
> >
> > I do want running latest BPF selftests to work on stable thought, made
> > some half-hearted attempts last April on trying bpf-next's BPF selftests
> > (during 6.15 phase) run on stable/linux-6.14.y, but wasn't able to get
> > pass the building phase.
> >
> > As far as I remember I ran into issue building bpf_testmod.ko (kernel
> > module) of bpf-next against 6.14, and other similar issues related data
> > structure or API changes. Pretty much the same set of problems we get
> > when trying to build any driver in the latest kernel against stable
> > kernels.
> >
> > Maybe it can work if BPF selftests that does not depends on
> > bpf_testmod.ko, and build failures of BPF programs are simply ignored
> > (not sure how feasible that is, probably would make Makefile much
> > complicated), so for now I'm sticking with backporting BPF selftests to
> > stable kernels.
>
> If a kernel test module is involved, then yes, you are right, those
> changes do need to be backported as mix/match of kernel modules to other
> trees is not something that we support.
>
> We only want this to be the rule for when we have purely userspace test
> cases/code.
Thanks for the confirmation regarding kernel test module.
For the record most BPF selftests probably doesn't depends on
bpf_testmod.ko, I don't think I have state that clear enough in the
exchange. So with some work running a subset of latest BPF selftest on
stable kernel may be possible, just that I have no idea the effort
required.
Thanks,
Shung-Hsi
