Re: [REGRESSION] bpf verifier slowdown due to vrealloc() change since 6.15-rc6

[Kees Cook <kees@kernel.org>]

On Thu, May 15, 2025 at 10:53:10AM -0700, Andrii Nakryiko wrote:
> On Thu, May 15, 2025 at 10:41 AM Kees Cook <kees@kernel.org> wrote:
> >
> > On Thu, May 15, 2025 at 10:18:21AM -0700, Pawan Gupta wrote:
> > > On Thu, May 15, 2025 at 09:51:15AM -0700, Kees Cook wrote:
> > > > On Thu, May 15, 2025 at 07:51:26AM -0700, Kees Cook wrote:
> > > > > On May 15, 2025 6:12:25 AM PDT, Shung-Hsi Yu <shung-hsi.yu@suse.com> wrote:
> > > > > >There is an observable slowdown when running BPF selftests on 6.15-rc6
> > > > > >kernel[1] built with tools/testing/selftests/bpf/{config,config.x86_64}.
> > > > > [...]
> > > > > Where can I find the .config for the slow runs?
> > > >
> > > > Oops, I can read. :) Doing a build now...
> > > >
> > > > > And how do I run the test myself directly?
> > > >
> > > > I found:
> > > > https://docs.kernel.org/bpf/bpf_devel_QA.html
> > > >
> > > > But it doesn't seem to cover a bunch of stuff (no way to prebuild the
> > > > tests, no info on building the test modules).
> > > >
> > > > This seems to be needed:
> > > >
> > > > make O=regression-bug -C tools/testing/selftests/bpf/test_kmods
> > > >
> > > > But then the booted kernel doesn't load it (missing signatures?)
> > > >
> > > > Anyway, I'll keep digging...
> > >
> > > After struggling with this for a while, I figured vmtest.sh is the easiest
> > > way to test bpf:
> > >
> > > ./tools/testing/selftests/bpf/vmtest.sh -i ./test_progs
> >
> > I can't even build the test_progs. :(
> >
> > $ make test_progs
> > ...
> >   CLNG-BPF [test_progs] bpf_iter_tasks.bpf.o
> > progs/bpf_iter_tasks.c:98:8: error: call to undeclared function 'bpf_copy_from_user_task_str'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
> >    98 |         ret = bpf_copy_from_user_task_str((char *)task_str1, sizeof(task_str1), ptr, task, 0
> > );
> >       |               ^
> > 1 error generated.
> >
> 
> BPF selftests expect that there was a successful kernel build done
> before that. So generally speaking:
> 
> 0) cd <linux/repo/path>
> 1) export O=/path/to/build
> 2) cat tools/testing/selftests/bpf/config >> /path/to/build/.config
> 3) make O=/path/to/build -j$(nproc) oldefconfig all
> 4) cd tools/testing/selftests/bpf # everything is built within this
> directory, we don't support KBUILD_PATH or O for BPF selftests build
> artifacts
> 5) make O=/path/to/build -j$(nproc)

Linux ToT fails to build, -next fails to build. v6.14.6 fails build,
each in different ways. :(

> But tbh, if the above causes you problems, I don't think you need to
> spend that much time trying to build BPF selftests, given you know
> what the issue is and you are fixing it.

Well, nothing I've proposed makes any sense as far as something that would
double execution time, so I don't really know what the issue is yet. :P

-- 
Kees Cook