Re: [PATCH v4 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Fri, Oct 04, 2024 at 04:56:57PM -0700, Song Liu wrote:
> On Fri, Oct 4, 2024 at 4:44 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> [...]
> > > diff --git i/kernel/bpf/helpers.c w/kernel/bpf/helpers.c
> > > index 3709fb142881..7311a26ecb01 100644
> > > --- i/kernel/bpf/helpers.c
> > > +++ w/kernel/bpf/helpers.c
> > > @@ -3090,7 +3090,7 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
> > >  BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
> > >  BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
> > >  BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
> > > -BTF_ID_FLAGS(func, bpf_get_kmem_cache, KF_RET_NULL)
> > > +BTF_ID_FLAGS(func, bpf_get_kmem_cache, KF_RET_NULL | KF_TRUSTED_ARGS
> > > | KF_RCU_PROTECTED)
> >
> > I don't think KF_TRUSTED_ARGS approach would fit here.
> > Namhyung's use case is tracing. The 'addr' will be some potentially
> > arbitrary address from somewhere. The chance to see a trusted pointer
> > is probably very low in such a tracing use case.
> 
> I thought the primary use case was to trace lock contention, for
> example, queued_spin_lock_slowpath(). Of course, a more
> general solution is better.

Right, my intended use case is the lock contention profiling so probably
it's ok to limit it for trusted pointers if it helps.  But as Song said,
a general solution should be better. :)

> 
> >
> > The verifier change can mainly be the following:
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 7d9b38ffd220..e09eb108e956 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -12834,6 +12834,9 @@ static int check_kfunc_call(struct
> > bpf_verifier_env *env, struct bpf_insn *insn,
> >                         regs[BPF_REG_0].type = PTR_TO_BTF_ID;
> >                         regs[BPF_REG_0].btf_id = ptr_type_id;
> >
> > +                       if (meta.func_id == special_kfunc_list[KF_get_kmem_cache])
> > +                               regs[BPF_REG_0].type |= PTR_UNTRUSTED;
> > +
> >                         if (is_iter_next_kfunc(&meta)) {
> >                                 struct bpf_reg_state *cur_iter;
> 
> This is easier than I thought.

Indeed!  Thanks for providing the code.

> 
> > The returned 'struct kmem_cache *' won't be refcnt-ed (acquired).
> > It will be readonly via ptr_to_btf_id logic.
> > s->flags;
> > s->size;
> > s->offset;
> > access will be allowed but the verifier will sanitize them
> > with an inlined version of probe_read_kernel.
> > Even KF_RET_NULL can be dropped.

Ok, I'll check this out.  By having PTR_UNTRUSTED, are the callers
still required to check NULL or is it handled by probe_read_kernel()?

Thanks,
Namhyung