Re: [PATCH bpf-next v1 8/8] bpf/selftests: Test for bpf_per_cpu_ptr()

[Hao Luo]

On Tue, Sep 1, 2020 at 11:12 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Aug 27, 2020 at 8:42 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
[...]
> > > >
> > > > -extern const struct rq runqueues __ksym; /* struct type global var. */
> > > > +extern const struct rq runqueues __ksym; /* struct type percpu var. */
> > > >  extern const int bpf_prog_active __ksym; /* int type global var. */
> > > > +extern const unsigned long process_counts __ksym; /* int type percpu var. */
> > > >
> > > >  SEC("raw_tp/sys_enter")
> > > >  int handler(const void *ctx)
> > > >  {
> > > > +       struct rq *rq;
> > > > +       unsigned long *count;
> > > > +
> > > >         out__runqueues = (__u64)&runqueues;
> > > >         out__bpf_prog_active = (__u64)&bpf_prog_active;
> > > >
> > > > +       rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, 1);
> > > > +       if (rq)
> > > > +               out__rq_cpu = rq->cpu;
> > >
> > > this is awesome!
> > >
> > > Are there any per-cpu variables that are arrays? Would be nice to test
> > > those too.
> > >
> > >
> >
> > There are currently per-cpu arrays, but not common. There is a
> > 'pmc_prev_left' in arch/x86, I can add that in this test.
>
> arch-specific variables are bad, because selftests will be failing on
> other architectures; let's not do this then.
>

Yeah, no problem. Though not going to add this arch-specific variable
in the posted patches, I tried array-typed ksyms locally in my test
environment. It worked fine, except that the array size is not
checked. For instance, if there is a percpu array in kernel as

DEFINE_PER_CPU(u32[64], foo);

we can declare a ksym of different size and it passes libbpf checks
and kernel verification.

extern u32 foo[128] __ksyms;

It seems that bpf_core_types_are_compat() doesn't check nr_elem. But
it seems the kernel verifier does check out-of-bounds accesses, so
this may not be a real problem. Just want to list what I saw.

> >
> > [...]