Re: [PATCH v5 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc

[Namhyung Kim]

Hi Alexei,

On Fri, Oct 11, 2024 at 12:14:14PM -0700, Namhyung Kim wrote:
> On Fri, Oct 11, 2024 at 11:35:27AM -0700, Alexei Starovoitov wrote:
> > On Thu, Oct 10, 2024 at 4:25 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > >
> > > The bpf_get_kmem_cache() is to get a slab cache information from a
> > > virtual address like virt_to_cache().  If the address is a pointer
> > > to a slab object, it'd return a valid kmem_cache pointer, otherwise
> > > NULL is returned.
> > >
> > > It doesn't grab a reference count of the kmem_cache so the caller is
> > > responsible to manage the access.  The returned point is marked as
> > > PTR_UNTRUSTED.  And the kfunc has KF_RCU_PROTECTED as the slab object
> > > might be protected by RCU.
> > 
> > ...
> > > +BTF_ID_FLAGS(func, bpf_get_kmem_cache, KF_RCU_PROTECTED)
> > 
> > This flag is unnecessary. PTR_UNTRUSTED can point to absolutely any memory.
> > In this case it likely points to a valid kmem_cache, but
> > the verifier will guard all accesses with probe_read anyway.
> > 
> > I can remove this flag while applying.
> 
> Ok, I'd be happy if you would remove it.

You will need to update the bpf_rcu_read_lock/unlock() in the test code
(patch 3).  I can send v6 with that and Vlastimil's Ack if you want.

Thanks,
Namhyung