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

[Alexei Starovoitov]

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.