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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Oct 4, 2024 at 3:57 PM Song Liu <song@kernel.org> wrote:
>
> On Fri, Oct 4, 2024 at 2:58 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Fri, Oct 04, 2024 at 02:36:30PM -0700, Song Liu wrote:
> > > On Fri, Oct 4, 2024 at 2:25 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> > > >
> > > > On Fri, Oct 04, 2024 at 01:10:58PM -0700, Song Liu wrote:
> > > > > On Wed, Oct 2, 2024 at 11:10 AM Namhyung Kim <namhyung@kernel.org> 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 intended use case for now is to
> > > > > > symbolize locks in slab objects from the lock contention tracepoints.
> > > > > >
> > > > > > Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> > > > > > Acked-by: Roman Gushchin <roman.gushchin@linux.dev> (mm/*)
> > > > > > Acked-by: Vlastimil Babka <vbabka@suse.cz> #mm/slab
> > > > > > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > > > > > ---
> > > > > >  kernel/bpf/helpers.c |  1 +
> > > > > >  mm/slab_common.c     | 19 +++++++++++++++++++
> > > > > >  2 files changed, 20 insertions(+)
> > > > > >
> > > > > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > > > > > index 4053f279ed4cc7ab..3709fb14288105c6 100644
> > > > > > --- a/kernel/bpf/helpers.c
> > > > > > +++ b/kernel/bpf/helpers.c
> > > > > > @@ -3090,6 +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_KFUNCS_END(common_btf_ids)
> > > > > >
> > > > > >  static const struct btf_kfunc_id_set common_kfunc_set = {
> > > > > > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > > > > > index 7443244656150325..5484e1cd812f698e 100644
> > > > > > --- a/mm/slab_common.c
> > > > > > +++ b/mm/slab_common.c
> > > > > > @@ -1322,6 +1322,25 @@ size_t ksize(const void *objp)
> > > > > >  }
> > > > > >  EXPORT_SYMBOL(ksize);
> > > > > >
> > > > > > +#ifdef CONFIG_BPF_SYSCALL
> > > > > > +#include <linux/btf.h>
> > > > > > +
> > > > > > +__bpf_kfunc_start_defs();
> > > > > > +
> > > > > > +__bpf_kfunc struct kmem_cache *bpf_get_kmem_cache(u64 addr)
> > > > > > +{
> > > > > > +       struct slab *slab;
> > > > > > +
> > > > > > +       if (!virt_addr_valid(addr))
> > > > > > +               return NULL;
> > > > > > +
> > > > > > +       slab = virt_to_slab((void *)(long)addr);
> > > > > > +       return slab ? slab->slab_cache : NULL;
> > > > > > +}
> > > > >
> > > > > Do we need to hold a refcount to the slab_cache? Given
> > > > > we make this kfunc available everywhere, including
> > > > > sleepable contexts, I think it is necessary.
> > > >
> > > > It's a really good question.
> > > >
> > > > If the callee somehow owns the slab object, as in the example
> > > > provided in the series (current task), it's not necessarily.
> > > >
> > > > If a user can pass a random address, you're right, we need to
> > > > grab the slab_cache's refcnt. But then we also can't guarantee
> > > > that the object still belongs to the same slab_cache, the
> > > > function becomes racy by the definition.
> > >
> > > To be safe, we can limit the kfunc to sleepable context only. Then
> > > we can lock slab_mutex for virt_to_slab, and hold a refcount
> > > to slab_cache. We will need a KF_RELEASE kfunc to release
> > > the refcount later.
> >
> > Then it needs to call kmem_cache_destroy() for release which contains
> > rcu_barrier. :(
> >
> > >
> > > IIUC, this limitation (sleepable context only) shouldn't be a problem
> > > for perf use case?
> >
> > No, it would be called from the lock contention path including
> > spinlocks. :(
> >
> > Can we limit it to non-sleepable ctx and not to pass arbtrary address
> > somehow (or not to save the result pointer)?
>
> I hacked something like the following. It is not ideal, because we are
> taking spinlock_t pointer instead of void pointer. To use this with void
> 'pointer, we will need some verifier changes.
>
> Thanks,
> Song
>
>
> 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.

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;

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.