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     | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 1a43d06eab286c26..bbc5800ec3afc899 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..e648b05a635b94bf 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1322,6 +1322,22 @@ 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;
+
+	slab = virt_to_slab((void *)(long)addr);
+	return slab ? slab->slab_cache : NULL;
+}
+
+__bpf_kfunc_end_defs();
+#endif /* CONFIG_BPF_SYSCALL */
+
 /* Tracepoints definitions. */
 EXPORT_TRACEPOINT_SYMBOL(kmalloc);
 EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
-- 
2.46.1.824.gd892dcdcdd-goog

On Fri, Sep 27, 2024 at 11:41 AM Namhyung Kim <namhyung@kernel.org> wrote:
>
> +__bpf_kfunc struct kmem_cache *bpf_get_kmem_cache(u64 addr)
> +{
> +       struct slab *slab;
> +
> +       slab = virt_to_slab((void *)(long)addr);
> +       return slab ? slab->slab_cache : NULL;
> +}

I think this needs more safety guards on 'addr'.
It needs to check the valid range of 'addr' before doing virt_to_slab.

On Sun, Sep 29, 2024 at 10:05:42AM -0700, Alexei Starovoitov wrote:
> On Fri, Sep 27, 2024 at 11:41 AM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > +__bpf_kfunc struct kmem_cache *bpf_get_kmem_cache(u64 addr)
> > +{
> > +       struct slab *slab;
> > +
> > +       slab = virt_to_slab((void *)(long)addr);
> > +       return slab ? slab->slab_cache : NULL;
> > +}
> 
> I think this needs more safety guards on 'addr'.
> It needs to check the valid range of 'addr' before doing virt_to_slab.

Ok, I think we can use virt_addr_valid() for that.

Thanks,
Namhyung