mm/bpf: Add bpf_get_kmem_cache() kfunc

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.

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>
Link: https://lore.kernel.org/r/20241010232505.1339892-3-namhyung@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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((void *)(long)addr))
+		return NULL;
+
+	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);