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

[Namhyung Kim]

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.

The intended use case for now is to symbolize locks in slab objects
from the lock contention tracepoints.

Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx> (mm/*)
Acked-by: Vlastimil Babka <vbabka@xxxxxxx> #mm/slab
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 kernel/bpf/helpers.c  |  1 +
 kernel/bpf/verifier.c |  5 +++++
 mm/slab_common.c      | 19 +++++++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 4053f279ed4cc7ab..7bfef9378ab21267 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_RCU_PROTECTED)
 BTF_KFUNCS_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index cfc62e0776bff2c8..f514247ba8ba8a57 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11259,6 +11259,7 @@ enum special_kfunc_type {
 	KF_bpf_preempt_enable,
 	KF_bpf_iter_css_task_new,
 	KF_bpf_session_cookie,
+	KF_bpf_get_kmem_cache,
 };
 
 BTF_SET_START(special_kfunc_set)
@@ -11324,6 +11325,7 @@ BTF_ID(func, bpf_session_cookie)
 #else
 BTF_ID_UNUSED
 #endif
+BTF_ID(func, bpf_get_kmem_cache)
 
 static bool is_kfunc_ret_null(struct bpf_kfunc_call_arg_meta *meta)
 {
@@ -12834,6 +12836,9 @@ 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_bpf_get_kmem_cache])
+				regs[BPF_REG_0].type |= PTR_UNTRUSTED;
+
 			if (is_iter_next_kfunc(&meta)) {
 				struct bpf_reg_state *cur_iter;
 
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;
+}
+
+__bpf_kfunc_end_defs();
+#endif /* CONFIG_BPF_SYSCALL */
+
 /* Tracepoints definitions. */
 EXPORT_TRACEPOINT_SYMBOL(kmalloc);
 EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
-- 
2.47.0.rc1.288.g06298d1525-goog