[PATCH bpf-next v2 1/3] bpf: Add KF_SPINLOCK_SAFE flag for kfuncs under bpf_spin_lock

[Kaitao Cheng]

From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>

Introduce the KF_SPINLOCK_SAFE kfunc metadata flag in BTF so kfuncs may
be explicitly marked as safe to call while holding bpf_spin_lock.

Allow kfuncs defined in kernel modules to be marked with KF_SPINLOCK_SAFE.

Example: BTF_ID_FLAGS(func, $kfunc_name, KF_SPINLOCK_SAFE)

Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
---
 include/linux/btf.h   |  1 +
 kernel/bpf/verifier.c | 20 +++++++++++++++-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index c09b7994de4e..3f5255d095a2 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -79,6 +79,7 @@
 #define KF_ARENA_ARG1   (1 << 14) /* kfunc takes an arena pointer as its first argument */
 #define KF_ARENA_ARG2   (1 << 15) /* kfunc takes an arena pointer as its second argument */
 #define KF_IMPLICIT_ARGS (1 << 16) /* kfunc has implicit arguments supplied by the verifier */
+#define KF_SPINLOCK_SAFE (1 << 17) /* kfunc is allowed inside bpf_spin_lock-ed region */
 
 /*
  * Tag marking a kernel function as a kfunc. This is meant to minimize the
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b274004fccfd..9c1f8d552655 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11832,11 +11832,21 @@ static bool is_bpf_stream_kfunc(u32 btf_id)
 	       btf_id == special_kfunc_list[KF_bpf_stream_print_stack];
 }
 
-static bool kfunc_spin_allowed(u32 btf_id)
+static bool kfunc_spin_allowed(struct bpf_verifier_env *env, s32 func_id, s16 offset)
 {
-	return is_bpf_graph_api_kfunc(btf_id) || is_bpf_iter_num_api_kfunc(btf_id) ||
-	       is_bpf_res_spin_lock_kfunc(btf_id) || is_bpf_arena_kfunc(btf_id) ||
-	       is_bpf_stream_kfunc(btf_id);
+	struct bpf_kfunc_meta kfunc;
+	int err;
+
+	if (is_bpf_graph_api_kfunc(func_id) || is_bpf_iter_num_api_kfunc(func_id) ||
+	    is_bpf_res_spin_lock_kfunc(func_id) || is_bpf_arena_kfunc(func_id) ||
+	    is_bpf_stream_kfunc(func_id))
+		return true;
+
+	err = fetch_kfunc_meta(env, func_id, offset, &kfunc);
+	if (err || !kfunc.flags)
+		return false;
+
+	return *kfunc.flags & KF_SPINLOCK_SAFE;
 }
 
 static bool is_sync_callback_calling_kfunc(u32 btf_id)
@@ -17415,7 +17425,7 @@ static int do_check_insn(struct bpf_verifier_env *env, bool *do_print_state)
 				     insn->imm != BPF_FUNC_spin_unlock &&
 				     insn->imm != BPF_FUNC_kptr_xchg) ||
 				    (insn->src_reg == BPF_PSEUDO_KFUNC_CALL &&
-				     (insn->off != 0 || !kfunc_spin_allowed(insn->imm)))) {
+				     !kfunc_spin_allowed(env, insn->imm, insn->off))) {
 					verbose(env,
 						"function calls are not allowed while holding a lock\n");
 					return -EINVAL;
-- 
2.50.1 (Apple Git-155)