Allow BPF to implement the loop_step callback to overwrite the main loop
logic. As described in the patch introducing the callback, it receives
iou_loop_params as an argument, which BPF can directly use to control
the loop.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/bpf-ops.c | 36 ++++++++++++++++++++++++++++++++++++
 io_uring/bpf-ops.h |  4 ++++
 2 files changed, 40 insertions(+)

diff --git a/io_uring/bpf-ops.c b/io_uring/bpf-ops.c
index a89d1dea60c7..7db07eda5a48 100644
--- a/io_uring/bpf-ops.c
+++ b/io_uring/bpf-ops.c
@@ -1,11 +1,22 @@
 #include <linux/mutex.h>
 #include <linux/bpf.h>
+#include <linux/bpf_verifier.h>
 
 #include "io_uring.h"
 #include "register.h"
 #include "bpf-ops.h"
+#include "loop.h"
+
+static const struct btf_type *loop_params_type;
+
+static int io_bpf_ops__loop_step(struct io_ring_ctx *ctx,
+				 struct iou_loop_params *lp)
+{
+	return IOU_LOOP_STOP;
+}
 
 static struct io_uring_bpf_ops io_bpf_ops_stubs = {
+	.loop_step = io_bpf_ops__loop_step,
 };
 
 static bool bpf_io_is_valid_access(int off, int size,
@@ -27,6 +38,14 @@ static int bpf_io_btf_struct_access(struct bpf_verifier_log *log,
 				    const struct bpf_reg_state *reg, int off,
 				    int size)
 {
+	const struct btf_type *t = btf_type_by_id(reg->btf, reg->btf_id);
+
+	if (t == loop_params_type) {
+		if (off >= offsetof(struct iou_loop_params, cq_wait_idx) &&
+		    off + size <= offsetofend(struct iou_loop_params, cq_wait_idx))
+			return SCALAR_VALUE;
+	}
+
 	return -EACCES;
 }
 
@@ -36,8 +55,25 @@ static const struct bpf_verifier_ops bpf_io_verifier_ops = {
 	.btf_struct_access = bpf_io_btf_struct_access,
 };
 
+static const struct btf_type *
+io_lookup_struct_type(struct btf *btf, const char *name)
+{
+	s32 type_id;
+
+	type_id = btf_find_by_name_kind(btf, name, BTF_KIND_STRUCT);
+	if (type_id < 0)
+		return NULL;
+	return btf_type_by_id(btf, type_id);
+}
+
 static int bpf_io_init(struct btf *btf)
 {
+	loop_params_type = io_lookup_struct_type(btf, "iou_loop_params");
+	if (!loop_params_type) {
+		pr_err("io_uring: Failed to locate iou_loop_params\n");
+		return -EINVAL;
+	}
+
 	return 0;
 }
 
diff --git a/io_uring/bpf-ops.h b/io_uring/bpf-ops.h
index a6756b391387..e8a08ae2df0a 100644
--- a/io_uring/bpf-ops.h
+++ b/io_uring/bpf-ops.h
@@ -5,6 +5,10 @@
 #include <linux/io_uring_types.h>
 
 struct io_uring_bpf_ops {
+	int (*loop_step)(struct io_ring_ctx *ctx, struct iou_loop_params *lp);
+
+	__u32 ring_fd;
+	void *priv;
 };
 
 #endif /* IOU_BPF_OPS_H */
-- 
2.52.0