io_uring/bpf-ops: restrict ctx access to BPF

BPF programs should have no need in looking into struct io_ring_ctx, if
anything, most of such cases would be anti patterns like looking up ring
indices directly via the context.

Replace it with a new empty structure, which is just an alias to struct
io_ring_ctx. It'll create a new BTF type and fail verification if a BPF
program tries to access it (beyond the first byte). It'll also give more
flexibility for the future, and otherwise it can be made aligned with
io_ring_ctx as before with struct groups if ever needed or extended in a
different way.

Fixes: d0e437b ("io_uring/bpf-ops: implement loop_step with BPF struct_ops")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/5f6ca3649e9e0bae8667db4357e28dd00cd07901.1780394491.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

include/linux/io_uring_types.h

@@ -290,6 +290,8 @@ enum {
 	IO_RING_F_IOWQ_LIMITS_SET	= BIT(12),
 };
 
+struct iou_ctx {};
+
 struct io_ring_ctx {
 	/* const or read-mostly hot data */
 	struct {
@@ -366,7 +368,7 @@ struct io_ring_ctx {
 		struct io_alloc_cache	rw_cache;
 		struct io_alloc_cache	cmd_cache;
 
-		int (*loop_step)(struct io_ring_ctx *ctx,
+		int (*loop_step)(struct iou_ctx *,
 				 struct iou_loop_params *);
 
 		/*

io_uring/bpf-ops.c

@@ -14,15 +14,18 @@ static const struct btf_type *loop_params_type;
 
 __bpf_kfunc_start_defs();
 
-__bpf_kfunc int bpf_io_uring_submit_sqes(struct io_ring_ctx *ctx, u32 nr)
+__bpf_kfunc int bpf_io_uring_submit_sqes(struct iou_ctx *loop_ctx, u32 nr)
 {
+	struct io_ring_ctx *ctx = io_loop_demangle_ctx(loop_ctx);
+
 	return io_submit_sqes(ctx, nr);
 }
 
 __bpf_kfunc
-__u8 *bpf_io_uring_get_region(struct io_ring_ctx *ctx, __u32 region_id,
+__u8 *bpf_io_uring_get_region(struct iou_ctx *loop_ctx, __u32 region_id,
 			      const size_t rdwr_buf_size)
 {
+	struct io_ring_ctx *ctx = io_loop_demangle_ctx(loop_ctx);
 	struct io_mapped_region *r;
 
 	lockdep_assert_held(&ctx->uring_lock);
@@ -58,7 +61,7 @@ static const struct btf_kfunc_id_set bpf_io_uring_kfunc_set = {
 	.set = &io_uring_kfunc_set,
 };
 
-static int io_bpf_ops__loop_step(struct io_ring_ctx *ctx,
+static int io_bpf_ops__loop_step(struct iou_ctx *ctx,
 				 struct iou_loop_params *lp)
 {
 	return IOU_LOOP_STOP;

io_uring/bpf-ops.h

@@ -11,7 +11,7 @@ enum {
 };
 
 struct io_uring_bpf_ops {
-	int (*loop_step)(struct io_ring_ctx *ctx, struct iou_loop_params *lp);
+	int (*loop_step)(struct iou_ctx *, struct iou_loop_params *lp);
 
 	__u32 ring_fd;
 	void *priv;

io_uring/loop.c

@@ -49,7 +49,7 @@ static int __io_run_loop(struct io_ring_ctx *ctx)
 		if (unlikely(!ctx->loop_step))
 			return -EFAULT;
 
-		step_res = ctx->loop_step(ctx, &lp);
+		step_res = ctx->loop_step(io_loop_mangle_ctx(ctx), &lp);
 		if (step_res == IOU_LOOP_STOP)
 			break;
 		if (step_res != IOU_LOOP_CONTINUE)

io_uring/loop.h

@@ -24,4 +24,14 @@ static inline bool io_has_loop_ops(struct io_ring_ctx *ctx)
 
 int io_run_loop(struct io_ring_ctx *ctx);
 
+static inline struct iou_ctx *io_loop_mangle_ctx(struct io_ring_ctx *ctx)
+{
+	return (struct iou_ctx *)ctx;
+}
+
+static inline struct io_ring_ctx *io_loop_demangle_ctx(struct iou_ctx *ctx)
+{
+	return (struct io_ring_ctx *)ctx;
+}
+
 #endif