[PATCH bpf-next 2/3] bpf/helpers: introduce bpf_dynptr_copy kfunc

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

From: Mykyta Yatsenko <yatsenko@meta.com>

Introducing bpf_dynptr_copy kfunc allowing copying data from one dynptr to
another. This functionality is useful in scenarios such as capturing XDP
data to a ring buffer.
The implementation consists of 4 branches:
  * A fast branch for contiguous buffer capacity in both source and
destination dynptrs
  * 3 branches utilizing __bpf_dynptr_read and __bpf_dynptr_write to copy
data to/from non-contiguous buffer

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
 kernel/bpf/helpers.c  | 37 +++++++++++++++++++++++++++++++++++++
 kernel/bpf/verifier.c |  3 +++
 2 files changed, 40 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 2833558c3009..ac5fbdfc504d 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2770,6 +2770,42 @@ __bpf_kfunc int bpf_dynptr_clone(const struct bpf_dynptr *p,
 	return 0;
 }
 
+__bpf_kfunc int bpf_dynptr_copy(struct bpf_dynptr *dst_ptr, u32 dst_off,
+				struct bpf_dynptr *src_ptr, u32 src_off, u32 size)
+{
+	struct bpf_dynptr_kern *dst = (struct bpf_dynptr_kern *)dst_ptr;
+	struct bpf_dynptr_kern *src = (struct bpf_dynptr_kern *)src_ptr;
+	__u8 *src_slice, *dst_slice;
+	int err = 0;
+
+	src_slice = bpf_dynptr_slice(src_ptr, src_off, NULL, size);
+	dst_slice = bpf_dynptr_slice_rdwr(dst_ptr, dst_off, NULL, size);
+
+	if (src_slice && dst_slice) {
+		memmove(dst_slice, src_slice, size);
+	} else if (src_slice) {
+		err = __bpf_dynptr_write(dst, dst_off, src_slice, size, 0);
+	} else if (dst_slice) {
+		err = __bpf_dynptr_read(dst_slice, size, src, src_off, 0);
+	} else {
+		u32 off = 0;
+		char buf[256];
+
+		if (bpf_dynptr_check_off_len(dst, dst_off, size) ||
+		    bpf_dynptr_check_off_len(src, src_off, size))
+			return -E2BIG;
+
+		while (err == 0 && off < size) {
+			u32 chunk_sz = min(sizeof(buf), size - off);
+
+			err = err ?: __bpf_dynptr_read(buf, chunk_sz, src, src_off + off, 0);
+			err = err ?: __bpf_dynptr_write(dst, dst_off + off, buf, chunk_sz, 0);
+			off += chunk_sz;
+		}
+	}
+	return err;
+}
+
 __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
 {
 	return obj;
@@ -3174,6 +3210,7 @@ BTF_ID_FLAGS(func, bpf_dynptr_is_null)
 BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
 BTF_ID_FLAGS(func, bpf_dynptr_size)
 BTF_ID_FLAGS(func, bpf_dynptr_clone)
+BTF_ID_FLAGS(func, bpf_dynptr_copy)
 #ifdef CONFIG_NET
 BTF_ID_FLAGS(func, bpf_modify_return_test_tp)
 #endif
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e7bc74171c99..3c567bfcc582 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11781,6 +11781,7 @@ enum special_kfunc_type {
 	KF_bpf_dynptr_slice,
 	KF_bpf_dynptr_slice_rdwr,
 	KF_bpf_dynptr_clone,
+	KF_bpf_dynptr_copy,
 	KF_bpf_percpu_obj_new_impl,
 	KF_bpf_percpu_obj_drop_impl,
 	KF_bpf_throw,
@@ -11819,6 +11820,7 @@ BTF_ID(func, bpf_dynptr_from_xdp)
 BTF_ID(func, bpf_dynptr_slice)
 BTF_ID(func, bpf_dynptr_slice_rdwr)
 BTF_ID(func, bpf_dynptr_clone)
+BTF_ID(func, bpf_dynptr_copy)
 BTF_ID(func, bpf_percpu_obj_new_impl)
 BTF_ID(func, bpf_percpu_obj_drop_impl)
 BTF_ID(func, bpf_throw)
@@ -11857,6 +11859,7 @@ BTF_ID_UNUSED
 BTF_ID(func, bpf_dynptr_slice)
 BTF_ID(func, bpf_dynptr_slice_rdwr)
 BTF_ID(func, bpf_dynptr_clone)
+BTF_ID(func, bpf_dynptr_copy)
 BTF_ID(func, bpf_percpu_obj_new_impl)
 BTF_ID(func, bpf_percpu_obj_drop_impl)
 BTF_ID(func, bpf_throw)
-- 
2.48.1