[PATCH v4 bpf-next 1/4] bpf: Add 'bpf_dynptr_get_data' helper

[Shmulik Ladkani <shmulik@metanetworks.com>]

The task of calculating bpf_dynptr_kern's available size, and the
current (offset) data pointer is common for BPF functions working with
ARG_PTR_TO_DYNPTR parameters.

Introduce 'bpf_dynptr_get_data' which returns the current data
(with properer offset), and the number of usable bytes it has.

This will void callers from directly calculating bpf_dynptr_kern's
data, offset and size.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
---
 include/linux/bpf.h  |  1 +
 kernel/bpf/helpers.c | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 39bd36359c1e..6d288dfc302b 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2576,6 +2576,7 @@ void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
 		     enum bpf_dynptr_type type, u32 offset, u32 size);
 void bpf_dynptr_set_null(struct bpf_dynptr_kern *ptr);
 int bpf_dynptr_check_size(u32 size);
+void *bpf_dynptr_get_data(struct bpf_dynptr_kern *ptr, u32 *avail_bytes);
 
 #ifdef CONFIG_BPF_LSM
 void bpf_cgroup_atype_get(u32 attach_btf_id, int cgroup_atype);
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 3c1b9bbcf971..91f406a9c37f 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1461,6 +1461,16 @@ void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
 	bpf_dynptr_set_type(ptr, type);
 }
 
+void *bpf_dynptr_get_data(struct bpf_dynptr_kern *ptr, u32 *avail_bytes)
+{
+	u32 size = bpf_dynptr_get_size(ptr);
+
+	if (!ptr->data || ptr->offset > size)
+		return NULL;
+	*avail_bytes = size - ptr->offset;
+	return ptr->data + ptr->offset;
+}
+
 void bpf_dynptr_set_null(struct bpf_dynptr_kern *ptr)
 {
 	memset(ptr, 0, sizeof(*ptr));
-- 
2.37.2