[PATCH v1 bpf-next 1/5] bpf: Add bpf_dynptr_trim and bpf_dynptr_advance

[Joanne Koong <joannelkoong@gmail.com>]

bpf_dynptr_trim decreases the size of a dynptr by the specified
number of bytes (offset remains the same). bpf_dynptr_advance advances
the offset of the dynptr by the specified number of bytes (size
decreases correspondingly).

Trimming or advancing the dynptr may be useful in certain situations.
For example, when hashing which takes in generic dynptrs, if the dynptr
points to a struct but only a certain memory region inside the struct
should be hashed, advance/trim can be used to narrow in on the
specific region to hash.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 kernel/bpf/helpers.c | 49 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b6a5cda5bb59..51b4c4b5dbed 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1448,6 +1448,13 @@ u32 bpf_dynptr_get_size(const struct bpf_dynptr_kern *ptr)
 	return ptr->size & DYNPTR_SIZE_MASK;
 }
 
+static void bpf_dynptr_set_size(struct bpf_dynptr_kern *ptr, u32 new_size)
+{
+	u32 metadata = ptr->size & ~DYNPTR_SIZE_MASK;
+
+	ptr->size = new_size | metadata;
+}
+
 int bpf_dynptr_check_size(u32 size)
 {
 	return size > DYNPTR_MAX_SIZE ? -E2BIG : 0;
@@ -2275,6 +2282,46 @@ __bpf_kfunc void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr_kern *ptr, u32 o
 	return bpf_dynptr_slice(ptr, offset, buffer, buffer__szk);
 }
 
+/* For dynptrs, the offset may only be advanced and the size may only be decremented */
+static int bpf_dynptr_adjust(struct bpf_dynptr_kern *ptr, u32 off_inc, u32 sz_dec)
+{
+	u32 size;
+
+	if (!ptr->data)
+		return -EINVAL;
+
+	size = bpf_dynptr_get_size(ptr);
+
+	if (sz_dec > size)
+		return -ERANGE;
+
+	if (off_inc) {
+		u32 new_off;
+
+		if (off_inc > size)
+			return -ERANGE;
+
+		if (check_add_overflow(ptr->offset, off_inc, &new_off))
+			return -ERANGE;
+
+		ptr->offset = new_off;
+	}
+
+	bpf_dynptr_set_size(ptr, size - sz_dec);
+
+	return 0;
+}
+
+__bpf_kfunc int bpf_dynptr_advance(struct bpf_dynptr_kern *ptr, u32 len)
+{
+	return bpf_dynptr_adjust(ptr, len, len);
+}
+
+__bpf_kfunc int bpf_dynptr_trim(struct bpf_dynptr_kern *ptr, u32 len)
+{
+	return bpf_dynptr_adjust(ptr, 0, len);
+}
+
 __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
 {
 	return obj;
@@ -2347,6 +2394,8 @@ BTF_ID_FLAGS(func, bpf_dynptr_slice_rdwr, KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_num_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_num_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
+BTF_ID_FLAGS(func, bpf_dynptr_trim)
+BTF_ID_FLAGS(func, bpf_dynptr_advance)
 BTF_SET8_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {
-- 
2.34.1