bpf: add bpf_dynptr_memset() kfunc

Currently there is no straightforward way to fill dynptr memory with a
value (most commonly zero). One can do it with bpf_dynptr_write(), but
a temporary buffer is necessary for that.

Implement bpf_dynptr_memset() - an analogue of memset() from libc.

Signed-off-by: Ihor Solodrai <[email protected]>

Author: theihor

kernel/bpf/helpers.c

@@ -2906,6 +2906,53 @@ __bpf_kfunc int bpf_dynptr_copy(struct bpf_dynptr *dst_ptr, u32 dst_off,
 	return 0;
 }
 
+/**
+ * bpf_dynptr_memset() - Fill dynptr memory with a constant byte.
+ * @ptr: Destination dynptr - where data will be filled
+ * @ptr_off: Offset into the dynptr to start filling from
+ * @size: Number of bytes to fill
+ * @val: Constant byte to fill the memory with
+ *
+ * Fills the size bytes of the memory area pointed to by ptr
+ * at offset ptr_off with the constant byte val.
+ * Returns 0 on success; negative error, otherwise.
+ */
+ __bpf_kfunc int bpf_dynptr_memset(struct bpf_dynptr *ptr, u32 ptr_off, u32 size, u8 val)
+ {
+	struct bpf_dynptr_kern *p = (struct bpf_dynptr_kern *)ptr;
+	char buf[256];
+	u32 chunk_sz;
+	void* slice;
+	u32 offset;
+	int err;
+
+	if (__bpf_dynptr_is_rdonly(p))
+		return -EINVAL;
+
+	err = bpf_dynptr_check_off_len(p, ptr_off, size);
+	if (err)
+		return err;
+
+	slice = bpf_dynptr_slice_rdwr(ptr, ptr_off, NULL, size);
+	if (likely(slice)) {
+		memset(slice, val, size);
+		return 0;
+	}
+
+	/* Non-linear data under the dynptr, write from a local buffer */
+	chunk_sz = min_t(u32, sizeof(buf), size);
+	memset(buf, val, chunk_sz);
+
+	for (offset = ptr_off; offset < ptr_off + size; offset += chunk_sz) {
+		chunk_sz = min_t(u32, sizeof(buf), size - offset);
+		err = __bpf_dynptr_write(p, offset, buf, chunk_sz, 0);
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
 __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
 {
 	return obj;
@@ -3364,6 +3411,7 @@ 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)
+BTF_ID_FLAGS(func, bpf_dynptr_memset)
 #ifdef CONFIG_NET
 BTF_ID_FLAGS(func, bpf_modify_return_test_tp)
 #endif