IB/core: uverbs copy to struct or zero helper

Add a helper to zero fill fields before copying data to
UVERBS_ATTR_STRUCT.

As UVERBS_ATTR_STRUCT can be used as an extensible struct, we want to make
sure that if the user supplies us with a struct that has new fields that
we are not aware of, we return them zeroed to the user.

This helper should be used when using UVERBS_ATTR_STRUCT for an extendable
data structure and there is a need to make sure that extended members of
the struct, that the kernel doesn't handle, are returned zeroed to the
user. This is needed due to the fact that UVERBS_ATTR_STRUCT allows
non-zero values for members after 'last' member.

Signed-off-by: Michael Guralnik <michaelgur@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/uverbs_ioctl.c

@@ -751,3 +751,14 @@ int _uverbs_get_const(s64 *to, const struct uverbs_attr_bundle *attrs_bundle,
 	return 0;
 }
 EXPORT_SYMBOL(_uverbs_get_const);
+
+int uverbs_copy_to_struct_or_zero(const struct uverbs_attr_bundle *bundle,
+				  size_t idx, const void *from, size_t size)
+{
+	const struct uverbs_attr *attr = uverbs_attr_get(bundle, idx);
+
+	if (clear_user(u64_to_user_ptr(attr->ptr_attr.data),
+		       attr->ptr_attr.len))
+		return -EFAULT;
+	return uverbs_copy_to(bundle, idx, from, size);
+}

include/rdma/uverbs_ioctl.h

@@ -871,6 +871,8 @@ static inline __malloc void *uverbs_zalloc(struct uverbs_attr_bundle *bundle,
 int _uverbs_get_const(s64 *to, const struct uverbs_attr_bundle *attrs_bundle,
 		      size_t idx, s64 lower_bound, u64 upper_bound,
 		      s64 *def_val);
+int uverbs_copy_to_struct_or_zero(const struct uverbs_attr_bundle *bundle,
+				  size_t idx, const void *from, size_t size);
 #else
 static inline int
 uverbs_get_flags64(u64 *to, const struct uverbs_attr_bundle *attrs_bundle,
@@ -906,6 +908,12 @@ _uverbs_get_const(s64 *to, const struct uverbs_attr_bundle *attrs_bundle,
 {
 	return -EINVAL;
 }
+static inline int
+uverbs_copy_to_struct_or_zero(const struct uverbs_attr_bundle *bundle,
+			      size_t idx, const void *from, size_t size)
+{
+	return -EINVAL;
+}
 #endif
 
 #define uverbs_get_const(_to, _attrs_bundle, _idx)                             \