Re: [PATCH v3] docs: net: add netlink attrs best practices

[Lin Ma]

Hello Randy,

> 
> Hi--
> 
> More corrections/comments:
> 
> On 8/10/23 20:15, Lin Ma wrote:
> > Provide some suggestions that who deal with Netlink code could follow
> > (of course using the word "best-practices" may sound somewhat
> > exaggerate).
> > 
> > According to my recent practices, the parsing of the Netlink attributes
> > lacks documents for kernel developers. Since recently the relevant docs
> > for Netlink user space get replenished, I guess is a good chance for
> > kernel space part to catch with.
> > 
> > First time to write a document and any reviews are appreciated.
> > 
> > Signed-off-by: Lin Ma <linma@xxxxxxxxxx>
> > ---
> > v1 -> v2: fix some typos in FOO example,
> >           add extra section "About General Netlink Case" to avoid any
> >           confusion for new code users.
> > v2 -> v3: move from staging to the networking directory and polish this document
> >           with the suggestions from Simon and Randy, including:
> >           * fix typos
> >           * wrap lines
> >           * use kernel-doc to refer to the parsers function
> >           * add an example for strict_start_type
> > 
> >  Documentation/networking/index.rst            |   1 +
> >  .../netlink-attrs-best-practices.rst          | 772 ++++++++++++++++++
> >  2 files changed, 773 insertions(+)
> >  create mode 100644 Documentation/networking/netlink-attrs-best-practices.rst
> > 
> 
> > diff --git a/Documentation/networking/netlink-attrs-best-practices.rst b/Documentation/networking/netlink-attrs-best-practices.rst
> > new file mode 100644
> > index 000000000000..38cbd47b2c99
> > --- /dev/null
> > +++ b/Documentation/networking/netlink-attrs-best-practices.rst
> > @@ -0,0 +1,772 @@
> > +.. SPDX-License-Identifier: BSD-3-Clause
> > +
> > +=================================
> > +Netlink Attributes Best Practices
> > +=================================
> > +
> > +Introduction
> > +============
> > +
> > +This document serves as a guide to the best practices, or cautions, for parsing
> > +user-space-provided Netlink attributes in kernel space. The intended audience
> > +is those who want to leverage the powerful Netlink interface
> > +(mainly for classic or legacy Netlink and the general Netlink users basically
> > +don't worry about these, see penultimate section) in their code. Additionally,
> > +for those who are curious about the parsing of Netlink attributes but may feel
> > +apprehensive about delving into the code itself, this document can serve as an
> > +excellent starting point.
> > +
> > +However, if you are concerned about how to prepare Netlink messages from a user
> > +space socket instead of writing kernel code, it is recommended to read the
> > +`Netlink Handbook <https://docs.kernel.org/userspace-api/netlink/intro.html>`_
> > +first. And after finishing reading this, remember to also read the brief
> > +introduction to `Generic Netlink <https://docs.kernel.org/networking/generic_netlink.html>`_.
> > +
> > +Background
> > +==========
> > +
> > +Data Structures
> > +---------------
> > +
> > +So what is a Netlink attribute? In simple terms, **the Netlink attribute is the
> > +structural payload carried by the Netlink message in TLV (Type-Length-Value)
> > +format** (At least it is suggested to do so). One can straightly read the
> 
> s/straigtly/directly/
> 
> > +comment and the code in ``include/net/netlink.h`` (most of the below content is
> > +derived from there). The following graph demonstrates the structure for the
> > +majority of messages.
> > +
> > +.. code-block::
> > +
> > +   +-----------------+------------+--------------------------
> > +   | Netlink Msg Hdr | Family Hdr |  Netlink Attributes  ...
> > +   +-----------------+------------+--------------------------
> > +                     ^            ^
> > +
> > +The ``^`` part will be padded to align to ``NLMSG_ALIGNTO`` (4 bytes for the
> > +Linux kernel).
> > +
> > +The term **majority** is used here because the structure is dependent on the
> > +specific Netlink family you are dealing with. For example, the general Netlink
> > +family (NETLINK_GENERIC) puts ``struct genlmsghdr`` in the Family Hdr location
> > +and is strictly followed by specified Netlink attributes TLV. As a
> > +counterexample, the connector Netlink family (NETLINK_CONNECTOR) does not use
> > +Netlink attributes for transiting the payload, but, rather, places a naked data
> 
> Grammatically "transiting" is correct. Personally I would use "carrying" or "passing"
> or even "transmitting". Not a big deal though.
> 
> > +structure immediately after its family header ``struct cn_msg``. In general,
> > +when working with Netlink-powered code, most developers opt for Netlink
> > +attributes due to their convenience and ease of maintenance. This means it is
> > +definitely okay to overlook the corner cases which may eventually be
> > +incorporated into Netlink attributes in the future.
> > +
> > +The Netlink attribute in the Linux kernel conforms to the following structure.
> > +
> > +.. code-block:: c
> > +
> > +   // >------- nla header --------<
> > +   // +-------------+-------------+----------- - - - ----------+
> > +   // |  Attr Len   |  Attr Type  |          Attr Data         |
> > +   // +-------------+-------------+----------- - - - ----------+
> > +   // >-- 2 bytes --|-- 2 bytes --|------ Attr Len bytes ------<
> > +
> > +   struct nlattr {
> > +       __u16           nla_len;
> > +       __u16           nla_type;
> > +   };
> > +
> > +The 2 bytes attr len (\ ``nla_len``\ ) indicates the entire attribute length
> > +(includes the nla header) and the other 2 bytes attr type (\ ``nla_type``\ )
> > +is used by the kernel code to identify the expected attribute. To process
> > +these attributes, the kernel code needs to locate the specific attribute and
> > +extract the payload from it, a process known as attribute parsing. This
> > +procedure can be tedious and error-prone when done manually, so the interface
> > +provides parsers to simplify the process.
> > +
> > +*It is worth mentioning that if you choose to use general Netlink without a
> > +nested data structure, you don't even need to call any parsers as the
> > +interface already does this and your task will be simplified to handling the
> > +parsed result.*
> > +
> > +Parsers
> > +-------
> > +
> > +There are several parsers available, each designed to handle a specific type
> > +of object being parsed. If you have a pointer to a Netlink message,
> > +specifically a (\ ``struct nlmsghdr *``\ ), it's likely that you'll want to
> > +call the ``nlmsg_parse`` function. The prototype for this function is as
> > +follows:
> > +
> > +.. kernel-doc:: include/net/netlink.h
> > +   :identifiers: nlmsg_parse
> > +
> > +Otherwise, if you have a pointer to Netlink attribute (\ ``struct nlattr *``\ )
> > +already, the ``nla_parse``\ , or  ``nla_parse_nested`` may be used.
> > +
> > +.. kernel-doc:: include/net/netlink.h
> > +   :identifiers: nla_parse
> > +
> > +.. kernel-doc:: include/net/netlink.h
> > +   :identifiers: nla_parse_nested
> > +
> > +Upon closer inspection, one will notice that the parameters for the various
> > +parsers bear a striking resemblance to one another. In fact, they share a
> > +commonality that goes beyond mere coincidence, as they all ultimately call
> > +upon the same internal parsing helper function, namely ``__nla_parse``.
> > +
> > +.. code-block:: c
> > +
> > +   int __nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
> > +           int len, const struct nla_policy *policy, unsigned int validate,
> > +           struct netlink_ext_ack *extack);
> > +
> > +The idea is straightforward since we know that adding an offset to either
> > +the Netlink message header or the nested attribute will yield the TLV format
> > +of the attributes. On this basis, we will learn how to utilize those parsers.
> > +
> > +The parser functions mentioned above require two inputs from the user: a
> > +destination array, ``tb``, and a maximum attribute type, ``maxtype``. Rest
> > +assured that the parsers will take care of clearing the buffer by calling
> 
> I would drop the "Rest assured that".
> 
> > +memset, so you don't need to worry about any dirty data.  After the parsing,
> > +parsers will store the parsed pointer in the provided array, allowing you to
> > +easily access the parsed data later on.  For clarity, you can refer to the
> > +example below to see how the parsed result is organized.
> > +
> > +.. code-block:: c
> > +
> > +   enum {
> > +       EXAMPLEA_1,
> > +       EXAMPLEA_2,
> > +       EXAMPLEA_3,
> > +       EXAMPLEA_4,
> > +       __EXAMPLEA_MAX
> > +
> > +   #define EXAMPLEA_MAX (__EXAMPLEA_MAX - 1)
> > +   };
> > +
> > +   /*
> > +    *     attributes being parsed and pointers:
> > +    *
> > +    *     +---+------------+- - -+---+------------+- - -+---+------------+- - -+
> > +    *     | X | EXAMPLEA_1 | ... | Y | EXAMPLEA_4 | ... | Z | EXAMPLEA_2 | ... |
> > +    *     +---+------------+- - -+---+------------+- - -+---+------------+- - -+
> > +    *     ^                      ^                      ^
> > +    *     |                      |                      |
> > +    *     ptr1                   |                      |
> > +    *     ptr2 -----------------------------------------+
> > +    *     ptr3 = NULL            |
> > +    *     ptr4 ------------------+
> > +    */
> > +
> > +   struct nlattr *tb[EXAMPLEA_MAX+1];
> > +   nla_parse(tb, EXAMPLEA_MAX, ....);
> > +   // After the parser returns, the tb contains the data shown below,
> > +   // with each pointer holding the value indicated in the above diagram.
> > +   // tb == {
> > +   //      ptr1, // EXAMPLEA_1
> > +   //      ptr2, // EXAMPLEA_2
> > +   //      NULL, // EXAMPLEA_3
> > +   //      ptr4, // EXAMPLEA_4
> > +   // };
> > +
> > +The Linux kernel's attribute parsing mechanism is notably flexible, enabling
> > +attributes to be presented in any order, regardless of their specific type
> > +(e.g., ``EXAMPLE_4`` can precede ``EXAMPLE_2``). This flexibility is not always
> > +the case in other Netlink implementations, such as FreeBSD, where attributes
> > +may be required to follow a specific order.
> > +
> > +With the parsing process complete, the kernel code can readily access the
> > +parsed attribute using the attribute pointer. To illustrate, let's say we
> > +anticipate the ``EXAMPLEA_2`` attribute to hold a ``u16`` value. In that case,
> > +we can derive it in the following manner:
> > +
> > +.. code-block:: c
> > +
> > +   u16 val;
> > +   if (tb[EXAMPLEA_2]) // check if user provides attribute EXAMPLEA_2
> > +       val = nla_get_u16(tb[EXAMPLEA_2]); // dereference it
> > +
> > +There are multiple helpers available to help us to derive the attribute, such
> > +as ``nla_get_u8/16/32/64``\ , ``nla_get_s8/16/32/64``\ , ``nla_get_flag``\ ,
> > +``nla_get_msecs``.
> > +
> > +Looks pretty good, right? However, the code still has a potential flaw, as it
> > +naively assumes that the user has indeed placed a ``u16`` value in the payload.
> > +We'll discuss this issue further in the #2 best practice.
> > +
> > +FOO example
> > +-----------
> > +
> > +In this part we will make a horizontal comparison to help readers better
> > +understand how to write a Netlink code to parse the user provided data. To
> 
>               drop the 'a' ^
> 
> > +begin with, let's take a look at how this is accomplished in ``ioctl``.
> > +
> > +.. code-block:: c
> > +
> > +   struct foo_d1 {
> > +       u32 a;
> > +       u32 b;
> > +   };
> > +
> > +   struct foo_d2 {
> > +       u32 c;
> > +       u32 d;
> > +       u8  haschild;
> > +       struct foo_d1 child; // optional
> > +   }
> > +
> > +   static long foo_ioctl(
> > +       struct file *flip, unsigned int cmd, unsigned long arg)
> > +   {
> > +       u32 s = offsetof(struct foo_d2, child);
> > +       struct foo_d2 *d2 = kmalloc(sizeof(struct foo_d2), GFP_KERNEL);
> > +       if (!d2) return -ENOMEM;
> > +
> > +       copy_from_user(d2, arg, s);
> > +       // access data like d2->c, d2->d
> > +
> > +       if (d2->haschild) {
> > +           copy_from_user(&d2->child, arg + s, sizeof(struct foo_d1));
> > +           // access data like d2->child.a, d2->child.b
> > +       }
> > +       // ...
> > +   }
> > +
> > +The ``ioctl`` code above reveals that two calls to ``copy_from_user`` are made,
> > +facilitating the transfer of data from user space to kernel space. After these
> > +calls, the code can manipulate values using pointers.
> > +
> > +In contrast, the second snippet showcases a similar solution when utilizing the
> > +Netlink interface. There are multiple ways to use the attributes to
> > +achieve the same outcome, the snippet below illustrates just one possible
> 
> s/,/;/                     ^
> 
> > +approach.
> > +
> > +.. code-block:: c
> > +
> > +
> > +   enum {
> > +       FOOCHILD_A,
> > +       FOOCHILD_B,
> > +       __FOOCHILD_MAX,
> > +
> > +   #define FOOCHILD_MAX (__FOOCHILD_MAX - 1)
> > +   };
> > +
> > +   enum {
> > +       FOOA_C,
> > +       FOOA_D,
> > +       FOOA_CHILD,
> > +       __FOOA_MAX,
> > +
> > +   #define FOOA_MAX (__FOOA_MAX - 1)
> > +   };
> > +
> > +   // just an example, different consumers have different handler prototype
> > +   static int foo_handler(struct nlmsghdr *nlh, struct netlink_ext_ack *extack)
> > +   {
> > +       struct nlattr *attrs[FOOA_MAX+1];
> > +       int err;
> > +       // ...
> > +       err = nlmsg_parse(nlh, FOOA_MAX, attrs, ..., extack);
> > +       // ...
> > +       if (!attrs[FOOA_C] || !attrs[FOOA_D])
> > +           return -EINVAL;
> > +       // access data via
> > +       // nla_get_u32(attrs[FOOA_C]) and nla_get_u32(attrs[FOOA_D])
> > +
> > +       // access child as nested
> > +       if (attrs[FOOA_CHILD])
> > +           err = foo_handler_internal(attrs[FOOA_ARRAY], extack);
> > +   }
> > +
> > +   static int foo_handler_internal(struct nlattr *nla,
> > +                                   struct netlink_ext_ack *extack)
> > +   {
> > +       struct nlattr *attrs[FOOCHILD_MAX+1];
> > +       int err;
> > +       // ...
> > +       err = nla_parse_nested(attrs, FOOCHILD_MAX, nla, ..., extack);
> > +       // access data via
> > +       // nla_get_u32(attrs[FOOCHILD_A]), nla_get_u32(attrs[FOOCHILD_B])
> > +   }
> > +
> > +By contrast, the Netlink version provides several benefits, including improved
> > +readability, thanks to the use of enums that clearly identify each value, and
> > +simplified writing, as the interface takes care of transferring data to kernel
> > +space, allowing programmers to focus on other parts of the code without
> > +worrying about bugs like double fetching.
> > +
> > +Now that we've covered the background information and basic data structure,
> > +it's time to dive deeper into the best practices for handling Netlink
> > +attributes. If you're interested in learning more about the underlying code,
> > +we recommend checking out the ``lib/nlattr.c`` file for additional details.
> > +With that said, let's move on to the tips and tricks for working with Netlink
> > +attributes.
> > +
> > +Best Practices
> > +==============
> > +
> > +#1 Use provided helpers to parse and access nlattr
> > +--------------------------------------------------
> > +
> > +As mentioned above, the Netlink attributes parsing procedure operates on
> > +pointers, which is rather error-prone. To avoid introducing bugs, as well as
> > +enhance code maintainability, it's recommended to utilize the provided parsers
> > +and helpers when working with Netlink attributes.
> > +
> > +While it's generally recommended to use existing parsers to parse Netlink
> > +attributes, there may be (old) cases where kernel code needs to manually parse
> > +the attributes. We'll discuss this further later on. Regardless, it's suggested
> > +to use helpers to perform the access action for better code maintainability and
> > +reliability.
> > +
> > +For example, one should never add the offset to the attribute pointer himself.
> 
> s/himself/itself/
> 
> > +
> > +.. code-block:: c
> > +
> > +   struct nlattr *nla = ...;
> > +   u8 *p;
> > +
> > +   p = (char*)nla + sizeof(struct nlattr); //  Correct but not suggested
> > +   p = nla_data(nla);  // Correct as well as concise
> > +
> > +Several helpers are particularly useful but often overlooked, including
> > +``nla_memcpy``\ , ``nla_memdup``\ , ``nla_memcmp``\ , and ``nla_strscpy``\ ,
> > +``nla_strdup``\ , ``nla_strcmp``. These helpers can greatly simplify the
> > +process of working with Netlink attributes and reduce the risk of errors.
> > +
> > +.. code-block:: c
> > +
> > +   struct nlattr *nla = ...;
> > +   u8 buffer[XXX];
> > +
> > +   memcpy(buffer, nla_data(nla), XXX);    // Wordy and could be buggy
> > +   nla_memcpy(buffer, nla, XXX);   // Concise and also safe
> > +
> > +When you need to access a Netlink attribute, it's recommended to check if
> > +there's already a helper function available in the ``include/net/netlink.h``
> > +header file. This will help you avoid introducing complex and error-prone
> > +pointer arithmetic into your code.
> > +
> > +
> > +#2 Never access nlattr without checking
> > +---------------------------------------
> > +
> > +Let's take another look at the example code for utilizing parsers.
> > +
> > +.. code-block:: c
> > +
> > +   u16 val;
> > +   if (tb[EXAMPLEA_2])
> > +       val = nla_get_u16(tb[EXAMPLEA_2]);
> > +
> > +Recall that we noted earlier that this access code has a bug since it naively
> > +trusts the input provided and assumes that a ``u16`` value is present. Now,
> > +imagine a scenario where a malicious user deliberately provides a malformed
> > +``EXAMPLEA_2``, as shown below.
> > +
> > +.. code-block::
> > +
> > +    nla_len   nla_type
> > +   +-------+------------+- - - - - - - - - +
> > +   |   0   | EXAMPLEA_2 |  Heap Dirty Data |
> > +   +-------+------------+- - - - - - - - - +
> > +                        ^
> > +             attributes end location
> > +
> > +In this scenario, the attribute ``EXAMPLEA_2`` is positioned as the final
> > +attribute, situated adjacent to the heap dirty data (since the nlmsg is
> > +allocated with the ``GFP_KERNEL`` flag without ``GFP_ZERO``\ ). Furthermore,
> > +although the nla_type is ``EXAMPLEA_2`` as expected, the nla_len is ``0``,
> > +rather than the expected ``sizeof(u16)``, in the malformed packet.
> > +
> > +The access code erroneously believes the parsed attribute has a valid length,
> > +leading to out-of-attribute access. This has serious consequences, as it causes
> > +the heap of dirty data to be loaded into the variable ``val``\ , which can be
> > +stored in a global state and accessed from user space. This leakage of heap
> > +information can be exploited by a skilled attacker using Heap Feng Shui, then
> > +gain unauthorized access to sensitive data, potentially allowing them to bypass
> > +protections like KASLR or SLAB_HARDEN.
> > +
> > +To prevent such a problem from occurring, it's essential to verify the
> > +attribute before attempting to access it. For instance, we can modify the
> > +example code as follows:
> > +
> > +.. code-block:: c
> > +
> > +   u16 val;
> > +   if (tb[EXAMPLEA_2] \
> > +       && nla_len(tb[EXAMPLEA_2]) >= sizeof(u16)) // length check
> > +       val = nla_get_u16(tb[EXAMPLEA_2]);
> > +
> > +The similar case is the example using ``memcpy`` to access the attribute.
> > +
> > +.. code-block:: c
> > +
> > +    u8 buffer[XXX];
> > +    memcpy(buffer, nla_data(nla), XXX);
> > +
> > +The code mistakenly assumes that the ``nla`` has a length of ``XXX``\ , leading
> > +to out-of-attribute access that potentially copies dirty data to the
> > +``buffer``\ . However, ``nla_memcpy`` is immune to such bugs because it
> > +internally calls ``nla_len`` to ensure that only data within the boundary is
> > +copied. This also underscores the importance of #1 best practice.
> > +
> > +While the length check, which ensures the attribute has enough data to be
> > +accessed by `nla_get_u16`, is a good starting point, it's not the most
> > +efficient approach to perform such checks extensively throughout the code.
> > +Moreover, there are other important factors to consider beyond data length,
> > +such as value ranges, when determining the validity of an attribute. In
> > +practice, data length is just one of the basic aspects we need to examine.
> > +
> > +To achieve more convenient and flexible validation, a very important data
> > +structure named ``nla_policy`` is employed.
> > +
> > +.. code-block:: c
> > +
> > +   struct nla_policy {
> > +       u8      type;
> > +       u8      validation_type;
> > +       u16     len;
> > +       union {
> > +           /**
> > +            * ... field for advanced validation ...
> > +            */
> > +       };
> > +   };
> > +
> > +There are two ways to employ ``nla_policy``: pass the policy to the parser,
> > +which will validate the parsing process (see the ``policy`` parameter in those
> > +parsers), or actively call validation helpers like ``nlmsg_validate`` and
> > +``nla_validate`` to check the attributes. The first option is generally
> > +preferred because it ensures that the pointer in the parsed destination array
> > +points to a valid attribute that can be safely accessed, eliminating the need
> > +for an additional check. Here's an example:
> > +
> > +.. code-block:: c
> > +
> > +   struct nla_policy example_policy[EXAMPLEA_MAX + 1] = {
> > +       [EXAMPLEA_2] = { .type = NLA_U8 },
> > +   };
> > +   // ...
> > +   struct nlattr *tb[EXAMPLEA_MAX+1];
> > +   u16 value;
> > +   nla_parse(tb, EXAMPLEA_MAX, nla, ..., example_policy, extack);
> > +
> > +   u16 val;
> > +   if (tb[EXAMPLEA_2]) // if not NULL, already pass the policy length check
> > +       val = nla_get_u16(tb[EXAMPLEA_2]);  // safe
> > +
> > +In addition to ``NLA_U8``\ , there are several other defined types, such as
> > +``NLA_BINARY`` and ``NLA_STRING``, that can be used to prepare a nla_policy.
> 
>                                                                   an
> 
> > +However, two attributes deserve special attention: ``NLA_NESTED`` and
> > +``NLA_NESTED_ARRAY``. These attributes are used to validate complex structures
> > +that contain nested elements, and they are particularly useful when dealing
> > +with hierarchical data structures. For instance, we can define a ``nla_policy``
> 
>                                                                   an
> 
> > +for the Foo example above using these attributes.
> > +
> > +.. code-block:: c
> > +
> > +   struct nla_policy foo_policy_nested[FOOCHILD_MAX + 1] = {
> > +       [FOOCHILD_A] = { .type = NLA_U32 },
> > +       [FOOCHILD_B] = { .type = NLA_U32 },
> > +   };
> > +
> > +   struct nla_policy foo_policy[EXAMPLEA_MAX + 1] = {
> > +       [FOOA_C] = { .type = NLA_U32 },
> > +       [FOOA_D] = { .type = NLA_U32 },
> > +       [FOOA_CHILD] = NLA_POLICY_NESTED(foo_policy_nested),
> > +   };
> > +
> > +``nla_policy`` by no means makes the validation clean and graceful hence should
> > +be employed first than manual checks. The general Netlink interface allows the
> 
>                first rather than manual checks.
> 
> > +user to specify the policy for each handler and finish the validation before
> > +calling it, we will talk more about this next section.
> 
>            it. We
> 
> > +
> > +While using ``nla_policy`` is generally straightforward and recommended, there
> > +are some important considerations to keep in mind.
> > +
> > +First and foremost, **Don't forget to update the nla_policy when a new
> > +attribute type is introduced**. There are cases when developers, no matter for
> > +classic Netlink or general Netlink interfaces, make such mistakes. If the
> > +parsing does not follow the strict mode (which will be discussed in #3), such a
> > +mistake will result in the out-of-attribute access bug.
> > +
> > +Second, **Perform manual checks for manual parsing**. Examine the kernel code,
> > +readers can find cases that prepare ``nla_policy`` in an unconventional manner
> > +such as the following.
> > +
> > +.. code-block:: c
> > +
> > +   // drivers/net/bonding/bond_netlink.c
> > +   static const struct nla_policy bond_policy[IFLA_BOND_MAX + 1] = {
> > +       // ...
> > +       [IFLA_BOND_ARP_IP_TARGET]   = { .type = NLA_NESTED },
> > +   };
> > +
> > +The attribute ``IFLA_BOND_ARP_IP_TARGET`` is defined with the ``NLA_NESTED``
> > +type, but without a nested ``nla_policy``. While it may seem logical to one to
> > +allow nested validation for this attribute, unfortunately, it is not.
> > +The code employs an undocumented and sophisticated approach to organizing
> > +attributes, where the array index is embedded within the attribute type.
> > +Currently, there is no validation mechanism available to handle such a case.
> > +
> > +Besides, there may be other cases where the kernel code also deviates from the
> > +conventional approach of using parsers and instead manually iterates through
> > +the attributes TLV, performing manual parsing.
> > +
> > +To validate cases where manual parsing is used, manual length checks based on
> > +``nla_len`` are required, as demonstrated in the parsing code for
> > +``IFLA_BOND_ARP_IP_TARGET`` below.
> > +
> > +.. code-block:: c
> > +
> > +   if (data[IFLA_BOND_ARP_IP_TARGET]) {
> > +       // ...
> > +       nla_for_each_nested(attr, data[IFLA_BOND_ARP_IP_TARGET], rem) {
> > +           __be32 target;
> > +
> > +           // manual length check here
> > +           if (nla_len(attr) < sizeof(target))
> > +               return -EINVAL;
> > +
> > +           target = nla_get_be32(attr);
> > +           // ...
> > +
> > +``NLA_POLICY_NESTED`` is just one of many helpful tools at your disposal. If
> > +you're looking to validate the value range of an attribute, consider using
> > +``NLA_POLICY_RANGE``\ , ``NLA_POLICY_MIN``\ , and ``NLA_POLICY_MAX``\ . And if
> > +you need to perform a strict length check, ``NLA_POLICY_EXACT_LEN`` is your
> > +best bet. Simply assigning a value to len in nla_policy will ensure that the
> > +attribute length is greater than or equal to your expected value, without
> > +strictly equaling it.
> > +
> > +#3 Embrace and take advantage of the strictness
> > +-----------------------------------------------
> > +
> > +In the previous section, we talked about **strict mode**, which is related to
> > +the last best practice we'll discuss. While reading through the helpers
> > +declared in ``include/net/netlink.h``\ , readers may have noticed some of them
> > +have the ``_deprecated`` suffix, such as ``nla_parse_deprecated`` and
> > +``nlmsg_parse_deprecated``. These deprecated functions are still widely used in
> > +the kernel, with over 300 call sites. This raises some questions: why are they
> > +marked as deprecated? What's the difference between them and the non-deprecated
> > +versions? And how do we choose between them?
> > +
> > +These deprecated functions in question were introduced in commit
> > +**8cb081746c03 ("netlink: make validation more configurable for future strictness")**,
> > +which aimed to enhance the configurability of parsing strictness. This commit
> > +split the parsing strictness into several options, as detailed in the commit
> > +message.
> > +
> > +* TRAILING     - check that there's no trailing data after parsing attributes (in message or nested)
> > +* MAXTYPE      - reject attrs greater than max known type
> > +* UNSPEC       - reject attributes with ``NLA_UNSPEC`` policy entries
> > +* STRICT_ATTRS - strictly validate attribute size
> > +
> > +and one more option follows up later
> > +
> > +* NESTED       - strictly validate ``NLA_F_NESTED``
> > +
> > +After the aforementioned commit, the code now features three distinct levels of
> > +strictness: Liberal, Deprecated Strict, and Strict. By examining the code, it
> > +becomes readily apparent which options are employed by each level.
> > +
> > +.. code-block:: c
> > +
> > +   enum netlink_validation {
> > +       NL_VALIDATE_LIBERAL = 0,
> > +       NL_VALIDATE_TRAILING = BIT(0),
> > +       NL_VALIDATE_MAXTYPE = BIT(1),
> > +       NL_VALIDATE_UNSPEC = BIT(2),
> > +       NL_VALIDATE_STRICT_ATTRS = BIT(3),
> > +       NL_VALIDATE_NESTED = BIT(4),
> > +   };
> > +
> > +   #define NL_VALIDATE_DEPRECATED_STRICT (NL_VALIDATE_TRAILING |\
> > +                          NL_VALIDATE_MAXTYPE)
> > +   #define NL_VALIDATE_STRICT (NL_VALIDATE_TRAILING |\
> > +                   NL_VALIDATE_MAXTYPE |\
> > +                   NL_VALIDATE_UNSPEC |\
> > +                   NL_VALIDATE_STRICT_ATTRS |\
> > +                   NL_VALIDATE_NESTED)
> > +
> > +The ``NL_VALIDATE_UNSPEC`` is interesting and deserves a note.. In the
> > +previous part, a buggy case caused by forgetting to update the policy is
> > +mentioned. Specifically, such a case only occurs when using the deprecated
> > +level. If choosing the latest strict level, which enables the
> > +``NL_VALIDATE_UNSPEC`` option, such a case can be prevented as the newly added
> > +attribute type will be directly rejected due to the absence of the policy. This
> > +is a compelling reason to use the modern strict level instead of the older ones.
> > +
> > +While the benefits of using ``NL_VALIDATE_STRICT`` are clear, simply replacing
> > +all deprecated parsers with this new option could result in compatibility
> > +issues due to the fact that older user space code may not follow a strict
> > +implementation. To address this concern, Johannes Berg, the author of the
> > +aforementioned commit, provided a subsequent commit
> > +**56738f460841 ("netlink: add strict parsing for future attributes")**,
> > +which introduces a new field, ``strict_start_type``, in the struct nla_policy
> > +structure. This new field allows for the definition of a "boundary type" for
> > +legacy code that utilizes deprecated parsers, thereby ensuring compatibility
> > +with older code while still providing the benefits of strict parsing.
> > +
> > +Let's see a good example, such as commit
> > +**1a432018c0cd ("net/sched: flower: Allow matching on layer 2 miss")**,
> > +to see how to add a new attribute type and update the policy with
> > +``strict_start_type``.
> > +
> > +.. code-block:: diff
> > +
> > +    @@ -594,6 +594,8 @@ enum {
> > +
> > +            TCA_FLOWER_KEY_L2TPV3_SID,      /* be32 */
> > +
> > +    +       TCA_FLOWER_L2_MISS,             /* u8 */
> > +    +
> > +            __TCA_FLOWER_MAX,
> > +    };
> > +
> > +    static const struct nla_policy fl_policy[TCA_FLOWER_MAX + 1] = {
> > +    -       [TCA_FLOWER_UNSPEC]             = { .type = NLA_UNSPEC },
> > +    +       [TCA_FLOWER_UNSPEC]             = { .strict_start_type =
> > +    +                                               TCA_FLOWER_L2_MISS },
> > +            [TCA_FLOWER_CLASSID]            = { .type = NLA_U32 },
> > +            [TCA_FLOWER_INDEV]              = { .type = NLA_STRING,
> > +                                                .len = IFNAMSIZ },
> > +    @@ -720,7 +724,7 @@ static const struct nla_policy fl_policy[TCA_FLOWER_MAX + 1] = {
> > +            [TCA_FLOWER_KEY_PPPOE_SID]      = { .type = NLA_U16 },
> > +            [TCA_FLOWER_KEY_PPP_PROTO]      = { .type = NLA_U16 },
> > +            [TCA_FLOWER_KEY_L2TPV3_SID]     = { .type = NLA_U32 },
> > +    -
> > +    +       [TCA_FLOWER_L2_MISS]            = NLA_POLICY_MAX(NLA_U8, 1),
> > +    };
> > +
> > +Because enforcing strict validation to the newly added attribute
> > +``TCA_FLOWER_L2_MISS`` raises no compatibility problems, this commit specifies
> > +the ``strict_start_type`` to ``fl_policy`` (for index 0 attribute only, see the
> > +comment in ``struct nla_policy``).    This approach allows the validation
> > +process to employ the ``NL_VALIDATE_STRICT`` strictness level for
> > +``TCA_FLOWER_L2_MISS`` and any future attribute types while keeping liberal for
> > +the old attributes, thereby ensuring compatibility and adhering to practice #3.
> > +
> > +
> > +About General Netlink Case
> > +==========================
> 
> I would s/General Netlink/Generic Netlink/ throughout the document.
> 
> > +
> > +The aforementioned content primarily presents best practices based on the
> > +author's experience with classic Netlink. However, these practices may not be
> > +relevant or could potentially cause confusion when working with new code that
> > +employs general Netlink. In the following section, we will explore how users of
> > +general Netlink can address these concerns and identify additional
> > +considerations that need to be taken into account.
> > +
> > +Let's revisit the FOO example, where the general Netlink alternative would look
> > +something like this.
> > +
> > +.. code-block:: c
> > +
> > +    static const struct genl_split_ops foo_nl_ops[] = {
> > +        {
> > +            .cmd        = FOO_CMD_TEST,
> > +            .doit       = foo_nl_test_doit,
> > +            .policy     = foo_policy,
> > +            .maxattr    = EXAMPLEA_MAX,
> > +            .flags      = GENL_ADMIN_PERM,
> > +            // .validate    = GENL_DONT_VALIDATE_STRICT
> > +        },
> > +    };
> > +
> > +    int foo_nl_test_doit(struct sk_buff *skb, struct genl_info *info)
> > +    {
> > +        struct nlattr *tb_child[FOOCHILD_MAX + 1];
> > +
> > +        /* can access parsed result in info->attrs[] */
> > +        // nla_get_u32(info->attrs[FOOA_C])
> > +        if (info->attrs[FOOA_CHILD]) {
> > +            if (nla_parse_nested(tb_child, FOOCHILD_MAX,
> > +                                 info->attrs[FOOA_CHILD], foo_policy_nested, NULL))
> > +                return -EINVAL
> > +            /* can access nested parsed result in tb_child[] */
> > +            // nla_get_u32(tb_child[FOOCHILD_A])
> > +        }
> > +    }
> > +
> > +    /* omit and struct genl_family and call to genl_register_family */
> > +
> > +In contrast to the classic implementation, which primarily involves registering
> > +a callback function, general Netlink utilizes the ``genl_split_ops`` structure
> > +(or ``genl_ops``, ``genl_small_ops`` depending on the situation) to provide
> > +additional details such as privileges, concurrency, and more. In the example
> > +mentioned above, when the ``policy`` and ``maxattr`` are known, the Netlink
> > +core will perform validation and parsing tasks before invoking the ``doit``
> > +callback. For more details, refer to `Generic Netlink <https://docs.kernel.org/networking/generic_netlink.html>`_
> > +
> > +As a result, the general Netlink user inadvertently follows the second best
> > +practice once they specify the ``policy`` field. Furthermore, in the case of
> > +modern families where the specification is generated
> > +(see `Netlink spec C code generation <https://docs.kernel.org/userspace-api/netlink/c-code-gen.html>`_ ),
> > +the problem of forgetting to update the ``nla_policy`` should never occur.
> > +This feature is incredibly beneficial as it enhances maintainability. It is
> > +highly recommended to incorporate it into your Netlink code.
> > +
> > +However, the general Netlink user should still consider the #1 and #3 best
> > +practices as areas of concern.
> > +
> > +Regarding the first best practice, regardless of the interface you are currently
> > +working with, it is essential to handle Netlink attributes and make use of the
> > +available helpers. In particular, don't forget the ``nla_memcpy`` helper.
> > +
> > +Regarding the third best practice, general Netlink employs a modern strictness
> > +approach by default for parsing, utilizing the user-provided policy and
> > +maxattr, which is perfect. However, for the sake of flexibility (and
> > +compatibility), the interface also allows users to specify
> > +``GENL_DONT_VALIDATE_STRICT`` if they prefer a more liberal strictness.
> > +Additionally, there are additional validate flags available, see
> > +``genl_validate_flags`` enum.
> > +
> > +.. code-block:: c
> > +
> > +    enum genl_validate_flags {
> > +        GENL_DONT_VALIDATE_STRICT		= BIT(0),
> > +        GENL_DONT_VALIDATE_DUMP			= BIT(1),
> > +        GENL_DONT_VALIDATE_DUMP_STRICT		= BIT(2),
> > +    };
> > +
> > +To be honest, the developer should avoid those ``DONT`` flags as possible.
> 
>                                                                  if possible.
> or                                                         as much as possible.
> ??
> 
> > +However, the ``GENL_DONT_VALIDATE_STRICT`` flag, which enables liberal
> > +strictness, is used over 400 times in the Linux kernel code. This observation
> > +suggests that a significant portion of the codebase has not fully adopted
> > +modern strictness principles yet.
> > +
> > +Additionally, also illustrated by the FOO example, the user has to do local
> > +parsing when processing nested attributes. In such scenarios, please
> > +
> > +* leverage the parsers instead of manual parsing.
> > +* leverage the parsers which are not marked as deprecated to adhere to modern strictness.
> > +
> > +Summary
> > +=======
> > +
> > +This document provides an overview of the data structure and parsers for
> > +Netlink attributes, along with three best practices for parsing these
> > +attributes. The parsing discussed in this document only applies to the
> > +"RX channel" (most ``doit`` functions), where the kernel needs to parse user
> > +space-provided Netlink messages. The opposite channel, where the kernel
> > +prepares Netlink messages for user space (most ``dumpit`` functions), is not
> > +covered in this document as it is considered less prone to implementation
> > +errors.
> > +
> > +The first best practice is "use provided helper to parse and access nlattr",
> > +which can make the parsing easier and increase the code maintainability.
> > +The second one is "never access nlattr without checking", otherwise an
> > +out-of-attribute access bug could occur. The last one is "embrace and take
> > +advantage of the strictness", which has been a topic of discussion since
> > +2019 but has yet to be fully adopted.
> > +
> > +An extra section is used to discuss the general Netlink interface case.
> > +Fortunately, this interface has evolved rapidly, becoming safer and safer.
> > +The latest general Netlink allows for the generation of specifications and
> > +policy automatically from YAML, with the generated policy defaulting to modern
> > +strictness. However, users still have to concern about the #1 and #3 practices
> > +when dealing with the attributes.
> > +
> > +It's important to note that correctly parsing Netlink attributes is only the
> > +first step but definitely not the last step to keep your code away from the
> > +bugs.
> 
> Thanks.
> -- 
> ~Randy

Thanks for pointing them out. Need to express my sincere gratitude for taking the
time to review and provide feedback on this document.

I will adopt these suggestions and prepare the next version.

Thanks
Lin Ma