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Date:   Fri, 11 Aug 2023 10:51:59 -0700
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Subject: Re: [PATCH v3] docs: net: add netlink attrs best practices
Content-Language: en-US
To:     Lin Ma <linma@zju.edu.cn>, corbet@lwn.net, davem@davemloft.net,
        edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
        void@manifault.com, jani.nikula@intel.com, horms@kernel.org
Cc:     linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org
References: <20230811031549.2011622-1-linma@zju.edu.cn>
From:   Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <20230811031549.2011622-1-linma@zju.edu.cn>
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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@zju.edu.cn>
> ---
> 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