Re: [RFC] netlink: add variable-length / auto integers

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 11 Oct 2023 15:11:15 +0200 Johannes Berg wrote:
> > +++ b/include/uapi/linux/netlink.h
> > @@ -298,6 +298,8 @@ struct nla_bitfield32 {
> >   *	entry has attributes again, the policy for those inner ones
> >   *	and the corresponding maxtype may be specified.
> >   * @NL_ATTR_TYPE_BITFIELD32: &struct nla_bitfield32 attribute
> > + * @NL_ATTR_TYPE_SINT: 32-bit or 64-bit signed attribute, aligned to 4B
> > + * @NL_ATTR_TYPE_UINT: 32-bit or 64-bit unsigned attribute, aligned to 4B  
> 
> This is only for exposing the policy (policy description), not sure the
> alignment thing matters here?
> 
> OTOH, there's nothing in this file that ever describes *any* of the
> attributes, yet in pracice all the uapi headers do refer to NLA_U8 and
> similar - so we should probably have a new comment section here in the
> UAPI that describes the various types as used by the documentation of
> other families?
> 
> Anyway, I think some kind of bigger "careful with alignment" here would
> be good, so people do the correct thing and not just "if (big)
> nla_get_u64()" which would get the alignment thing problematic again.

I was planning to add the docs to Documentation/userspace-api/netlink/
Is that too YNL-specific?

diff --git a/Documentation/userspace-api/netlink/specs.rst b/Documentation/userspace-api/netlink/specs.rst
index cc4e2430997e..a8218284e67a 100644
--- a/Documentation/userspace-api/netlink/specs.rst
+++ b/Documentation/userspace-api/netlink/specs.rst
@@ -408,10 +408,21 @@ This section describes the attribute types supported by the ``genetlink``
 compatibility level. Refer to documentation of different levels for additional
 attribute types.
 
-Scalar integer types
+Common integer types
 --------------------
 
-Fixed-width integer types:
+``sint`` and ``uint`` represent signed and unsigned 64 bit integers.
+If the value can fit on 32 bits only 32 bits are carried in netlink
+messages, otherwise full 64 bits are carried. Note that the payload
+is only aligned to 4B, so the full 64 bit value may be unaligned!
+
+Common integer types should be preferred over fix-width types in majority
+of cases.
+
+Fix-width integer types
+-----------------------
+
+Fixed-width integer types include:
 ``u8``, ``u16``, ``u32``, ``u64``, ``s8``, ``s16``, ``s32``, ``s64``.
 
 Note that types smaller than 32 bit should be avoided as using them
@@ -421,6 +432,9 @@ See :ref:`pad_type` for padding of 64 bit attributes.
 The payload of the attribute is the integer in host order unless ``byte-order``
 specifies otherwise.
 
+64 bit values are usually aligned by the kernel but it is recommended
+that the user space is able to deal with unaligned values.
+
 .. _pad_type:
 
 pad