Date: Mon, 2 Nov 2020 14:13:30 -0800 From: John-Mark Gurney <jmg@funkthat.com> To: "Alexander V. Chernikov" <melifaro@ipfw.ru> Cc: freebsd-arch <freebsd-arch@freebsd.org> Subject: Re: Versioning support for kernel<>userland sysctl interface Message-ID: <20201102221330.GS31099@funkthat.com> In-Reply-To: <356181604233241@mail.yandex.ru> References: <356181604233241@mail.yandex.ru>
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Alexander V. Chernikov wrote this message on Sun, Nov 01, 2020 at 12:47 +0000:
> I would like to propose a change [1] that introduces versioning support for the data structures exposed to userland by sysctl interface.
>
> We have dozens of interfaces exposing various statistics and control data by filling in and exporting structures.
> net.inet6.icmp6.stats or net.inet6.icmp6.nd6_prlist can be a good examples of such interaction.
We also need to decide the policy on dealing w/ support for these
data structures going forward... Because if we do the simple, default
policy of all userland apps can handle all structures, and kernel can
produce all structures, we now have an unbounded growth of complexity
and testing...
I do understand the desire to solve this problem, but IMO, this solution
is too simple, and dangerous to unbounded growth above.
While I do like it's simplicity, one idea that I've had, while being a
bit more complex, has the ability to handle modification in a more
compatible way.
Since we have dtrace, one of the outputs of dtrace is ctf, which allows
use to convey the type and structure information in a machine parseable
format. The idea is that each sysctl oid (that supports this) would
have the ability to fetch the ctf data for that oid. The userland would
then be able to convert the members to the local members of a similar
struct. A set of defaults could also be provided, allowing new fields
to have sane initial values.
As long as the name of a structure member is never reused for a different
meaning, this will get us most of the way there, in a much cleaner
method...
I do realize that this isn't the easiest thing, but the tools to do this
are in the tree, and would solve this problem, IMO, in a way that is a
lot more maintainable, and long term than the current proposal.
Other solution, use ctf data to produce nvlist generation/consumption
code for a structure... The data transfered would be larger, but also
more compatible...
Overall, using bare structures is an ABI compatibility nightmare that
should be fixed in a better method.
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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