Re: [PATCH v7 08/12] xen: add /buildinfo/config entry to hypervisor filesystem

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.04.2020 11:43, Julien Grall wrote:
> Hi Jan,
> 
> On 28/04/2020 09:39, Jan Beulich wrote:
>> On 28.04.2020 10:24, George Dunlap wrote:
>>>> On Apr 28, 2020, at 8:20 AM, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 27.04.2020 18:25, George Dunlap wrote:
>>>>> If Jan is OK with it simply being outside CONFIG_EXPERT, then great.  But 
>>>>> if he insists on some kind of testing for it to be outside of 
>>>>> CONFIG_EXPERT, then again, the people who want it to be security 
>>>>> supported should be the ones who do the work to make it happen.
>>>>
>>>> I don't understand this part, I'm afraid: Without a config option,
>>>> the code is going to be security supported as long as it doesn't
>>>> get marked otherwise (experimental or what not). With an option
>>>> depending on EXPERT, what would become security unsupported is the
>>>> non-default (i.e. disabled) setting. There's not a whole lot to
>>>> test there, it's merely a formal consequence of our general rules.
>>>> (Of course, over time dependencies of other code may develop on
>>>> the information being available e.g. to Dom0 userland. Just like
>>>> there's Linux userland code assuming the kernel config is
>>>> available in certain ways [I don't necessarily mean the equivalent
>>>> of hypfs here], to then use it in what I'd call abusive ways in at
>>>> least some cases.)
>>>
>>> Here’s an argument you might make:
>>>
>>> “As a member of the security team, I don’t want to be on the hook for 
>>> issuing XSAs for code which isn’t at least smoke-tested.  Therefore, I 
>>> oppose any patch adding CONFIG_HYPFS outside of CONFIG_EXPERT, *unless* 
>>> there is a concrete plan for getting regular testing for CONFIG_HYPFS=n.”
>>>
>>> I’m not saying that’s an argument you *should* make.  But personally I 
>>> don’t have a strong argument against such an argument. So, it seems to me, 
>>> if you did make it, you have a reasonable chance of carrying your point.
>>>
>>> Now consider this hypothetical universe where you made that argument and 
>>> nobody opposed it.  In order to get a particular feature (CONFIG_HYPFS=n 
>>> security supported), there is extra work that needs to be done (getting 
>>> CONFIG_HYPFS=n tested regularly).  My point was, the expectation should be 
>>> that the extra work will be done by the people who want or benefit from the 
>>> feature; the series shouldn’t be blocked until Juergen implements 
>>> CONFIG_HYPFS=n testing (since he doesn’t personally have a stake in that 
>>> feature).
>>>
>>> Now obviously, doing work to help someone else out in the community is of 
>>> course a good thing to do; it builds goodwill, uses our aggregate resources 
>>> more efficiently, and makes our community more enjoyable to work with.  But 
>>> the goodwill primarily comes from the fact that it was done as a voluntary 
>>> choice, not as a requirement.
>>>
>>> Juergen was balking at having to do what he saw as extra work to implement 
>>> CONFIG_HYPFS.  I wanted to make it clear that even though I see value in 
>>> having CONFIG_HYPFS, *he* doesn’t have to do the work if he doesn’t want to 
>>> (although it would certainly be appreciated if he did).  And this paragraph 
>>> was extending the same principle into the hypothetical universe where 
>>> someone insisted that CONFIG_HYPFS=n had to be tested before being security 
>>> supported.
>>>
>>> Hope that makes sense. :-)
>>
>> Yes, it does, thanks for the clarification. I can see what you describe
>> as a valid perspective to take, but really in my request to Jürgen I
>> took another: Now that we have Kconfig, additions of larger bodies of
>> code (possibly also just in terms of binary size) should imo generally
>> be questioned whether they want/need to be built for everyone. I.e. it
>> is not to be left to people being worried about binary sizes to arrange
>> for things to not be built, but for people contributing new but not
>> entirely essential code to consider making it option from the very
>> beginning.
> 
> I like the idea to have a more configurable Xen but this also comes at the 
> expense of the testing/support.
> 
> At the moment, we are getting around the problem by gating the new config 
> options with CONFIG_EXPERT. I have stoppped counting the number of time I 
> sweared because my config got rewritten when using 'make clean' or explain to 
> someone else how to use it.
> 
> As it stands, CONFIG_EXPERT is unusable and most likely anything behind it 
> will rot quite quickly. So if we want to add more stuff behind it, then I 
> would suggest to make it more accessible so any developper can experiment 
> with it.

This complaint is not new; what I'm missing are concrete suggestions
on how to improve the situation.

> Going forward, I would expect the embedded folks to want more part of Xen 
> configurable. Requesting them to use CONFIG_EXPERT may be an issue as this 
> means we would not security support them. At the same time, I understand that 
> exposing a CONFIG increase the testing matrix. How about declaring we are 
> supporting/testing a given set of .config? On Arm it would be defconfig and 
> tiny.

We could do this, sure, but it would end up being rather limiting at
least on the x86 side.

Considering how frequently this is coming up, perhaps instead we
should drop use of EXPERT mostly or altogether, and declare that
we're willing to live with the fallout? We could document options
or option combinations we specifically exclude from being supported
then ...

Jan