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

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

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.

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.


Julien Grall



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