Date:   Mon, 2 Mar 2020 19:45:09 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Brendan Higgins <brendanhiggins@...gle.com>, g@...do-not-panic.com
Cc:     Alan Maguire <alan.maguire@...cle.com>,
        Frank Rowand <frowand.list@...il.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        David Gow <davidgow@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, rppt@...ux.ibm.com,
        Greg KH <gregkh@...uxfoundation.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Knut Omang <knut.omang@...cle.com>,
        linux-um <linux-um@...ts.infradead.org>,
        linux-arch@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        KUnit Development <kunit-dev@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v1 0/6] kunit: create a centralized executor to dispatch
 all KUnit tests

On Wed, Jan 29, 2020 at 01:28:19PM -0800, Brendan Higgins wrote:
> On Wed, Jan 29, 2020 at 5:07 AM Alan Maguire <alan.maguire@...cle.com> wrote:
> >
> > On Tue, 28 Jan 2020, Frank Rowand wrote:
> >
> > > On 1/28/20 1:19 AM, Brendan Higgins wrote:
> > > > On Mon, Jan 27, 2020 at 9:40 AM Frank Rowand <frowand.list@...il.com> wrote:
> > > >>
> > > >> On 1/23/20 4:40 PM, Brendan Higgins wrote:
> > > >>> Sorry for the late reply. I am still catching up from being on vacation.
> > > >>>>> On Mon, Jan 6, 2020 at 2:40 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
> > > >>>> It does beg the question if this means kunit is happy to not be a tool
> > > >>>> to test pre basic setup stuff (terminology used in init.c, meaning prior
> > > >>>> to running all init levels). I suspect this is the case.
> > > >>>
> > > >>> Not sure. I still haven't seen any cases where this is necessary, so I
> > > >>> am not super worried about it. Regardless, I don't think this patchset
> > > >>> really changes anything in that regard, we are moving from late_init
> > > >>> to after late_init, so it isn't that big of a change for most use
> > > >>> cases.
> > > >>>
> > > >>> Please share if you can think of some things that need to be tested in
> > > >>> early init.
> > > >>
> > > >> I don't have a specific need for this right now.  I had not thought about
> > > >> how the current kunit implementation forces all kunit tests to run at a
> > > >> specific initcall level before reading this email thread.
> > > >>
> > > >> I can see the value of being able to have some tests run at different
> > > >> initcall levels to verify what functionality is available and working
> > > >> at different points in the boot sequence.
> > > >
> > > > Let's cross that bridge when we get there. It should be fairly easy to
> > > > add that functionality.
> > >
> > > Yes. I just wanted to add the thought to the back of your mind so that
> > > it does not get precluded by future changes to the kunit architecture.
> > >
> > > >
> > > >> But more important than early initcall levels, I do not want the
> > > >> framework to prevent using or testing code and data that are marked
> > > >> as '__init'.  So it is important to retain a way to invoke the tests
> > > >> while __init code and data are available, if there is also a change
> > > >> to generally invoke the tests later.
> > > >
> > > > Definitely. For now that still works as long as you don't build KUnit
> > > > as a module, but I think Alan's new patches which allow KUnit to be
> > > > run at runtime via debugfs could cause some difficulty there. Again,
> > >
> > > Yes, Alan's patches are part of what triggered me thinking about the
> > > issues I raised.
> > >
> > >
> >
> > As Brendan says, any such tests probably shouldn't be buildable
> > as modules, but I wonder if we need to add some sort of way
> > to ensure execution from debugfs is not allowed for such cases?

The kernel's linker will ensure this doesn't happen by default, ie
__init data called from non __init code gets a complaint at linker
time today.

*Iff* you are sure the code is proper, you *whitelist* it by adding the
__ref tag to it.

> > Even if a test suite is builtin, it can be executed via debugfs
> > in the patches I sent out, allowing suites to be re-run.  Sounds
> > like we need a way to control that behaviour based on the
> > desired test suite execution environment.
> 
> I think that's true.
> 
> > Say, for example, the "struct kunit_suite" definitions associated
> > with the tests was marked as __initdata; are there any handy macros to
> > identify it as being in the __init section? If so, we could simply
> > avoid adding a "run" file to the debugfs representation for such
> > suites.

> > Failing that, perhaps we need some sort of flags field
> > in "struct kunit_suite" to specify execution environment constraints?
> 
> I think the former would be ideal, but the latter is acceptable as
> well, assuming neither results in complaints from the compiler (I
> guess we will find out for sure once we get a hold of the device tree
> KUnit test).

I'd split out tests in two different arrays, one with __init or
__initdata  one without. Likewise two dispatches, one for init and
one for non-init data.

> Luis, you mentioned your linker table work might be applicable for
> dynamic post boot configuring of dispatching. Do you think this work
> could help solve this problem?

The Linux kernel table / section ranges code helps aggregate data into
ELF sections in a generic way, that is, hacks we have been doing over
years into a generic way.

So it would be easier to read and implement. For instance see how in
this commit the intent/goal of kprobe blacklists is a bit easier to
read:

https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux-next.git/commit/?h=20170620-linker-tables-v8&id=b2662efa7c6a3c436961c07fa3082e8640f0e352

In particular DEFINE_LINKTABLE_INIT_DATA() use. I think Youd' want to
use DEFINE_LINKTABLE_INIT_DATA() for code which you want to use to
dispatch on init and and a DEFINE_LINKTABLE_DATA() for non-init code.

If a dynamic dispatcher is used you'd opt out of the using for instance
linktable_for_each() and instead use the data structure defined for
however you want to disaptch your run time.

  Luis

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