linux-kernel - Re: [PATCH 0/4] selftests/nolibc: add user-space 'efault' handler

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Date:   Sun, 4 Jun 2023 21:07:25 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Willy Tarreau <w@....eu>
Cc:     Zhangjin Wu <falcon@...ylab.org>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 0/4] selftests/nolibc: add user-space 'efault' handler

On 2023-06-04 13:05:18+0200, Willy Tarreau wrote:
> Hi Zhangjin,
> 
> On Tue, May 30, 2023 at 06:47:38PM +0800, Zhangjin Wu wrote:
> > Hi, Willy, Thomas
> > 
> > This is not really for merge, but only let it work as a demo code to
> > test whether it is possible to restore the next test when there is a bad
> > pointer access in user-space [1].
> > 
> > Besides, a new 'run' command is added to 'NOLIBC_TEST' environment
> > variable or arguments to control the running iterations, this may be
> > used to test the reentrancy issues, but no failures found currently ;-)
> 
> Since the tests we're running are essentially API tests, I'm having
> a hard time seeing in which case it can be useful to repeat the tests.
> I'm not necessarily against doing it, I'm used to repeating tests for
> example in anything sensitive to timing or race conditions, it's just
> that here I'm not seeing the benefit. And the fact you found no failure
> is rather satisfying because the opposite would have surprised me.
> 
> Regarding the efault handler, I don't think it's a good idea until we
> have signal+longjmp support in nolibc. Because running different tests
> with different libcs kind of defeats the purpose of the test in the
> first place. The reason why I wanted nolibc-test to be portable to at
> least one other libc is to help the developer figure if a failure is in
> the nolibc syscall they're implementing or in the test itself. Here if
> we start to say that some parts cannot be tested similarly, the benefit
> disappears.
> 
> I mentioned previously that I'm not particularly impatient to work on
> signals and longjmp. But in parallel I understand how this can make the
> life of some developers easier and even allow to widen the spectrum of
> some tests. Thus, maybe in the end it could be beneficial to make progress
> on this front and support these. We should make sure that this doesn't
> inflate the code base however. I guess I'd be fine with ignoring libc-
> based restarts on EINTR, alt stacks and so on and keeping this minimal
> (i.e. catch a segfault/bus error/sigill in a test program, or a Ctrl-C
> in a tiny shell).
> 
> Just let us know if you think that's something you could be interested
> in exploring. There might be differences between architectures, I have
> not checked.

If the goal is to handle hard errors like segfaults more gracefully,
would it not be easier to run each testcase in a subprocess?

Then we can just check if the child exited successfully.

It should also be completely architecture agnostic.

Thomas