linux-kernel - Re: [LTP] [fs] ce436509a8: ltp.openat203.fail

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Message-ID: <20200428153558.r32pgvymgk56urtu@yavin.dot.cyphar.com>
Date:   Wed, 29 Apr 2020 01:35:58 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Cyril Hrubis <chrubis@...e.cz>
Cc:     kernel test robot <rong.a.chen@...el.com>,
        Josh Triplett <josh@...htriplett.org>,
        linux-arch@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        lkp@...ts.01.org, Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        ltp@...ts.linux.it
Subject: Re: [LTP] [fs] ce436509a8: ltp.openat203.fail

On 2020-04-28, Cyril Hrubis <chrubis@...e.cz> wrote:
> Hi!
> > > > commit: ce436509a8e109330c56bb4d8ec87d258788f5f4 ("[PATCH v4 2/3] fs: openat2: Extend open_how to allow userspace-selected fds")
> > > > url: https://github.com/0day-ci/linux/commits/Josh-Triplett/Support-userspace-selected-fds/20200414-102939
> > > > base: https://git.kernel.org/cgit/linux/kernel/git/shuah/linux-kselftest.git next
> > > 
> > > This commit adds fd parameter to the how structure where LTP test was
> > > previously passing garbage, which obviously causes the difference in
> > > errno.
> > > 
> > > This could be safely ignored for now, if the patch gets merged the test
> > > needs to be updated.
> > 
> > It wouldn't be a bad idea to switch the test to figure out the ksize of
> > the struct, so that you only add bad padding after that. But then again,
> > this would be a bit ugly -- having CHECK_FIELDS would make this simpler.
> 
> Any pointers how can be the size figured out without relying on the
> E2BIG we are testing for? Does the kernel export it somewhere?

No, you would have to effectively binary search on -E2BIG at the moment.
CHECK_FIELDS is a proposal I have which would allow you to get get the
size of the in-kernel struct, but it's still a proposal.

In theory you could get the size through BTF, but it's probably more
effort than it's worth to implement that.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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