Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 21 Sep 2020 12:34:05 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Souptick Joarder <jrdr.linux@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, alex.bou9@gmail.com,
        gustavoars@kernel.org, Ira Weiny <ira.weiny@intel.com>,
        linux-kernel@vger.kernel.org, madhuparnabhowmik10@gmail.com,
        mporter@kernel.crashing.org, Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] mm/gup: protect unpin_user_pages() against npages==-ERRNO
Message-ID: <20200921093405.GV18329@kadam>
References: <20200916100232.GF18329@kadam>
 <20200917065706.409079-1-jhubbard@nvidia.com>
 <20200917074054.GO18329@kadam>
 <CAFqt6zYFjGTJ=KUjHd5jf-ri5tCmo4HOLP1j3nsELFgcoEoLHQ@mail.gmail.com>
 <7812dfc5-698d-e765-ad62-abf23d19d3cf@nvidia.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <7812dfc5-698d-e765-ad62-abf23d19d3cf@nvidia.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Precedence: bulk

On Sat, Sep 19, 2020 at 09:13:17PM -0700, John Hubbard wrote:
> On 9/19/20 8:03 PM, Souptick Joarder wrote:
> > On Thu, Sep 17, 2020 at 1:11 PM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> > > On Wed, Sep 16, 2020 at 11:57:06PM -0700, John Hubbard wrote:
> > > > As suggested by Dan Carpenter, fortify unpin_user_pages() just a bit,
> > > > against a typical caller mistake: check if the npages arg is really a
> > > > -ERRNO value, which would blow up the unpinning loop: WARN and return.
> > > > 
> > > > If this new WARN_ON() fires, then the system *might* be leaking pages
> > > > (by leaving them pinned), but probably not. More likely, gup/pup
> > > > returned a hard -ERRNO error to the caller, who erroneously passed it
> > > > here.
> ...
> > 
> > Do we need a similar check inside unpin_user_pages_dirty_lock(),
> > when make_dirty set to false ?
> 
> 
> Maybe not. This call is rarely if ever used for error handling, but
> rather, for finishing up a successful use of the pages.
> 
> There is a balance between protecting against buggy callers and just
> fixing any buggy callers. There is also a limit to how much code one can
> write in hopes of avoiding bugs in...code that one writes. :)  Which is
> why static analysis, unit and regression tests, code reviews are
> important too.
> 
> Here, I submit that that we're about to cross the line and go too far.
> But if you have any examples of buggy callers for
> unpin_user_pages_dirty_lock(), that might shift the line.

I checked for buggy uses of unpin_user_pages_dirty_lock() using Smatch
and didn't find anything.  (Which doesn't mean that there aren't any).

regards,
dan carpenter