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Date:   Tue, 8 Dec 2020 22:54:51 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Eric Biggers <ebiggers@kernel.org>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH V2 2/2] mm/highmem: Lift memcpy_[to|from]_page to core
Message-ID: <20201208225451.GM7338@casper.infradead.org>
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On Tue, Dec 08, 2020 at 02:45:55PM -0800, Darrick J. Wong wrote:
> On Tue, Dec 08, 2020 at 10:32:34PM +0000, Matthew Wilcox wrote:
> > On Tue, Dec 08, 2020 at 02:23:10PM -0800, Dan Williams wrote:
> > > On Tue, Dec 8, 2020 at 1:51 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > >
> > > > On Tue, Dec 08, 2020 at 01:32:55PM -0800, Ira Weiny wrote:
> > > > > On Mon, Dec 07, 2020 at 03:49:55PM -0800, Dan Williams wrote:
> > > > > > On Mon, Dec 7, 2020 at 3:40 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > > > > >
> > > > > > > On Mon, Dec 07, 2020 at 03:34:44PM -0800, Dan Williams wrote:
> > > > > > > > On Mon, Dec 7, 2020 at 3:27 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > > > > > > >
> > > > > > > > > On Mon, Dec 07, 2020 at 02:57:03PM -0800, ira.weiny@intel.com wrote:
> > > > > > > > > > +static inline void memcpy_page(struct page *dst_page, size_t dst_off,
> > > > > > > > > > +                            struct page *src_page, size_t src_off,
> > > > > > > > > > +                            size_t len)
> > > > > > > > > > +{
> > > > > > > > > > +     char *dst = kmap_local_page(dst_page);
> > > > > > > > > > +     char *src = kmap_local_page(src_page);
> > > > > > > > >
> > > > > > > > > I appreciate you've only moved these, but please add:
> > > > > > > > >
> > > > > > > > >         BUG_ON(dst_off + len > PAGE_SIZE || src_off + len > PAGE_SIZE);
> > > > > > > >
> > > > > > > > I imagine it's not outside the realm of possibility that some driver
> > > > > > > > on CONFIG_HIGHMEM=n is violating this assumption and getting away with
> > > > > > > > it because kmap_atomic() of contiguous pages "just works (TM)".
> > > > > > > > Shouldn't this WARN rather than BUG so that the user can report the
> > > > > > > > buggy driver and not have a dead system?
> > > > > > >
> > > > > > > As opposed to (on a HIGHMEM=y system) silently corrupting data that
> > > > > > > is on the next page of memory?
> > > > > >
> > > > > > Wouldn't it fault in HIGHMEM=y case? I guess not necessarily...
> > > > > >
> > > > > > > I suppose ideally ...
> > > > > > >
> > > > > > >         if (WARN_ON(dst_off + len > PAGE_SIZE))
> > > > > > >                 len = PAGE_SIZE - dst_off;
> > > > > > >         if (WARN_ON(src_off + len > PAGE_SIZE))
> > > > > > >                 len = PAGE_SIZE - src_off;
> > > > > > >
> > > > > > > and then we just truncate the data of the offending caller instead of
> > > > > > > corrupting innocent data that happens to be adjacent.  Although that's
> > > > > > > not ideal either ... I dunno, what's the least bad poison to drink here?
> > > > > >
> > > > > > Right, if the driver was relying on "corruption" for correct operation.
> > > > > >
> > > > > > If corruption actual were happening in practice wouldn't there have
> > > > > > been screams by now? Again, not necessarily...
> > > > > >
> > > > > > At least with just plain WARN the kernel will start screaming on the
> > > > > > user's behalf, and if it worked before it will keep working.
> > > > >
> > > > > So I decided to just sleep on this because I was recently told to not introduce
> > > > > new WARN_ON's[1]
> > > > >
> > > > > I don't think that truncating len is worth the effort.  The conversions being
> > > > > done should all 'work'  At least corrupting users data in the same way as it
> > > > > used to...  ;-)  I'm ok with adding the WARN_ON's and I have modified the patch
> > > > > to do so while I work through the 0-day issues.  (not sure what is going on
> > > > > there.)
> > > > >
> > > > > However, are we ok with adding the WARN_ON's given what Greg KH told me?  This
> > > > > is a bit more critical than the PKS API in that it could result in corrupt
> > > > > data.
> > > >
> > > > zero_user_segments contains:
> > > >
> > > >         BUG_ON(end1 > page_size(page) || end2 > page_size(page));
> > > >
> > > > These should be consistent.  I think we've demonstrated that there is
> > > > no good option here.
> > > 
> > > True, but these helpers are being deployed to many new locations where
> > > they were not used before.
> > 
> > So what's your preferred poison?
> > 
> > 1. Corrupt random data in whatever's been mapped into the next page (which
> >    is what the helpers currently do)
> 
> Please no.
> 
> > 2. Copy less data than requested
> 
> This sounds like the germination event for a research paper showing that
> 63% of callers never notice. ;)
> 
> > 3. Crash
> 
> Useful as a debug tool?
> 
> > 4. Something else
> 
> Return bytes copied like we do for writes that didn't quite work?

... to learn that 87% of callers never check the return value, 10%
of them do the wrong thing and the remainder have never been tested?