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Date:   Sat, 11 Jun 2022 15:01:16 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Dominique Martinet <asmadeus@codewreck.org>,
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        linux-erofs@lists.ozlabs.org, linux-cachefs@redhat.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: mainline build failure due to 6c77676645ad ("iov_iter: Fix
 iter_xarray_get_pages{,_alloc}()")
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On Sat, Jun 11, 2022 at 07:00:52AM -0700, Guenter Roeck wrote:
> On Sat, Jun 11, 2022 at 12:56:27PM +0000, Al Viro wrote:
> > On Sat, Jun 11, 2022 at 12:37:44PM +0000, Al Viro wrote:
> > > On Sat, Jun 11, 2022 at 12:12:47PM +0000, Al Viro wrote:
> > > 
> > > 
> > > > At a guess, should be
> > > > 	return min((size_t)nr * PAGE_SIZE - offset, maxsize);
> > > > 
> > > > in both places.  I'm more than half-asleep right now; could you verify that it
> > > > (as the last lines of both iter_xarray_get_pages() and iter_xarray_get_pages_alloc())
> > > > builds correctly?
> > > 
> > > No, I'm misreading it - it's unsigned * unsigned long - unsigned vs. size_t.
> > > On arm it ends up with unsigned long vs. unsigned int; functionally it *is*
> > > OK (both have the same range there), but it triggers the tests.  Try 
> > > 
> > > 	return min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
> > > 
> > > there (both places).
> > 
> > The reason we can't overflow on multiplication there, BTW, is that we have
> > nr <= count, and count has come from weirdly open-coded
> > 	DIV_ROUND_UP(size + offset, PAGE_SIZE)
> 
> That is often done to avoid possible overflows. Is size + offset
> guaranteed to be smaller than ULONG_MAX ?

You'd need iter->count and maxsize argument to be within PAGE_SIZE of
ULONG_MAX.  How would you populate that xarray, anyway?