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Date:   Fri, 30 Jul 2021 12:30:57 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-block@vger.kernel.org
Subject: Re: [PATCH v15 01/17] block: Add bio_add_folio()
Message-ID: <YQPi8R+Mjh3MxToC@casper.infradead.org>
References: <20210719184001.1750630-1-willy@infradead.org>
 <20210719184001.1750630-2-willy@infradead.org>
 <YQO3bXpibArh37fH@T590>
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On Fri, Jul 30, 2021 at 04:25:17PM +0800, Ming Lei wrote:
> > +size_t bio_add_folio(struct bio *bio, struct folio *folio, size_t len,
> > +		size_t off)
> > +{
> > +	if (len > UINT_MAX || off > UINT_MAX)
> > +		return 0;
> 
> The added page shouldn't cross 4G boundary, so just wondering why not
> check 'if (len > UINT_MAX - off)'?

That check is going to be vulnerable to wrapping, eg
	off = 2^32, len = 512

It would be less vulnerable to wrapping if we cast both sides to
signed long.  But at that point, we're firmly into obscuring the
intent of the check.