linux-kernel - Re: [PATCH V2 2/2] mm/highmem: Lift memcpy_[to|from]

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Message-ID: <CAPcyv4hkY-9V5Rq5s=BRku2AeWYtgs9DuVXnhdEkara2NiN9Tg@mail.gmail.com>
Date:   Mon, 7 Dec 2020 15:34:44 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Weiny, Ira" <ira.weiny@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Eric Biggers <ebiggers@...nel.org>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V2 2/2] mm/highmem: Lift memcpy_[to|from]_page to core

On Mon, Dec 7, 2020 at 3:27 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Dec 07, 2020 at 02:57:03PM -0800, ira.weiny@...el.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?