linux-kernel - Re: [PATCH 03/16] mm: simplify gup_fast

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Date:   Sat, 1 Jun 2019 09:14:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Nicholas Piggin <npiggin@...il.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-mips@...r.kernel.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Linux-MM <linux-mm@...ck.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/16] mm: simplify gup_fast_permitted

On Sat, Jun 1, 2019 at 12:50 AM Christoph Hellwig <hch@....de> wrote:
>
> Pass in the already calculated end value instead of recomputing it, and
> leave the end > start check in the callers instead of duplicating them
> in the arch code.

Good cleanup, except it's wrong.

> -       if (nr_pages <= 0)
> +       if (end < start)
>                 return 0;

You moved the overflow test to generic code - good.

You removed the sign and zero test on nr_pages - bad.

The zero test in particular is _important_ - the GUP range operators
know and depend on the fact that they are passed a non-empty range.

The sign test it less so, but is definitely appropriate. It might be
even better to check that the "<< PAGE_SHIFT" doesn't overflow in
"long", of course, but with callers being supposed to be trusted, the
sign test at least checks for stupid underflow issues.

So at the very least that "(end < start)" needs to be "(end <=
start)", but honestly, I think the sign of the nr_pages should be
continued to be checked.

                      Linus