linux-kernel - Re: [PATCH] [iov_iter] use memmove() when copying to/from user page

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Message-ID: <CACT4Y+bk_LUSxm1DrGQTpXOUtkNvhJAbxmYi67wX7+mEb3h_gA@mail.gmail.com>
Date:   Tue, 16 May 2017 15:15:16 -0700
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [iov_iter] use memmove() when copying to/from user page

On Tue, May 16, 2017 at 2:33 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, May 16, 2017 at 02:01:51PM -0700, Dmitry Vyukov wrote:
>
>> > In that case your patch does not suffice.  Overlapping move _forwards_ still
>> > yields unexpected results, doesn't it?
>>
>> Why? memmove can move both ways. Do we need to change more memcpy's to
>> memmove's?
>
> Because it's not going to be *one* call of memcpy() or memmove().  It's
> one per page.


I missed that.

I assumed that in the case of sendfile from memfd to memfd data will
be copied directly. But it goes through a pipe with multiple buffers.
Does not look easily fixable.