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    SubjectRe: [PATCH] [iov_iter] use memmove() when copying to/from user page
    On Thu 2017-05-25 10:15:11, Eric Dumazet wrote:
    > On Thu, May 25, 2017 at 10:04 AM, Pavel Machek <pavel@ucw.cz> wrote:
    > > Hi!
    > >
    > > On Tue 2017-05-16 23:48:54, Al Viro wrote:
    > >> On Tue, May 16, 2017 at 03:15:16PM -0700, Dmitry Vyukov wrote:
    > >> > > 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.
    > >>
    > >> Which leaves us only with "will nasal demons really fly there?".
    > >
    > > It seems so:
    > >
    > > Date: Tue, 16 May 2017 14:27:34 +0200
    > > From: Alexander Potapenko <glider@google.com>
    > > Subject: [PATCH] [iov_iter] use memmove() when copying to/from user
    > > page
    > >
    > > BUG: memcpy-param-overlap in generic_perform_write+0x551/0xa20
    > > __msan_memcpy(ffff88013c6e3001, ffff88013c6e3000, 105)
    > > CPU: 0 PID: 1040 Comm: probe Not tainted 4.11.0-rc5+ #2562
    > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
    > > 01/01/2011
    > >
    > > At the very least, we do not want userland to trigger kernel
    > > BUG()s... so this needs fixes beyond documentation.
    >
    > To be fair, this BUG() only happens because Alexander added one in memcpy() ,
    > testing for the cases where memmove() should have been used.
    >
    > Kind of a debugging trap if you prefer.
    >
    > This is not something that a pristine kernel would do.

    Aha, so BUG() is not realy a problem. Problem is that memcpy() may not
    be called on overlapping regions, and may do something stupid; but we
    don't have evidence that it does.

    Pavel
    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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