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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 28 Aug 2020 22:15:24 +0200
Message-ID: <CAK8P3a0rWjvSTDO_p7mKDv24OcdnWvMaM+_pHPEDGZGwn5BG=Q@mail.gmail.com>
Subject: Re: [PATCH v8 3/4] mm/madvise: introduce process_madvise() syscall:
 an external memory hinting API
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
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        Suren Baghdasaryan <surenb@google.com>,
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On Fri, Aug 28, 2020 at 9:27 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 8/28/20 12:24 PM, Jens Axboe wrote:

> >> @@ -1683,8 +1683,13 @@ ssize_t import_iovec(int type, const struct
> >> iovec __user * uvector,
> >>  {
> >>         ssize_t n;
> >>         struct iovec *p;
> >> -       n = rw_copy_check_uvector(type, uvector, nr_segs, fast_segs,
> >> -                                 *iov, &p);
> >> +
> >> +       if (in_compat_syscall())
> >> +               n = compat_rw_copy_check_uvector(type, uvector, nr_segs,
> >> +                                                fast_segs, *iov, &p);
> >> +       else
> >> +               n = rw_copy_check_uvector(type, uvector, nr_segs,
> >> +                                         fast_segs, *iov, &p);
> >>         if (n < 0) {
> >>                 if (p != *iov)
> >>                         kfree(p);
> >
> > Doesn't work for the async case, where you want to be holding on to the
> > allocated iovec. But in general I think it's a good helper for the sync
> > case, which is by far the majority.
>
> Nevermind, I'm an idiot for reading this totally wrong.
>

I think you are right about the need to pick the compat vs native
behavior based on req->ctx->compat instead of in_compat_syscall()
inside of io_import_iovec().

That one can probably call a lower-level version and when all other
callers get changed to calling

ssize_t import_iovec(int type, const struct iovec __user * uvector,
                 unsigned nr_segs, unsigned fast_segs,
                 struct iovec **iov, struct iov_iter *i)
{
       return __import_iovec(type, uvector, nr_segs, fast_segs, iov,
i, in_compat_syscall());
}

      Arnd