netdev - Re: [PATCH v3 12/26] compat_ioctl: move more drivers to compat_ptr

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Message-ID: <CAK8P3a1qCQJEkne3uAw-Ep-taNU9t5Y2ZmgoiCGmOz1BSwmzEw@mail.gmail.com>
Date:   Thu, 25 Apr 2019 17:32:05 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
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Subject: Re: [PATCH v3 12/26] compat_ioctl: move more drivers to compat_ptr_ioctl

On Thu, Apr 25, 2019 at 5:22 PM Mauro Carvalho Chehab
<mchehab+samsung@...nel.org> wrote:
> Em Tue, 16 Apr 2019 22:25:33 +0200 Arnd Bergmann <arnd@...db.de> escreveu:
>
> If I understand your patch description well, using compat_ptr_ioctl
> only works if the driver is not for s390, right?

No, the purpose of compat_ptr_ioctl() is to make sure it works
everywhere including s390.

Even on s390 it tends to work most of the time, but for correctness
the upper bit of a 32-bit pointer needs to be cleared, as
compat_ptr_ioctl does, in case some application passes a pointer
with that bit set. [IIRC, in the instruction pointer, the high bit is set, in
data references it is ignored but usually cleared, but it may be left
on for IP-relative address generation]

       Arnd