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

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Message-ID: <5511420228cb38d08a67c0f6a614b7671d7d23d4.camel@sipsolutions.net>
Date:   Thu, 25 Apr 2019 23:25:43 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Arnd Bergmann <arnd@...db.de>, Al Viro <viro@...iv.linux.org.uk>
Cc:     Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
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        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
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Subject: Re: [PATCH v3 12/26] compat_ioctl: move more drivers to
 compat_ptr_ioctl

On Thu, 2019-04-25 at 17:55 +0200, Arnd Bergmann wrote:
> On Thu, Apr 25, 2019 at 5:35 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> > 
> > On Thu, Apr 25, 2019 at 12:21:53PM -0300, Mauro Carvalho Chehab wrote:
> > 
> > > If I understand your patch description well, using compat_ptr_ioctl
> > > only works if the driver is not for s390, right?
> > 
> > No; s390 is where "oh, just set ->compat_ioctl same as ->unlocked_ioctl
> > and be done with that; compat_ptr() is a no-op anyway" breaks.  IOW,
> > s390 is the reason for having compat_ptr_ioctl() in the first place;
> > that thing works on all biarch architectures, as long as all stuff
> > handled by ->ioctl() takes pointer to arch-independent object as
> > argument.  IOW,
> >         argument ignored => OK
> >         any arithmetical type => no go, compat_ptr() would bugger it
> >         pointer to int => OK
> >         pointer to string => OK
> >         pointer to u64 => OK
> >         pointer to struct {u64 addr; char s[11];} => OK
> 
> To be extra pedantic, the 'struct {u64 addr; char s[11];} '
> case is also broken on x86, because sizeof (obj) is smaller
> on i386, even though the location of the members are
> the same. i.e. you can copy_from_user() this

Actually, you can't even do that because the struct might sit at the end
of a page and then you'd erroneously fault in this case.

We had this a while ago with struct ifreq, see commit 98406133dd and its
parents.

johannes