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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 25 Apr 2019 17:55:23 +0200
Message-ID: <CAK8P3a2HmiYQJ2FV2FgLiFsD8M9UKteC9Jetx7ja06PQVZWYfA@mail.gmail.com>
Subject: Re: [PATCH v3 12/26] compat_ioctl: move more drivers to compat_ptr_ioctl
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
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On Thu, Apr 25, 2019 at 5:35 PM Al Viro <viro@zeniv.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, but not
copy_to_user(), which overwrites 4 bytes after the end of
the 20-byte user structure.

       Arnd