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Date:   Thu, 25 Apr 2019 17:42:35 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
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Subject: Re: [PATCH v3 12/26] compat_ioctl: move more drivers to
 compat_ptr_ioctl
Message-ID: <20190425164235.GT2217@ZenIV.linux.org.uk>
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On Thu, Apr 25, 2019 at 05:55:23PM +0200, Arnd Bergmann wrote:
> 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.

D'oh!  FWIW, it might be worth putting into Documentation/ somewhere;
basically, what is and what isn't biarch-neutral.

Or arch-neutral, for that matter - it's very close.  The only real
exception, IIRC, is an extra twist on m68k, where int behaves
like x86 long long - its alignment is only half its size, so
sizeof(struct {char c; int x;}) is 6, not 8 as everywhere
else.  Irrelevant for biarch, thankfully (until somebody gets insane
enough to implement 64bit coldfire, kernel port for it *and* biarch
support for m68k binaries on that thing, that is)...