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Date:   Thu, 25 Apr 2019 16:35:34 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     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: <20190425153534.GS2217@ZenIV.linux.org.uk>
References: <20190416202013.4034148-1-arnd@arndb.de>
 <20190416202701.127745-1-arnd@arndb.de>
 <20190425122153.450fc094@coco.lan>
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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
	pointer to long => needs explicit handler
	pointer to struct {void *addr; char s[11];} => needs explicit handler
	pointer to struct {int x; u64 y;} => needs explicit handler on amd64
For "just use ->unlocked_ioctl for ->ioctl" we have
	argument ignored => OK
	any arithmetical type => OK
	any pointer => instant breakage on s390, in addtion to cases that break
with compat_ptr_ioctl().

Probably some form of that ought to go into commit message for compat_ptr_ioctl()
introduction...