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SubjectRe: [PATCH v3 12/26] compat_ioctl: move more drivers to compat_ptr_ioctl
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()

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