linux-kernel - RE: [PATCH] fat: Use pointer to d_name[0] in put

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Date:   Mon, 14 Feb 2022 11:11:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Helge Deller' <deller@....de>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>
Subject: RE: [PATCH] fat: Use pointer to d_name[0] in put_user() for compat
 case

From: Helge Deller
> Sent: 14 February 2022 11:05
> 
> On 2/14/22 10:26, David Laight wrote:
> > From: David Laight
> >> Sent: 14 February 2022 09:12
> >>
> >> From: Helge Deller
> >>> Sent: 13 February 2022 22:10
> >>>
> >>> The put_user(val,ptr) macro wants a pointer in the second parameter, but in
> >>> fat_ioctl_filldir() the d_name field references a whole "array of chars".
> >>> Usually the compiler automatically converts it and uses a pointer to that
> >>> array, but it's more clean to explicitly give the real pointer to where someting
> >>> is put, which is in this case the first character of the d_name[] array.
> >>
> >> That just isn't true.
> >>
> >> In C both x->char_array and &x->char_array[0] have the same type
> >> 'char *'.
> >>
> >> The 'bug' is caused by put_user() trying to do:
> >> 	__typeof__(ptr) __ptr = ptr;
> >> where __typeof__ is returning char[n] not char *.
> >>
> >> I've tried a few things but can't get __typeof__ to
> >> generate a suitable type for both a simple type and array.
> >
> > Actually the issue is that put_user() writes a single variable
> > and needs a pointer to one.
> > So changing to:
> > 	put_user(0, &array[0]);
> > is probably fine.
> 
> Ok.
> 
> > But the description is all wrong.
> 
> I agree it can be improved.
> Would you mind proposing a better description?

put_user() needs a pointer to a simple type.

	David

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