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

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Date:   Mon, 14 Feb 2022 12:05:00 +0100
From:   Helge Deller <deller@....de>
To:     David Laight <David.Laight@...LAB.COM>,
        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

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?

Helge