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

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Date:   Mon, 14 Feb 2022 16:59:50 +0900
From:   OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:     Helge Deller <deller@....de>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-parisc@...r.kernel.org
Subject: Re: [PATCH] fat: Use pointer to d_name[0] in put_user() for compat
 case

Helge Deller <deller@....de> writes:

> 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.
>
> I noticed that issue while trying to optimize the parisc put_user() macro
> and used an intermediate variable to store the pointer. In that case I
> got this error:
>
> In file included from include/linux/uaccess.h:11,
>                  from include/linux/compat.h:17,
>                  from fs/fat/dir.c:18:
> fs/fat/dir.c: In function ‘fat_ioctl_filldir’:
> fs/fat/dir.c:725:33: error: invalid initializer
>   725 |                 if (put_user(0, d2->d_name)                     ||         \
>       |                                 ^~
> include/asm/uaccess.h:152:33: note: in definition of macro ‘__put_user’
>   152 |         __typeof__(ptr) __ptr = ptr;                            \
>       |                                 ^~~
> fs/fat/dir.c:759:1: note: in expansion of macro ‘FAT_IOCTL_FILLDIR_FUNC’
>   759 | FAT_IOCTL_FILLDIR_FUNC(fat_ioctl_filldir, __fat_dirent)
>
> The patch below cleans it up.
>
> Signed-off-by: Helge Deller <deller@....de>

Looks good.

Acked-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>

> diff --git a/fs/fat/dir.c b/fs/fat/dir.c
> index c4a274285858..249825017da7 100644
> --- a/fs/fat/dir.c
> +++ b/fs/fat/dir.c
> @@ -722,7 +722,7 @@ static int func(struct dir_context *ctx, const char *name, int name_len,   \
>  		if (name_len >= sizeof(d1->d_name))			   \
>  			name_len = sizeof(d1->d_name) - 1;		   \
>  									   \
> -		if (put_user(0, d2->d_name)			||	   \
> +		if (put_user(0, &d2->d_name[0])			||	   \
>  		    put_user(0, &d2->d_reclen)			||	   \
>  		    copy_to_user(d1->d_name, name, name_len)	||	   \
>  		    put_user(0, d1->d_name + name_len)		||	   \

-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>