Re: [PATCH 4/7] fs: Change error checking logic for fsdrv->lseek() call

[Sascha Hauer <s.hauer@pengutronix.de>]

On Tue, Jan 22, 2019 at 05:13:35PM -0800, Andrey Smirnov wrote:
> On 32-bit systems, cheking for IS_ERR_VALUE(pos) is not
> correct. Expanding that code we get (loff_t cast is added for clarity):
> 
>  (loff_t)pos >= (unsigned long)-MAX_ERRNO
> 
> given that loff_t is a 64-bit signed value, any perfectly valid seek
> offset that is greater than 0xffffc000 will result in false
> positive. Change the logic to check if position returned by
> fsdrv->lseek() is what's been requested. If it is, we can assume that
> operation was succesfull. If not, that's likely means failure and
> return value is a negative error code.
> 
> This should accomodate both 32-bit systems, where we /dev/mem doesn't
> present any range problems, as well as 64-bit systems where both file
> offset and size of /dev/mem couldn't really be correctly captured by
> loff_t and we have to rely on 2's complement and overflow.
> 
> Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
> ---
>  fs/fs.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/fs.c b/fs/fs.c
> index a304bf186..6a62fb98b 100644
> --- a/fs/fs.c
> +++ b/fs/fs.c
> @@ -405,8 +405,7 @@ loff_t lseek(int fildes, loff_t offset, int whence)
>  {
>  	struct fs_driver_d *fsdrv;
>  	FILE *f;
> -	loff_t pos;
> -	int ret;
> +	loff_t pos, ret;
>  
>  	if (check_fd(fildes))
>  		return -1;
> @@ -442,13 +441,11 @@ loff_t lseek(int fildes, loff_t offset, int whence)
>  		goto out;
>  	}
>  
> -	pos = fsdrv->lseek(&f->fsdev->dev, f, pos);
> -	if (IS_ERR_VALUE(pos)) {
> -		errno = -pos;
> -		return -1;
> -	}
> +	ret = fsdrv->lseek(&f->fsdev->dev, f, pos);
> +	if (ret != pos)
> +		goto out;

There's no point in returning the current position from fsdrv->lseek
when the desired position is already an input parameter. I think we
should change the prototype of fsdrv->lseek to just return an error code.

Sascha

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