[RFC] mtd: Fix error code loss in mtdchar_read() function.

[Miquel Raynal]

Hello,

Richard, your advice is welcome here.

wangzhaolong1 at huawei.com wrote on Sat, 23 Sep 2023 08:58:56 +0800:

> In the first while loop, if the mtd_read() function returns -EBADMSG

			      s/the//	     s/function//
			      ,

> and 'retlen' returns 0, the loop break and the function returns value

s/and//		remains to 0. The loop breaks and the function
returns 'total_retlen' which is 0 instead of the error code.

> 'total_retlen' is 0, not the error code.

Actually after looking at the code, I have no strong opinion
regarding whether we should return 0 or an error code in this case. 

There is this comment right above, and I'm not sure it is still up to
date because I believe many drivers just don't provide the data upon
ECC error:

                /* Nand returns -EBADMSG on ECC errors, but it returns
                 * the data. For our userspace tools it is important
                 * to dump areas with ECC errors!
                 * For kernel internal usage it also might return -EUCLEAN
                 * to signal the caller that a bitflip has occurred and has
                 * been corrected by the ECC algorithm.
                 * Userspace software which accesses NAND this way
                 * must be aware of the fact that it deals with NAND
                 */

> This problem causes the user-space program to encounter EOF when it has
> not finished reading the mtd partion, and this also violates the read
> system call standard in POSIX.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217939
> Signed-off-by: ZhaoLong Wang <wangzhaolong1 at huawei.com>
> ---
>  drivers/mtd/mtdchar.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
> index 8dc4f5c493fc..ba60dc6bef98 100644
> --- a/drivers/mtd/mtdchar.c
> +++ b/drivers/mtd/mtdchar.c
> @@ -211,7 +211,7 @@ static ssize_t mtdchar_read(struct file *file, char __user *buf, size_t count,
>  	}
>  
>  	kfree(kbuf);
> -	return total_retlen;
> +	return total_retlen ? total_retlen : ret;

This is kind of wrong, if ret is 0 then you return ret while you should
return total_retlen. In practice it does not really matter, the result
is the same, but it makes it harder to understand the code IMHO.

>  } /* mtdchar_read */
>  
>  static ssize_t mtdchar_write(struct file *file, const char __user *buf, size_t count,


Thanks,
Miquèl