linux-kernel - Re: [RFC] mtd: Fix error code loss in mtdchar

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Date:   Mon, 25 Sep 2023 11:28:53 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Richard Weinberger <richard@....at>
Cc:     ZhaoLong Wang <wangzhaolong1@...wei.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        chengzhihao1 <chengzhihao1@...wei.com>,
        yi zhang <yi.zhang@...wei.com>, yangerkun@...wei.com
Subject: Re: [RFC] mtd: Fix error code loss in mtdchar_read() function.

Hi Richard,

richard@....at wrote on Mon, 25 Sep 2023 11:14:40 +0200 (CEST):

> ----- Ursprüngliche Mail -----
> >> '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.  
> 
> This is a special purpose device file and not a regular file.
> Please explain in detail why this violates POSIX and which program breaks.
> 
> As pointed out by Miquel, the comment makes it clean that this behavior is
> on purpose. If we return now all of a sudden -EBADMSG for the described
> scenario we might even break existing MTD userspace.

The bugzilla link in the commit log [1] mentions:

* dd would just stop in the middle without showing errors
  -> we probably don't care, we expect the userspace to know this is
  NAND when dealing with mtd devices directly, dd is not mtd-aware
  anyway.

* ubiformat would loop forever
  -> that one needs attention I guess :)

[1] https://bugzilla.kernel.org/show_bug.cgi?id=217939

Thanks,
Miquèl