Date:	Thu, 6 Mar 2014 00:49:09 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	Christian Riesch <christian.riesch@...cron.at>
Cc:	Amul Kumar Saha <amul.saha@...sung.com>,
	Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 RESEND 2/2] mtd: Fix the behavior of otp write if
 there is not enough room for data

Hi,

On Wed, Mar 05, 2014 at 09:50:35AM +0100, Christian Riesch wrote:
> On March 04, 2014 23:20 -0800 Brian Norris <computersforpeace@...il.com> wrote:
> >On Tue, Jan 28, 2014 at 09:29:45AM +0100, Christian Riesch wrote:
> >>An OTP write shall write as much data as possible to the OTP memory
> >>and return the number of bytes that have actually been written.
> >>If no data could be written at all due to lack of OTP memory,
> >>return -ENOSPC.
> >>
> >>Signed-off-by: Christian Riesch <christian.riesch@...cron.at>
> >>Cc: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> >>Cc: Kyungmin Park <kyungmin.park@...sung.com>
> >>Cc: Amul Kumar Saha <amul.saha@...sung.com>
> >>---
> >> drivers/mtd/chips/cfi_cmdset_0001.c |   13 +++++++++++--
> >> drivers/mtd/devices/mtd_dataflash.c |   13 +++++--------
> >> drivers/mtd/mtdchar.c               |    7 +++++++
> >> drivers/mtd/onenand/onenand_base.c  |   10 +++++++++-
> >> 4 files changed, 32 insertions(+), 11 deletions(-)
> >>
> >>diff --git a/drivers/mtd/chips/cfi_cmdset_0001.c
> >>b/drivers/mtd/chips/cfi_cmdset_0001.c index 7aa581f..cf423a6 100644
> >>--- a/drivers/mtd/chips/cfi_cmdset_0001.c
> >>+++ b/drivers/mtd/chips/cfi_cmdset_0001.c
> >>@@ -2387,8 +2387,17 @@ static int
> >>cfi_intelext_write_user_prot_reg(struct mtd_info *mtd, loff_t from,
> >> 					    size_t len, size_t *retlen,
> >> 					     u_char *buf)
> >> {
> >>-	return cfi_intelext_otp_walk(mtd, from, len, retlen,
> >>-				     buf, do_otp_write, 1);
> >>+	int ret;
> >>+
> >>+	ret = cfi_intelext_otp_walk(mtd, from, len, retlen,
> >>+				    buf, do_otp_write, 1);
> >>+
> >>+	/* if no data could be written due to lack of OTP memory,
> >>+	   return ENOSPC */
> >
> >/*
> > * Can you use this style of mult-line comments please?
> > * It's in Documentation/CodingStyle
> > */
> >
> 
> Ok, I will change that.
> 
> >>+	if (!ret && len && !(*retlen))
> >>+		return -ENOSPC;
> >
> >Couldn't (shouldn't) this check be pushed to the common
> >mtd_write_user_prot_reg() helper in mtdcore.c?
> 
> Yes, I don't see why this wouldn't work. But I thought the code
> would be easier to understand if we return the correct error code as
> soon as the error is detected, not using some additional logic in
> some other function. What do you think?

No, this is the purpose of the mtd_xxx() wrappers for the mtd->_xxx
implementations. That way we don't have to inconsistently implement the
same checks in every driver. This caught a few bugs for
mtd_{read,write}() when we unified the bounds checking, I think.

> >And once you do that, you
> >will see that cfi_intelext_write_user_prot_reg() (and other
> >mtd->_write_user_prot_reg() implementations) will never be called with
> >len == 0. So this just becomes (in mtdcore.c):
> >
> >diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
> >index 0a7d77e65335..ee6730748f7e 100644
> >--- a/drivers/mtd/mtdcore.c
> >+++ b/drivers/mtd/mtdcore.c
> >@@ -909,11 +909,16 @@ EXPORT_SYMBOL_GPL(mtd_read_fact_prot_reg);
> > int mtd_get_user_prot_info(struct mtd_info *mtd, size_t len, size_t
> >*retlen,  			   struct otp_info *buf)
> > {
> >+	int ret;
> >+
> > 	if (!mtd->_get_user_prot_info)
> > 		return -EOPNOTSUPP;
> > 	if (!len)
> > 		return 0;
> >-	return mtd->_get_user_prot_info(mtd, len, retlen, buf);
> >+	ret = mtd->_get_user_prot_info(mtd, len, retlen, buf);
> >+	if (ret)
> >+		return ret;
> >+	return !(*retlen) ? -ENOSPC: 0;
> > }
> > EXPORT_SYMBOL_GPL(mtd_get_user_prot_info);
> >
> >
> >>+
> >>+	return ret;
> >> }
> >>
> >> static int cfi_intelext_lock_user_prot_reg(struct mtd_info *mtd,
> >>diff --git a/drivers/mtd/devices/mtd_dataflash.c
> >>b/drivers/mtd/devices/mtd_dataflash.c index 09c69ce..5236d85 100644
> >>--- a/drivers/mtd/devices/mtd_dataflash.c
> >>+++ b/drivers/mtd/devices/mtd_dataflash.c
> >>@@ -545,14 +545,11 @@ static int dataflash_write_user_otp(struct
> >>mtd_info *mtd, struct dataflash	*priv = mtd->priv;
> >> 	int			status;
> >>
> >
> >I'm not sure I quite follow the logic for the following hunk. I think it
> >deserves some more explanation, either in your commit or in a comment.
> >As it stands, you're deleting a comment and potentially changing the
> >return code behavior subtly.
> >
> >>-	if (len > 64)
> >>-		return -EINVAL;
> >>-
> >>-	/* Strictly speaking, we *could* truncate the write ... but
> >>-	 * let's not do that for the only write that's ever possible.
> >>-	 */
> >>-	if ((from + len) > 64)
> >>-		return -EINVAL;
> >>+	if ((from + len) > 64) {
> >>+		len = 64 - from;
> >
> >Why are you reassigning len? Are you trying to undo the comment above,
> >so that you *can* truncate the write? (It looks like there are other
> >implmentations which will truncate the write and return -ENOSPC, FWIW.)
> 
> Currently we have two kind of implementations: We have
> implementations like this one which will refuse to write any data if
> the write requests more data to be written than space is available.
> And we have implementations like cfi_intelext_write_user_prot_reg
> that will truncate the write and write as much data that is possible
> (and return the number of bytes that actually have been written,
> -ENOSPC shall only be returned if no data could be written at all).
> 
> For a harmonization one of the implementations and their behavior
> must be changed. I chose to change it to "write as much as
> possible/truncate the write" since this is how a write should behave (http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html).
> And yes, this is why I try to undo the comment.

OK, that makes sense. Then I think you should add a comment here in
dataflash_write_user_otp() to say that you *are* truncating (or possibly
rearrange the logic?), as it's not 100% clear what you're trying to do
here. And add a small blurb to note this in the commit description. Some
version of the above two paragraphs would make a nice
addition/replacement to the patch description.

> But if you are afraid that this will break things for current users
> of the functions, I would keep the old behavior. What do you think?

No, I believe your semantics make sense, and it's not a major breakage.

> >
> >>+		if (len <= 0)
> >>+			return -ENOSPC;
> >>+	}
> >>
> >> 	/* OUT: OP_WRITE_SECURITY, 3 zeroes, 64 data-or-zero bytes
> >> 	 * IN:  ignore all
> >>diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
> >>index 0edb0ca..db99031 100644
> >>--- a/drivers/mtd/mtdchar.c
> >>+++ b/drivers/mtd/mtdchar.c
> >>@@ -323,6 +323,13 @@ static ssize_t mtdchar_write(struct file *file,
> >>const char __user *buf, size_t c default:
> >> 			ret = mtd_write(mtd, *ppos, len, &retlen, kbuf);
> >> 		}
> >>+		/* return -ENOSPC only if no data was written */
> >>+		if ((ret == -ENOSPC) && (total_retlen)) {
> >>+			ret = 0;
> >>+			retlen = 0;
> >>+			/* drop the remaining data */
> >>+			count = 0;
> >
> >This block can just be a 'break' statement, no?
> 
> No. mtdchar_write may split the write into several calls of
> mtd_write, mtd_write_user_prot_reg... It will call mtd_write,
> mtd_write_user_prot_reg as long as there is data to be written. If
> the write hits the boundary of the memory, the last call of
> mtd_write_user_prot_reg will return -ENOSPC. If this was the only
> call of mtd_write_user_prot_reg (so no data could be written at
> all), returning -ENOSPC to the user is fine. However, if data has
> been written before, we must not return -ENOSPC, we must return the
> number of bytes that have actually been written.
> 
> So at least it must be
> 
> if ((ret == -ENOSPC) && (total_retlen)) {
> 	ret = 0;

^^^ this line will have no effect, since 'ret' is not used outside the
while loop.

> 	break;
> }
> 
> Which one do you prefer?

I prefer the following :) It's fewer lines and more straightforward, I
think.

if ((ret == -ENOSPC) && total_retlen)
	break;

> >
> >>+		}
> >
> >I'm a bit wary of changing the behavior of non-OTP writes. At a minimum,
> >the patch description needs to acknowledge that this affects more than
> >just OTP writes. But after a cursory review of mtd->_write()
> >implementations, it looks like there's no driver which could be
> >returning -ENOSPC already, so this change is probably OK.
> 
> The behavior of non-OTP writes is not changed at all. At the begin
> of mtdchar_write, a check against mtd->size is done, and the write
> is truncated. Therefore, non-OTP writes will never hit the end of
> memory in the write function.

Right, and actually mtd_write() never gives -ENOSPC; it returns -EINVAL.
So this is fine.

Brian
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