linux-kernel - Re: [PATCH net-next v2 4/7] net: phy: mscc: read 'vsc8531,edge-slowdown' as an u32

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Message-ID: <20180904072630.zc6sdz2xdti5nku4@qschulz>
Date:   Tue, 4 Sep 2018 09:26:30 +0200
From:   Quentin Schulz <quentin.schulz@...tlin.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     davem@...emloft.net, robh+dt@...nel.org, mark.rutland@....com,
        f.fainelli@...il.com, allan.nielsen@...rochip.com,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com,
        rf@...nsource.wolfsonmicro.com
Subject: Re: [PATCH net-next v2 4/7] net: phy: mscc: read
 'vsc8531,edge-slowdown' as an u32

Hi Andrew,

On Mon, Sep 03, 2018 at 10:05:54PM +0200, Andrew Lunn wrote:
> > Just to be sure, we're talking here about making sure the value stored
> > in the DT is not bigger than the specified value (here an u8)? If so,
> > that isn't the reason why I'm suggesting those two patches.
> > 
> > Without /bits 8/ in the DT property, whatever were the values I put in
> > the property, I'd always get a 0. So I need to fix it either in the DT
> > (but Rob does not really like it) or in the driver.
> 
> Hi Quentin
> 
> Ah, you are fixing endian issues. That was not clear to me from the
> commit message.
> 
> I don't know enough about how DT stores values in the blob. Is there
> type info? Can the DT core tell if a value in the blob is a u8 or a
> u32?  It would be nice if it warned about reading a u8 from a u32
> blob.
> 

From my quick research, the lower bound checking is performed by
of_property_read_u* functions but not the higher bound checking (the
internal function of_find_property_value_of_size allows higher bound
checking but it seems it's never used by those functions (see 0 in
sz_max of of_property_read_variable_u*_array)).

sz_max is used by of_property_read_variable_u*_array to copy at a
maximum of sz_max values in the output buffer. If sz_max is 0, it takes
sz_min so it's an array of definite size.
So since sz_max is 0 for all calls to of_property_read_variable_u*_array
by of_property_read_u*_array, we basically know we'll get a buffer of
sz_min values but we don't actually make use of the higher bound
checking of of_find_property_value_of_size.

We could enforce this higher bound check by, instead of setting sz_max
to 0, setting sz_max to sz_min in calls to of_property_read_u*_array.

But I guess there is a reason for sz_max being 0. Rob, Richard (commit
signer of this code) do you know why? Could you explain?

> Anyway, this change still removes some bounds checking. Are they
> important? Do they need to be added back?
> 

The edge-slowdown and the vddmac values are compared against a const
array so we´re fine with those ones.

For the led-X-mode, I added a constant for supported modes that gets
checked when retrieving the DT property. So we´re fine here too.

Quentin

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