linux-kernel - Re: [PATCH] of: Split up name & type in modalias generation

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Date:   Fri, 7 Sep 2018 11:35:14 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] of: Split up name & type in modalias generation

On 09/07/18 07:22, Thierry Reding wrote:
> From: Thierry Reding <treding@...dia.com>
> 
> The kernel's vsnprintf() implementation discards all alpha-numeric
> characters following a %p conversion specifier. This is done in order to
> generically skip any of the various modifiers that the kernel supports.
> Unfortunately, the OF modalias is generated with a format string that
> violates the assumption made by vsnprintf():
> 
> 	of:N%pOFnT%s
> 
> While processing the above format string, vsnprintf() will eat the 'T'
> character, assuming that it belongs to the preceeding %p specifier. This
> results in a modalias with an incompatible format, which in turn causes
> the automatic loading of drivers based on modalias to no longer work.
> 
> To fix this, split up the generation of the name & type fields into two
> separate snprintf() calls to avoid confusing the parser.
> 
> Fixes: 73813f8483b1 ("of: Convert to using %pOFn instead of device_node.name")
> Signed-off-by: Thierry Reding <treding@...dia.com>
> ---
> Note that a more elegant fix would be to make the %p format specifier
> parser report back the exact number of characters consumed. I briefly
> tried to implement it, but quickly ran into numerous special cases
> that make this solution rather involved.
> 
> I can spend some more time to improve this in general if that's what we
> ultimately want, but I think this patch is a better short-term fix to
> workaround the issue.
> ---
>  drivers/of/device.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/of/device.c b/drivers/of/device.c
> index daa075d87317..dabef9fc8538 100644
> --- a/drivers/of/device.c
> +++ b/drivers/of/device.c
> @@ -218,14 +218,20 @@ static ssize_t of_device_get_modalias(struct device *dev, char *str, ssize_t len
>  	if ((!dev) || (!dev->of_node))
>  		return -ENODEV;
>  
> -	/* Name & Type */
> -	csize = snprintf(str, len, "of:N%pOFnT%s", dev->of_node,
> -			 dev->of_node->type);
> +	/* Name */
> +	csize = snprintf(str, len, "of:N%pOFn", dev->of_node);
>  	tsize = csize;
>  	len -= csize;
>  	if (str)
>  		str += csize;
>  
> +	/* Type */
> +	csize = snprintf(str, len, "T%s", dev->of_node->type);
> +	tsize += csize;
> +	len -= csize;
> +	if (str)
> +		str += csize;
> +
>  	of_property_for_each_string(dev->of_node, "compatible", p, compat) {
>  		csize = strlen(compat) + 1;
>  		tsize += csize;
> 

Reviewed-by: Frank Rowand <frank.rowand@...y.com>