linux-kernel - Re: [PATCH 1/2] ASoC: cs35l32: avoid uninitialized variable access

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Message-ID: <20160305145400.GF19428@n2100.arm.linux.org.uk>
Date:	Sat, 5 Mar 2016 14:54:01 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Mark Brown <broonie@...nel.org>,
	Brian Austin <brian.austin@...rus.com>,
	Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Paul Handrigan <Paul.Handrigan@...rus.com>,
	linux-kernel@...r.kernel.org, alsa-devel@...a-project.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] ASoC: cs35l32: avoid uninitialized variable access

On Mon, Jan 25, 2016 at 06:07:32PM +0100, Arnd Bergmann wrote:
> gcc warns about the possibilty of accessing a property read from
> devicetree in cs35l32_i2c_probe() when it has not been initialized
> because CONFIG_OF is disabled:
> 
> sound/soc/codecs/cs35l32.c: In function 'cs35l32_i2c_probe':
> sound/soc/codecs/cs35l32.c:278:2: warning: 'val' may be used uninitialized in this function [-Wmaybe-uninitialized]
> 
> The code is actually correct because it checks the dev->of_node
> variable first and we know this is NULL here, but by adding a
> check for IS_ENABLED(CONFIG_OF), we can let the compiler know
> as well, and also generate smaller object code.

No, the code is buggy, and the compiler is very correct in warning about
it.

The code there is:

        of_property_read_u32(np, "cirrus,boost-manager", &val);
        switch (val) {

of_property_read_u32() is aliased to of_property_read_u32_array() via:

static inline int of_property_read_u32(const struct device_node *np,
                                       const char *propname,
                                       u32 *out_value)
{
        return of_property_read_u32_array(np, propname, out_value, 1);
}

which does this:

int of_property_read_u32_array(const struct device_node *np,
                               const char *propname, u32 *out_values,
                               size_t sz)
{
        const __be32 *val = of_find_property_value_of_size(np, propname,
                                                (sz * sizeof(*out_values)));

        if (IS_ERR(val))
                return PTR_ERR(val);

        while (sz--)
                *out_values++ = be32_to_cpup(val++);
        return 0;
}

Note that 'out_values' is not written to if of_find_property_value_of_size()
returns an error.  Therefore, if cirrus,boost-manager is missing, the
resulting value of 'val' is left uninitialised.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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