linux-kernel - Re: [PATCH] backlight: pwm_bl: Fix uninitialized variable

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Message-ID: <20180723072534.GE4213@dell>
Date:   Mon, 23 Jul 2018 08:25:34 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Marcel Ziswiler <marcel.ziswiler@...adex.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "b.zolnierkie@...sung.com" <b.zolnierkie@...sung.com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "patches@...aro.org" <patches@...aro.org>
Subject: Re: [PATCH] backlight: pwm_bl: Fix uninitialized variable

On Wed, 18 Jul 2018, Daniel Thompson wrote:

> On Wed, Jul 18, 2018 at 04:55:44PM +0100, Lee Jones wrote:
> > On Wed, 18 Jul 2018, Daniel Thompson wrote:
> > 
> > > On Wed, Jul 18, 2018 at 02:08:53PM +0100, Lee Jones wrote:
> > > > > > > > No, then we are back to the initial issue of num_steps
> > > > > > > > potentially not
> > > > > > > > being initialised. We really want both of_property_read_u32() to
> > > > > > > > succeed AND num_steps to actually be set.
> > > > > > > 
> > > > > > > I also think num_steps should be pre-initialised.
> > > > > 
> > > > > Yes, I guess it definitely does not hurt.
> > > > > 
> > > > > > > Then it will only be set if of_property_read_u32() succeeds.
> > > > > 
> > > > > Yes, but we still need to check for both, the function not failing and
> > > > > num_steps to actually be non zero.
> > > > 
> > > > Why?  You don't do anything differently if it fails.
> > > 
> > > Only if you initialize num_steps...
> > > 
> > > We should either initialize to zero and not worry about the return
> > > code[1] or we check the return code and not worry about
> > > initialization[2]. I don't think both are worthwhile.
> > > 
> > > Whilst initialization can fix this specific instance we generally avoid
> > > overusing it since it messes up static analysis and, in this instance,
> > > distance from declaration to use is >25 lines, hence current patchset.
> > > 
> > > 
> > > Daniel.
> > > 
> > > 
> > > [1] https://lkml.org/lkml/2018/7/16/399
> > > [2] https://lkml.org/lkml/2018/7/16/1042
> > > 
> > > Or...
> > > 
> > > We check the return code and leave number
> > > 
> > > num_steps is uninitialized and stack allocated so it only has a valid
> > > value if of_property_read_u32() succeeds.
> > > 
> > > We can (and I originally did) fix the bug by initializing num_steps to 0
> > > but its quite some distance between declaration and use so I accepted
> > > Marcel's counter proposal to check the return code instead.
> > 
> > Only checking the return value of of_property_read_u32() is also
> > suitable.
> 
> I did think about that case... I concluded that it isn't wrong for a
> DT to set to this property to 0 (effectively meaning "no interpolated
> steps please").
> 
> If we take the branch when num_steps is zero we get a bunch of pointless
> housekeeping that amounts to no more than an extremely elaborate
> malloc/memcpy/free.

Yet in the latest patch, you do it anyway?  Or have I misread it?

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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