linux-kernel - Re: [PATCH 1/4] counter: Adjust final parameter type in function and signal callbacks

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Date:   Wed, 2 Nov 2022 17:30:06 -0400
From:   William Breathitt Gray <william.gray@...aro.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, linux-iio@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Sami Tolvanen <samitolvanen@...gle.com>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Patrick Havelange <patrick.havelange@...ensium.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabrice Gasnier <fabrice.gasnier@...s.st.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Julien Panis <jpanis@...libre.com>,
        David Lechner <david@...hnology.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-omap@...r.kernel.org
Subject: Re: [PATCH 1/4] counter: Adjust final parameter type in function and
 signal callbacks

On Wed, Nov 02, 2022 at 01:23:51PM -0700, Nathan Chancellor wrote:
> On Wed, Nov 02, 2022 at 12:21:23PM -0700, Kees Cook wrote:
> > On Wed, Nov 02, 2022 at 10:22:14AM -0700, Nathan Chancellor wrote:
> > > The ->signal_u32_read(), ->count_u32_read(), and ->count_u32_write()
> > > callbacks in 'struct counter_comp' expect the final parameter to have a
> > > type of 'u32' or 'u32 *' but the ops functions that are being assigned
> > > to those callbacks have an enumerated type as the final parameter. While
> > > these are compatible from an ABI perspective, they will fail the
> > > aforementioned CFI checks.
> > > 
> > > Adjust the type of the final parameter in the ->signal_read(),
> > > ->function_read(), and ->function_write() callbacks in 'struct
> > > counter_ops' and their implementations to match the prototypes in
> > > 'struct counter_comp' to clear up these warnings and CFI failures.
> > 
> > I don't understand these changes. Where do 'struct counter_comp'
> > and 'struct counter_ops' get confused? I can only find matching
> > ops/assignments/calls, so I must be missing something. This looks like
> > a loss of CFI granularity instead of having wrappers added if there is
> > an enum/u32 conversion needed somewhere.
> 
> Right, I am not the biggest fan of this change myself and it is entirely
> possible that I am misreading the warnings from the commit message but I
> do not see how
> 
>         comp_node.comp.signal_u32_read = counter->ops->signal_read;
> 
> and
> 
>         comp_node.comp.count_u32_read = counter->ops->function_read;
> 
> in counter_add_watch(),
> 
>         comp.signal_u32_read = counter->ops->signal_read;
> 
> in counter_signal_attrs_create(), and
> 
>         comp.count_u32_read = counter->ops->function_read;
>         comp.count_u32_write = counter->ops->function_write;
> 
> in counter_count_attrs_create() are currently safe under kCFI, since the
> final parameter type of the prototypes in 'struct counter_ops' does not
> match the final parameter type of the prototypes in 'struct
> counter_comp'. I would expect the indirect calls in counter_get_data()
> and counter_comp_u32_show() to fail currently.
> 
> I briefly looked at making the 'struct counter_comp' callbacks match the
> 'struct counter_ops' ones but the COUNTER_COMP macros in
> include/linux/counter.h made it seem like these callbacks might be used
> by implementations that might use different enumerated types as the
> final parameter. I can look a little closer to see if we can make
> everything match.
> 
> I am not sure how wrappers would work here, I can take a look into how
> feasible that is.
> 
> Cheers,
> Nathan

The intention of the code here is to treat the last parameter as an
makeshift generic; the u32 will always be some corresponding enum type
provided by the driver. The expectation is for drivers to define
components via respective COUNTER_COMP_* macros, such that the
assignments of the *_u32_read/*_u32_write callbacks are abstracted away
and the driver can treat the respective last parameter as of the desired
enum type.

For example, COUNTER_COMP_DIRECTION is expected to be used with enum
counter_count_direction, COUNTER_COMP_POLARITY is expected to be used
with enum counter_signal_polarity, etc.

What would be nice is if there is a way to ensure the enum type of the
last parameter of the callback provided to these COUNTER_COMP_* macros
matches the particular respective COUNTER_COMP_* macro's expectation;
e.g. we should get some sort of error if COUNTER_COMP_DIRECTION is used
for a enum counter_signal_level, etc.

William Breathitt Gray

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