linux-kernel - Re: [PATCH] mfd: arizona: fix undefined behavior

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Date:   Fri, 22 Mar 2019 08:57:04 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Lee Jones <lee.jones@...aro.org>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Sapthagiri Baratam <sapthagiri.baratam@...rus.com>,
        Colin Ian King <colin.king@...onical.com>,
        patches@...nsource.cirrus.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mfd: arizona: fix undefined behavior

On Fri, Mar 22, 2019 at 03:33:37PM +0100, Arnd Bergmann wrote:
> When the driver is used with a subdevice that is disabled in the
> kernel configuration, clang gets a little confused about the
> control flow and fails to notice that n_subdevs is only
> uninitialized when subdevs is NULL, and we check for that,
> leading to a false-positive warning:
> 
> drivers/mfd/arizona-core.c:1423:19: error: variable 'n_subdevs' is uninitialized when used here
>       [-Werror,-Wuninitialized]
>                               subdevs, n_subdevs, NULL, 0, NULL);
>                                        ^~~~~~~~~
> drivers/mfd/arizona-core.c:999:15: note: initialize the variable 'n_subdevs' to silence this warning
>         int n_subdevs, ret, i;
>                      ^
>                       = 0
> 
> Ideally, we would rearrange the code to avoid all those early
> initializations and have an explicit exit in each disabled case,
> but it's much easier to chicken out and add one more initialization
> here to shut up the warning.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Took me a bit to follow the flow of this function. I agree that without
restructuring it, zero initializing this variable to shut up the warning
is the path of least resistance (clang must evaluate variables in
isolation like I did until I fully read the commit message *facepalm*).

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
>  drivers/mfd/arizona-core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mfd/arizona-core.c b/drivers/mfd/arizona-core.c
> index 27b61639cdc7..0ca0fc9a67fd 100644
> --- a/drivers/mfd/arizona-core.c
> +++ b/drivers/mfd/arizona-core.c
> @@ -996,7 +996,7 @@ int arizona_dev_init(struct arizona *arizona)
>  	unsigned int reg, val;
>  	int (*apply_patch)(struct arizona *) = NULL;
>  	const struct mfd_cell *subdevs = NULL;
> -	int n_subdevs, ret, i;
> +	int n_subdevs = 0, ret, i;
>  
>  	dev_set_drvdata(arizona->dev, arizona);
>  	mutex_init(&arizona->clk_lock);
> -- 
> 2.20.0
>