Re: [PATCH] mfd: arizona: fix undefined behavior

Nathan Chancellor
2019-03-22T15:57:11Z

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@arndb.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@gmail.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
>