[PATCH v2][next] hwmon: (aspeed-pwm-tacho) Fix -Wstringop-overflow warning in aspeed_create_fan_tach_channel()

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 14, 2023 at 01:53:08PM -0600, Gustavo A. R. Silva wrote:
> Based on the documentation below, the maximum number of Fan tach
> channels is 16:
> 
> Documentation/devicetree/bindings/hwmon/aspeed-pwm-tacho.txt:45:
>  45 - aspeed,fan-tach-ch : should specify the Fan tach input channel.
>  46                 integer value in the range 0 through 15, with 0 indicating
>  47                 Fan tach channel 0 and 15 indicating Fan tach channel 15.
>  48                 At least one Fan tach input channel is required.
> 
> However, the compiler doesn't know that, and legitimaly warns about a potential
> overwrite in array `u8 fan_tach_ch_source[16]` in `struct aspeed_pwm_tacho_data`,
> in case `index` takes a value outside the boundaries of the array:
> 
> drivers/hwmon/aspeed-pwm-tacho.c:
> 179 struct aspeed_pwm_tacho_data {
> ...
> 184         bool fan_tach_present[16];
> ...
> 193         u8 fan_tach_ch_source[16];
> 196 };
> 
> In function ?aspeed_create_fan_tach_channel?,
>     inlined from ?aspeed_create_fan? at drivers/hwmon/aspeed-pwm-tacho.c:877:2,
>     inlined from ?aspeed_pwm_tacho_probe? at drivers/hwmon/aspeed-pwm-tacho.c:936:9:
> drivers/hwmon/aspeed-pwm-tacho.c:751:49: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
>   751 |                 priv->fan_tach_ch_source[index] = pwm_source;
>       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
> drivers/hwmon/aspeed-pwm-tacho.c: In function ?aspeed_pwm_tacho_probe?:
> drivers/hwmon/aspeed-pwm-tacho.c:193:12: note: at offset [48, 255] into destination object ?fan_tach_ch_source? of size 16
>   193 |         u8 fan_tach_ch_source[16];
>       |            ^~~~~~~~~~~~~~~~~~
> 
> Fix this by sanity checking `index` before using it to index arrays of
> size 16 elements in `struct aspeed_pwm_tacho_data`. Also, pass `dev` as
> argument to function `aspeed_create_fan_tach_channel()`, and add an error
> message in case `index` is out-of-bounds, in which case return `-EINVAL`.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks for the v2! This looks good; it's able to pass back the error
now.

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook