RE: [PATCH wireless-next v3 2/2] wifi: rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior

[Ping-Ke Shih]

> -----Original Message-----
> From: Su Hui <suhui@xxxxxxxxxxxx>
> Sent: Monday, November 27, 2023 9:35 AM
> To: Ping-Ke Shih <pkshih@xxxxxxxxxxx>; dan.carpenter@xxxxxxxxxx; kvalo@xxxxxxxxxx; nathan@xxxxxxxxxx;
> ndesaulniers@xxxxxxxxxx; trix@xxxxxxxxxx
> Cc: Su Hui <suhui@xxxxxxxxxxxx>; lizetao1@xxxxxxxxxx; linville@xxxxxxxxxxxxx; Larry.Finger@xxxxxxxxxxxx;
> linux-wireless@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; llvm@xxxxxxxxxxxxxxx;
> kernel-janitors@xxxxxxxxxxxxxxx
> Subject: [PATCH wireless-next v3 2/2] wifi: rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior
> 
> Clang staic checker warning:
> drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c:184:49:
>         The result of the left shift is undefined due to shifting by '32',
>         which is greater or equal to the width of type 'u32'.
>         [core.UndefinedBinaryOperatorResult]
> 
> If the value of the right operand is negative or is greater than or
> equal to the width of the promoted left operand, the behavior is
> undefined.[1][2]
> 
> For example, when using different gcc's compilation optimizaation options
> (-O0 or -O2), the result of '(u32)data << 32' is different. One is 0, the
> other is old value of data. Let _rtl8821ae_phy_calculate_bit_shift()'s
> return value less than 32 to fix this problem. Warn if bitmask is zero.
> 
> [1]:https://stackoverflow.com/questions/11270492/what-does-the-c-
> standard-say-about-bitshifting-more-bits-than-the-width-of-type
> [2]:https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
> 
> Fixes: 21e4b0726dc6 ("rtlwifi: rtl8821ae: Move driver from staging to regular tree")
> Signed-off-by: Su Hui <suhui@xxxxxxxxxxxx>

Acked-by: Ping-Ke Shih <pkshih@xxxxxxxxxxx>

> +       if (WARN_ON_ONCE(!bitmask))

I test this patch with real hardware to connect 2GHz and 5GHz AP
respectively and keep ping for a while. It doesn't throw warning
by this statement.