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SubjectRe: [PATCH wireless-next 2/2] rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior
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On 2023/11/22 21:02, Dan Carpenter wrote:
> On Wed, Nov 22, 2023 at 05:02:12PM +0800, Su Hui wrote:
>> 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. Adding an u64 cast to fix this problem.
>>
>> [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@nfschina.com>
>> ---
>> drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c | 8 ++++----
>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
>> index 6df270e29e66..89713e0587b5 100644
>> --- a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
>> +++ b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
>> @@ -106,7 +106,7 @@ u32 rtl8821ae_phy_query_bb_reg(struct ieee80211_hw *hw, u32 regaddr,
>> regaddr, bitmask);
>> originalvalue = rtl_read_dword(rtlpriv, regaddr);
>> bitshift = _rtl8821ae_phy_calculate_bit_shift(bitmask);
>> - returnvalue = (originalvalue & bitmask) >> bitshift;
>> + returnvalue = (u64)(originalvalue & bitmask) >> bitshift;
> This is a right shift, not a left shift. << vs >>.

Hi,

It's same for right shift and having a really weird result.

The result of '(u32)data >> 32' is different when using different compiler.
Clang: "(unsigned int)41 >> 32" = 2077469672
Gcc: "(unsigned int)41 >> 32" = 0
>
>>
>> rtl_dbg(rtlpriv, COMP_RF, DBG_TRACE,
>> "BBR MASK=0x%x Addr[0x%x]=0x%x\n",
>> @@ -128,7 +128,7 @@ void rtl8821ae_phy_set_bb_reg(struct ieee80211_hw *hw,
>> originalvalue = rtl_read_dword(rtlpriv, regaddr);
>> bitshift = _rtl8821ae_phy_calculate_bit_shift(bitmask);
>> data = ((originalvalue & (~bitmask)) |
>> - ((data << bitshift) & bitmask));
>> + (((u64)data << bitshift) & bitmask));
> The checker is printing an accurate warning, however, I'm not sure the
> fix is correct. Obviously, shift wrapping is bad and your patch would
> eliminate that possibility. However, data is a u32 so we end up
> discarding the high 32 bits. I can imagine a different static checker
> would complain about that.

Oh, it's my negligence...

Su Hui

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