linux-kernel - Re: [PATCH wireless-next 2/2] rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior

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Message-ID: <caf95d1d-f893-4553-9d0a-91d16f04182d@suswa.mountain>
Date:   Thu, 23 Nov 2023 08:48:36 +0300
From:   Dan Carpenter <dan.carpenter@...aro.org>
To:     Su Hui <suhui@...china.com>
Cc:     pkshih@...ltek.com, kvalo@...nel.org, nathan@...nel.org,
        ndesaulniers@...gle.com, trix@...hat.com, lizetao1@...wei.com,
        linville@...driver.com, Larry.Finger@...inger.net,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH wireless-next 2/2] rtlwifi: rtl8821ae: phy: fix an
 undefined bitwise shift behavior

On Thu, Nov 23, 2023 at 09:33:06AM +0800, Su Hui wrote:
> 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@...china.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

Ah.  Sorry.  I had forgotten that it was undefined either way...

regards,
dan carpenter