linux-kernel - [PATCH v2 2/2] wifi: rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior

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Date:   Fri, 24 Nov 2023 10:03:54 +0800
From:   Su Hui <suhui@...china.com>
To:     dan.carpenter@...aro.org, pkshih@...ltek.com, kvalo@...nel.org,
        nathan@...nel.org, ndesaulniers@...gle.com, trix@...hat.com
Cc:     Su Hui <suhui@...china.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: [PATCH v2 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@...china.com>
---
v2: 
 - fix the subject prefix problem
 - silence the warning by not return 32 bits rather than adding a type cast.(Thanks to Dan and Ping-Ke)

By the way, there some similar problems in
_rtl88e_phy_calculate_bit_shift(), _rtl92c_phy_calculate_bit_shift() and
so on...

 drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
index 6df270e29e66..52ab1b0761c0 100644
--- a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
+++ b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
@@ -31,7 +31,12 @@ static u32 _rtl8821ae_phy_calculate_bit_shift(u32 bitmask)
 {
 	u32 i = ffs(bitmask);
 
-	return i ? i - 1 : 32;
+	if (!i) {
+		WARN_ON_ONCE(1);
+		return 0;
+	}
+
+	return i - 1;
 }
 static bool _rtl8821ae_phy_bb8821a_config_parafile(struct ieee80211_hw *hw);
 /*static bool _rtl8812ae_phy_config_mac_with_headerfile(struct ieee80211_hw *hw);*/
-- 
2.30.2