linux-kernel - Re: [PATCH] selftests: timers: Fix valid-adjtimex signed left-shift undefined behavior

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Date: Thu, 11 Apr 2024 00:13:50 +0500
From: Muhammad Usama Anjum <usama.anjum@...labora.com>
To: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>
Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>,
 Thomas Gleixner <tglx@...utronix.de>, Stephen Boyd <sboyd@...nel.org>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>, Shuah Khan <shuah@...nel.org>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Lee Jones <joneslee@...gle.com>,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] selftests: timers: Fix valid-adjtimex signed left-shift
 undefined behavior

On 4/10/24 1:22 AM, John Stultz wrote:
> So, the struct adjtimex freq field takes a signed value who's
> units are in shifted (<<16) parts-per-million.
> 
> Unfortunately for negative adjustments, the straightforward use
> of:
> 	freq = ppm<<16
> will trip undefined behavior warnings with clang:
> 
> valid-adjtimex.c:66:6: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
>         -499<<16,
>         ~~~~^
> valid-adjtimex.c:67:6: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
>         -450<<16,
>         ~~~~^
> ...
> 
> So fix our use of shifting negative values in the valid-adjtimex
> test case to use multiply by (1<<16) to avoid this.
> 
> The patch also aligns the values a bit to make it look nicer.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Stephen Boyd <sboyd@...nel.org>
> Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Lee Jones <joneslee@...gle.com>
> Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>
> Cc: linux-kselftest@...r.kernel.org
> Reported-by: Lee Jones <joneslee@...gle.com>
> Reported-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> Link: https://lore.kernel.org/lkml/0c6d4f0d-2064-4444-986b-1d1ed782135f@collabora.com/
> Signed-off-by: John Stultz <jstultz@...gle.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@...labora.com>

> ---
>  .../testing/selftests/timers/valid-adjtimex.c | 69 ++++++++++---------
>  1 file changed, 35 insertions(+), 34 deletions(-)
> 
> diff --git a/tools/testing/selftests/timers/valid-adjtimex.c b/tools/testing/selftests/timers/valid-adjtimex.c
> index 48b9a803235a..9606d45767e7 100644
> --- a/tools/testing/selftests/timers/valid-adjtimex.c
> +++ b/tools/testing/selftests/timers/valid-adjtimex.c
> @@ -62,45 +62,46 @@ int clear_time_state(void)
>  #define NUM_FREQ_OUTOFRANGE 4
>  #define NUM_FREQ_INVALID 2
>  
> +#define SHIFTED_PPM (1 << 16)
>  long valid_freq[NUM_FREQ_VALID] = {
> -	-499<<16,
> -	-450<<16,
> -	-400<<16,
> -	-350<<16,
> -	-300<<16,
> -	-250<<16,
> -	-200<<16,
> -	-150<<16,
> -	-100<<16,
> -	-75<<16,
> -	-50<<16,
> -	-25<<16,
> -	-10<<16,
> -	-5<<16,
> -	-1<<16,
> +	 -499 * SHIFTED_PPM,
> +	 -450 * SHIFTED_PPM,
> +	 -400 * SHIFTED_PPM,
> +	 -350 * SHIFTED_PPM,
> +	 -300 * SHIFTED_PPM,
> +	 -250 * SHIFTED_PPM,
> +	 -200 * SHIFTED_PPM,
> +	 -150 * SHIFTED_PPM,
> +	 -100 * SHIFTED_PPM,
> +	  -75 * SHIFTED_PPM,
> +	  -50 * SHIFTED_PPM,
> +	  -25 * SHIFTED_PPM,
> +	  -10 * SHIFTED_PPM,
> +	   -5 * SHIFTED_PPM,
> +	   -1 * SHIFTED_PPM,
>  	-1000,
> -	1<<16,
> -	5<<16,
> -	10<<16,
> -	25<<16,
> -	50<<16,
> -	75<<16,
> -	100<<16,
> -	150<<16,
> -	200<<16,
> -	250<<16,
> -	300<<16,
> -	350<<16,
> -	400<<16,
> -	450<<16,
> -	499<<16,
> +	    1 * SHIFTED_PPM,
> +	    5 * SHIFTED_PPM,
> +	   10 * SHIFTED_PPM,
> +	   25 * SHIFTED_PPM,
> +	   50 * SHIFTED_PPM,
> +	   75 * SHIFTED_PPM,
> +	  100 * SHIFTED_PPM,
> +	  150 * SHIFTED_PPM,
> +	  200 * SHIFTED_PPM,
> +	  250 * SHIFTED_PPM,
> +	  300 * SHIFTED_PPM,
> +	  350 * SHIFTED_PPM,
> +	  400 * SHIFTED_PPM,
> +	  450 * SHIFTED_PPM,
> +	  499 * SHIFTED_PPM,
>  };
>  
>  long outofrange_freq[NUM_FREQ_OUTOFRANGE] = {
> -	-1000<<16,
> -	-550<<16,
> -	550<<16,
> -	1000<<16,
> +	-1000 * SHIFTED_PPM,
> +	 -550 * SHIFTED_PPM,
> +	  550 * SHIFTED_PPM,
> +	 1000 * SHIFTED_PPM,
>  };
>  
>  #define LONG_MAX (~0UL>>1)

-- 
BR,
Muhammad Usama Anjum