linux-kernel - Re: [PATCH] x86/lib: Fix overflow of variable m when val >= 1410065408

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Date:   Wed, 1 Nov 2023 16:50:57 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Colin Ian King <colin.i.king@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/lib: Fix overflow of variable m when val >=
 1410065408



On 11/1/23 08:32, Colin Ian King wrote:
> There is an overflow in variable m in function num_digits when val
> is >= 1410065408 which leads to the digit calculation loop to
> iterate more times than required. This results in either more
> digits being counted or in some cases (for example where val is
> 1932683193) the value of m eventually overflows to zero and the
> while loop spins forever).
> 
> Currently the function num_digits is currently only being used for
> small values of val in the SMP boot stage for digit counting on the
> number of cpus and NUMA nodes, so the overflow is never encounterd.

                                                          encountered.

> However it is useful to fix the overflow issue in case the function
> is used for other purposes in the future. (The issue was discovered
> while investigating the digit counting performance in various
> kernel helper functions rather than any real-world use-case).
> 
> The simplest fix is to make m a long int, the overhead in
> multiplication speed for a long is very minor for small values
> of val less than 10000 on modern processors. The alternative
> fix is to replace the multiplication with a constant division
> by 10 loop (this compiles down to an multiplication and shift)
> without needing to make m a long int, but this is slightly slower
> than the fix in this commit when measured on a range of x86
> processors).
> 
> Fixes: 646e29a1789a ("x86: Improve the printout of the SMP bootup CPU table")
> Signed-off-by: Colin Ian King <colin.i.king@...il.com>

Reviewed-by: Randy Dunlap <rdunlap@...radead.org>
Tested-by: Randy Dunlap <rdunlap@...radead.org>

num_digits() now works for all int values.
Thanks.

> ---
>  arch/x86/lib/misc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
> index 92cd8ecc3a2c..41e26e246d8f 100644
> --- a/arch/x86/lib/misc.c
> +++ b/arch/x86/lib/misc.c
> @@ -8,7 +8,7 @@
>   */
>  int num_digits(int val)
>  {
> -	int m = 10;
> +	long m = 10;
>  	int d = 1;
>  
>  	if (val < 0) {

-- 
~Randy