Re: [PATCH][V2] x86/lib: Fix overflow of variable m when val >= 1410065408

[Su Hui]

On 2023/11/3 01:49, 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 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 long, the overhead in
> multiplication speed for a long 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 long, 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@xxxxxxxxx>
> ---
>
> V2: Make m long long instead of long to fix issue for i386 as well as
>      x86-64
> ---
>   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)

Hi,

num_digits() still has a problem when val = INT_MIN.
 14         if (val < 0) {
 15                 d++;
 16                 val = -val;
                    ^^^^^^^^^^^
When val = INT_MIN, -val still equal to INT_MIN.
Make 'val' long long instead of int to fix this issue.

Su Hui


>   {
> -	int m = 10;
> +	long long m = 10;
>   	int d = 1;
>   
>   	if (val < 0) {