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@xxxxxxxxx>
Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Tested-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
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
