Re: [PATCH] x86/lib: Fix num_digits() signed overflow for INT_MIN

[David Desobry]

Fair point! I've sent a v2 that replaces the loop with a switch 
statement (using GCC ranges). It's faster, handles INT_MIN properly via 
an unsigned cast, and I've cleaned up that mobile-submission comment 
while I was at it.

Le 20/01/2026 à 17:23, H. Peter Anvin a écrit :
> On January 20, 2026 1:42:58 AM PST, David Desobry <david.desobry@xxxxxxxxxxxxx> wrote:
> > In num_digits(), the negation of the input value "val = -val"
> > causes undefined behavior when val is INT_MIN, as its absolute
> > value cannot be represented as a signed 32-bit integer.
> >
> > This leads to incorrect results (returning 2 instead of 11).
> > By promoting the value to long long before negation, we ensure
> > the absolute value is correctly handled.
> >
> > Signed-off-by: David Desobry <david.desobry@xxxxxxxxxxxxx>
> > ---
> > arch/x86/lib/misc.c | 7 ++++---
> > 1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
> > index 40b81c338ae5..c975db6ccb9f 100644
> > --- a/arch/x86/lib/misc.c
> > +++ b/arch/x86/lib/misc.c
> > @@ -8,15 +8,16 @@
> >   */
> > int num_digits(int val)
> > {
> > +	long long v = val;
> > 	long long m = 10;
> > 	int d = 1;
> >
> > -	if (val < 0) {
> > +	if (v < 0) {
> > 		d++;
> > -		val = -val;
> > +		v = -v;
> > 	}
> >
> > -	while (val >= m) {
> > +	while (v >= m) {
> > 		m *= 10;
> > 		d++;
> > 	}
> That has got to be the dumbest possible implementation of that task, bug or no bug.
>
> A switch statement would be simpler and faster.