linux-kernel - Re: [PATCH] x86/lib: Fix num_digits() signed overflow for INT

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Message-ID: <53DB5B87-BA96-44B1-AE1B-9055CB7B9350@zytor.com>
Date: Tue, 20 Jan 2026 08:23:16 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: David Desobry <david.desobry@...malgen.com>, tglx@...nel.org,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com
CC: x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/lib: Fix num_digits() signed overflow for INT_MIN

On January 20, 2026 1:42:58 AM PST, David Desobry <david.desobry@...malgen.com> 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@...malgen.com>
>---
> 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.