linux-kernel - Re: [PATCH] vdso: Define NSEC_PER_SEC as 64-bit to prevent overflow

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Message-ID: <87ikj0esnv.ffs@tglx>
Date: Wed, 06 Aug 2025 10:55:00 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Wake Liu <wakel@...gle.com>, Andy Lutomirski <luto@...nel.org>, Vincenzo
 Frascino <vincenzo.frascino@....com>
Cc: linux-kernel@...r.kernel.org, wakel@...gle.com
Subject: Re: [PATCH] vdso: Define NSEC_PER_SEC as 64-bit to prevent overflow

On Wed, Aug 06 2025 at 00:21, Wake Liu wrote:
> The constant NSEC_PER_SEC (1,000,000,000) is defined as a long literal.
> On 32-bit systems like arm32, where long is 32 bits, this value can
> overflow when used in 64-bit calculations.

How so? Where is the overflow exactly?

The only usage of NSEC_PER_SEC in the VDSO is:

# git grep NSEC_PER_SEC lib/vdso/
lib/vdso/gettimeofday.c:        ts->tv_sec = sec + __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);

and __iter_div_u64_rem() is an inline:

static __always_inline u32
__iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)

So how exactly is this causing an overflow?

Also by your description this would be a problem throughout the kernel.

Thanks,

        tglx