linux-kernel - Re: [PATCH] jiffies: Cast to unsigned long for secs_to

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Message-ID: <3bdffe1d-9355-4fad-8d4e-6526c094e3a3@linux.microsoft.com>
Date: Fri, 31 Jan 2025 09:59:50 -0800
From: Easwar Hariharan <eahariha@...ux.microsoft.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: eahariha@...ux.microsoft.com, Jiri Slaby <jirislaby@...nel.org>,
 David Laight <david.laight.linux@...il.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Luiz Augusto von Dentz <luiz.von.dentz@...el.com>,
 Miguel Ojeda <ojeda@...nel.org>, open list <linux-kernel@...r.kernel.org>,
 stable@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 kernel test robot <lkp@...el.com>, linux-xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH] jiffies: Cast to unsigned long for secs_to_jiffies()
 conversion

On 1/31/2025 9:55 AM, Easwar Hariharan wrote:
> On 1/31/2025 12:10 AM, Geert Uytterhoeven wrote:
>> Hi Jiri,
>>
>> CC linux-xfs
>>
>> On Fri, 31 Jan 2025 at 08:05, Jiri Slaby <jirislaby@...nel.org> wrote:
>>> On 30. 01. 25, 21:14, David Laight wrote:
>>>> On Thu, 30 Jan 2025 18:43:17 +0000
>>>> Easwar Hariharan <eahariha@...ux.microsoft.com> wrote:
>>>>
>>>>> While converting users of msecs_to_jiffies(), lkp reported that some
>>>>> range checks would always be true because of the mismatch between the
>>>>> implied int value of secs_to_jiffies() vs the unsigned long
>>>>> return value of the msecs_to_jiffies() calls it was replacing. Fix this
>>>>> by casting secs_to_jiffies() values as unsigned long.
>>>>
>>>> Surely 'unsigned long' can't be the right type ?
>>>> It changes between 32bit and 64bit systems.
>>>> Either it is allowed to wrap - so should be 32bit on both,
>>>> or wrapping is unexpected and it needs to be 64bit on both.
>>>
>>> But jiffies are really ulong.
>>
>> That's a good reason to make the change.
>> E.g. msecs_to_jiffies() does return unsigned long.
>>
>> Note that this change may cause fall-out, e.g.
>>
>>     int val = 5.
>>
>>     pr_debug("timeout = %u jiffies\n", secs_to_jiffies(val));
>>                         ^^
>>                         must be changed to %lu

That was wrong even before the conversion to secs_to_jiffies() (or this
patch) because as Jiri says, jiffies are ulong.

<snip>

- Easwar (he/him)