linux-kernel - Re: [PATCH 0/1] Slightly relax the type checking done by min() and max().

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Message-ID: <Y4Dcnha/JYoRl4HH@smile.fi.intel.com>
Date:   Fri, 25 Nov 2022 17:17:50 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        'Joe Perches' <joe@...ches.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/1] Slightly relax the type checking done by min() and
 max().

On Fri, Nov 25, 2022 at 03:00:40PM +0000, David Laight wrote:
> The min() and max() defines include a type check to avoid the unexpected
>   behaviour when a negative value is compared against and unsigned value.
> However a lot of code hits this check and uses min_t() to avoid the error.
> Many of these are just plain wrong.
> 
> Those casting to u8 or u16 are particularly suspect, eg:
> drivers/usb/misc/usb251xb.c:528:
> 		hub->max_current_sp = min_t(u8, property_u32 / 2000, 50);
> 
> This patch does two changes:
> - Replace typeof(x) with typeof((x) + 0) to promote char/short to int.
> - Add an (int) cast to constants between 0 and MAX_INT so the compiler
>   doesn't promote the 'other side' of the comparison to an unsinged type.
>   If this is done the type test is arranged to always succeed.
> 
> The following can also be done (with some lateral thought):
> - Allow all comparisons where both types are signed. 
> - Allow all comparisons where both types are unsigned. 
> - Allow comparisons where the larger type is signed.
> 
> In addition most of the min_t() calls are there to compare a signed type
> (that holds a non-negative value) with an unsigned value.
> The definition:
> #define min_unsigned(x,y) min((x) + 0u + 0ull, (y) + 0u + 0ull)
> will do an unsigned comparision without 'accidentally' masking off
> any non-zero high bits.
> 
> With those extra changes there can be a 'duck shoot' on min_t().

You have an issue with your email setup, i.e. you send two independent messages
(not a chain). It probably shows that either you don't use `git send-email` for
sending patch, or you missed --thread option to it.

-- 
With Best Regards,
Andy Shevchenko