linux-kernel - Re: [PATCH 1/2] [PATCH 1/2] Introduce new macros min_lt and max

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Message-ID: <CAHz2CGXmo87ZNha-Kzy_1Vk8vuWg_9CY5oYqtNGdw1iat1F_5A@mail.gmail.com>
Date:	Fri, 11 Mar 2016 15:19:40 +0800
From:	Jianyu Zhan <nasa4836@...il.com>
To:	dyoung@...hat.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, d.hatayama@...fujitsu.com,
	bhe@...hat.com, vgoyal@...hat.com, kexec@...ts.infradead.org
Subject: Re: [PATCH 1/2] [PATCH 1/2] Introduce new macros min_lt and max_lt
 for comparing with larger type

On Fri, Mar 11, 2016 at 2:21 PM,  <dyoung@...hat.com> wrote:
> A useful use case for min_t and max_t is comparing two values with larger
> type. For example comparing an u64 and an u32, usually we do not want to
> truncate the u64, so we need use min_t or max_t with u64.
>
> To simplify the usage introducing two more macros min_lt and max_lt,
> 'lt' means larger type.
>
> Signed-off-by: Dave Young <dyoung@...hat.com>
> ---
>  include/linux/kernel.h |   13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> --- linux.orig/include/linux/kernel.h
> +++ linux/include/linux/kernel.h
> @@ -798,6 +798,19 @@ static inline void ftrace_dump(enum ftra
>         type __max2 = (y);                      \
>         __max1 > __max2 ? __max1: __max2; })
>
> +/*
> + * use type of larger size in min_lt and max_lt
> + */
> +#define min_lt(x, y) ({                        \
> +       int sx = sizeof(typeof(x));             \
> +       int sy = sizeof(typeof(y));             \
> +       sx > sy ? min_t(typeof(x), x, y) : min_t(typeof(y), x, y); })
> +
> +#define max_lt(x, y) ({                        \
> +       int sx = sizeof(typeof(x));             \
> +       int sy = sizeof(typeof(y));             \
> +       sx > sy ? max_t(typeof(x), x, y) : max_t(typeof(y), x, y); })
> +
>  /**
>   * clamp_t - return a value clamped to a given range using a given type
>   * @type: the type of variable to use
>
>

No no!

C standard has defined "usual arithmetic conversions" rules[1], which
decides the type promotion rules in binary operators.

The interfaces in this patch just bluntly overrides this rule to
choose the bigger type size
for operation.  Most of time it might work well, because most time the
operands used in min_t()/max_t() in Linux kernel
have same sign'ness and this rule works.

But if two operands have same size type but have different different
sign'ness,  this interfaces will exhibit Undefind Behavior,
i.e.  you choose the typeof(y) as the final type to use in operation
when they have the same type size,  so it might be signed
or unsigned, depending on the type of y.

So,  in this /proc/fs/vmcore case you should rather just explicit cast
the operand to avoid truncation.


[1] http://www.tti.unipa.it/~ricrizzo/KS/Data/PBurden/chap4.usual.conversions.html

Regards,
Jianyu Zhan