linux-kernel - RE: [PATCH next v4 1/5] minmax: Add umin(a, b) and umax(a, b)

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Message-ID: <8e45b321c49b4c27a61b2db076ed5383@AcuMS.aculab.com>
Date: Fri, 12 Jan 2024 13:40:30 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Dan Carpenter' <dan.carpenter@...aro.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Linus
 Torvalds" <torvalds@...ux-foundation.org>, 'Andy Shevchenko'
	<andriy.shevchenko@...ux.intel.com>, 'Andrew Morton'
	<akpm@...ux-foundation.org>, "'Matthew Wilcox (Oracle)'"
	<willy@...radead.org>, 'Christoph Hellwig' <hch@...radead.org>, "'Jason A.
 Donenfeld'" <Jason@...c4.com>
Subject: RE: [PATCH next v4 1/5] minmax: Add umin(a, b) and umax(a, b)

From: Dan Carpenter
> Sent: 12 January 2024 12:50
> 
> On Mon, Sep 18, 2023 at 08:16:30AM +0000, David Laight wrote:
> > +/**
> > + * umin - return minimum of two non-negative values
> > + *   Signed types are zero extended to match a larger unsigned type.
> > + * @x: first value
> > + * @y: second value
> > + */
> > +#define umin(x, y)	\
> > +	__careful_cmp((x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull, <)
> 
> Why do we match "a larger unsigned type" instead of ULL_MAX?  Presumably
> it helps performance somehow...  I agree that it's probably fine but I
> would be more comfortable if it skipped UINT_MAX and jumped directly to
> ULONG_MAX.  These days 4 gigs is small potatoes.  The vmalloc() function
> can allocate 4G so we've had integer overflow bugs with this before.

The '+ 0ul*' carefully zero extend signed values without changing
unsigned values.
The compiler detects when it has zero-extended both sides and
uses the smaller compare.
In essence:
	x + 0u converts 'int' to 'unsigned int'.
		Avoids the sign extension adding 0ul on 64bit.
	x + 0ul converts a 'long' to 'unsigned long'.
		Avoids the sign extension adding 0ull on 32bit
	x + 0ull converts a 'long long' to 'unsigned long long'.
You need all three to avoid sign extensions and get an unsigned
compare.
If the type is __int128 (signed or unsigned) then nothing happens.
(which means you can still get a signed v unsigned error.)
You could add in (__uint128)0 on 64bit systems that support it,
but it is so uncommon it really isn't worth the hassle.

Unlike any kind of cast the arithmetic cannot discard high bits.
I've found a few min_t() with dubious types.
One was a real bug found by someone else at much the same time.

	David

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