linux-kernel - Re: [PATCH v3 00/16] Introduce and use generic parity16/32/64 helper

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Message-ID: <20250307195310.58abff8c@pumpkin>
Date: Fri, 7 Mar 2025 19:53:10 +0000
From: David Laight <david.laight.linux@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
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Subject: Re: [PATCH v3 00/16] Introduce and use generic parity16/32/64
 helper

On Fri, 07 Mar 2025 11:30:35 -0800
"H. Peter Anvin" <hpa@...or.com> wrote:

> On March 7, 2025 10:49:56 AM PST, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
> >> (int)true most definitely is guaranteed to be 1.  
> >
> >That's not technically correct any more.
> >
> >GCC has introduced hardened bools that intentionally have bit patterns
> >other than 0 and 1.
> >
> >https://gcc.gnu.org/gcc-14/changes.html
> >
> >~Andrew  
> 
> Bit patterns in memory maybe (not that I can see the Linux kernel using them) but
> for compiler-generated conversations that's still a given, or the manager isn't C
> or anything even remotely like it.
> 

The whole idea of 'bool' is pretty much broken by design.
The underlying problem is that values other than 'true' and 'false' can
always get into 'bool' variables.

Once that has happened it is all fubar.

Trying to sanitise a value with (say):
int f(bool v)
{
	return (int)v & 1;
}    
just doesn't work (see https://www.godbolt.org/z/MEndP3q9j)

I really don't see how using (say) 0xaa and 0x55 helps.
What happens if the value is wrong? a trap or exception?, good luck recovering
from that.

	David