linux-kernel - Re: [patch V3 07/12] uaccess: Provide scoped masked user access regions

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Message-ID: <871pn122qd.ffs@tglx>
Date: Fri, 17 Oct 2025 13:21:14 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Andrew Cooper <andrew.cooper@...rix.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Cooper
 <andrew.cooper3@...rix.com>, Linus Torvalds
 <torvalds@...ux-foundation.org>, kernel test robot <lkp@...el.com>,
 Russell King <linux@...linux.org.uk>,
 linux-arm-kernel@...ts.infradead.org, x86@...nel.org, Madhavan Srinivasan
 <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Nicholas
 Piggin <npiggin@...il.com>, linuxppc-dev@...ts.ozlabs.org, Paul Walmsley
 <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
 linux-riscv@...ts.infradead.org, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>, Sven Schnelle
 <svens@...ux.ibm.com>, linux-s390@...r.kernel.org, Julia Lawall
 <Julia.Lawall@...ia.fr>, Nicolas Palix <nicolas.palix@...g.fr>, Peter
 Zijlstra <peterz@...radead.org>, Darren Hart <dvhart@...radead.org>,
 Davidlohr Bueso <dave@...olabs.net>, André Almeida
 <andrealmeid@...lia.com>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 linux-fsdevel@...r.kernel.org
Subject: Re: [patch V3 07/12] uaccess: Provide scoped masked user access
 regions

On Fri, Oct 17 2025 at 12:08, Andrew Cooper wrote:

> On 17/10/2025 11:09 am, Thomas Gleixner wrote:
>> --- a/include/linux/uaccess.h
>> +++ b/include/linux/uaccess.h
>> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
>> +for (bool ____stop = false; !____stop; ____stop = true)						\
>> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\
>> +	     !____stop; ____stop = true)							\
>> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
>> +		     ____stop = true)					\
>> +			/* Force modified pointer usage within the scope */			\
>> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\
>> +				if (1)
>> +
>
> Truly a thing of beauty.  At least the end user experience is nice.
>
> One thing to be aware of is that:
>
>     scoped_masked_user_rw_access(ptr, efault) {
>         unsafe_get_user(rval, &ptr->rval, efault);
>         unsafe_put_user(wval, &ptr->wval, efault);
>     } else {
>         // unreachable
>     }
>
> will compile.  Instead, I think you want the final line of the macro to
> be "if (0) {} else" to prevent this.

Duh. yes. But I can just remove the 'if (1)' completely. That's a
leftover from some earlier iteration of this.

> While we're on the subject, can we find some C standards people to lobby.
>
> C2Y has a proposal to introduce "if (int foo =" syntax to generalise the
> for() loop special case.  Can we please see about fixing the restriction
> of only allowing a single type per loop?   This example could be a
> single loop if it weren't for that restriction.

That'd be nice. But we can't have nice things, can we?

Thanks,

        tglx