linux-kernel - Re: [patch V5 07/12] uaccess: Provide scoped user access regions

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Message-ID: <20251107191753.7433d2dc@pumpkin>
Date: Fri, 7 Nov 2025 19:17:53 +0000
From: David Laight <david.laight.linux@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, 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 V5 07/12] uaccess: Provide scoped user access regions

On Mon, 27 Oct 2025 09:43:55 +0100 (CET)
Thomas Gleixner <tglx@...utronix.de> wrote:

> User space access regions are tedious and require similar code patterns all
> over the place:
...
> There have been issues with using the wrong user_*_access_end() variant in
> the error path and other typical Copy&Pasta problems, e.g. using the wrong
> fault label in the user accessor which ends up using the wrong accesss end
> variant. 
> 
> These patterns beg for scopes with automatic cleanup. The resulting outcome
> is:
>     	scoped_user_read_access(from, Efault)
> 		unsafe_get_user(val, from, Efault);
> 	return 0;
>   Efault:
> 	return -EFAULT;
> 
> The scope guarantees the proper cleanup for the access mode is invoked both
> in the success and the failure (fault) path.
> 
...

The code doesn't work if the 'from' (above) is 'const foo __user *from'.
Due to assigning away constness.

The changes below fix the build, I suspect the code is then correct.

...
> +/* Define RW variant so the below _mode macro expansion works */
> +#define masked_user_rw_access_begin(u)	masked_user_access_begin(u)
> +#define user_rw_access_begin(u, s)	user_access_begin(u, s)
> +#define user_rw_access_end()		user_access_end()
> +
> +/* Scoped user access */
> +#define USER_ACCESS_GUARD(_mode)				\

#define USER_ACCESS_GUARD(_mode, void)
(but change all the void below to a different name...)

> +static __always_inline void __user *				\
> +class_user_##_mode##_begin(void __user *ptr)			\
> +{								\
> +	return ptr;						\
> +}								\
> +								\
> +static __always_inline void					\
> +class_user_##_mode##_end(void __user *ptr)			\
> +{								\
> +	user_##_mode##_access_end();				\
> +}								\
> +								\
> +DEFINE_CLASS(user_ ##_mode## _access, void __user *,		\
> +	     class_user_##_mode##_end(_T),			\
> +	     class_user_##_mode##_begin(ptr), void __user *ptr)	\
> +								\
> +static __always_inline class_user_##_mode##_access_t		\
> +class_user_##_mode##_access_ptr(void __user *scope)		\
> +{								\
> +	return scope;						\
> +}
> +
> +USER_ACCESS_GUARD(read)
> +USER_ACCESS_GUARD(write)
> +USER_ACCESS_GUARD(rw)

USER_ACCESS_GUARD(read, const void)
USER_ACCESS_GUARD(write, void)
USER_ACCESS_GUARD(rw, void)

> +#undef USER_ACCESS_GUARD
...
> +#define __scoped_user_access(mode, uptr, size, elbl)					\
> +for (bool done = false; !done; done = true)						\
> +	for (void __user *_tmpptr = __scoped_user_access_begin(mode, uptr, size, elbl); \

	for (typeof(uptr) _tmpptr = ...

> +	     !done; done = true)							\
> +		for (CLASS(user_##mode##_access, scope)(_tmpptr); !done; done = true)	\
> +			/* Force modified pointer usage within the scope */		\
> +			for (const typeof(uptr) uptr = _tmpptr; !done; done = true)
> +

	David