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Date: Wed, 28 May 2025 21:01:21 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Alice Ryhl <aliceryhl@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
	=?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
	Benno Lossin <benno.lossin@proton.me>,
	Andreas Hindborg <a.hindborg@kernel.org>,
	Trevor Gross <tmgross@umich.edu>,
	Danilo Krummrich <dakr@kernel.org>, rust-for-linux@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] uaccess: rust: use newtype for user pointers
Message-ID: <20250528200121.GD2023217@ZenIV>
References: <20250527-userptr-newtype-v2-1-a789d266f6b0@google.com>
 <20250527221211.GB2023217@ZenIV>
 <aDbpsB3ayj6tFfbI@google.com>
 <20250528174546.GC2023217@ZenIV>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Wed, May 28, 2025 at 06:45:46PM +0100, Al Viro wrote:
> On Wed, May 28, 2025 at 10:47:12AM +0000, Alice Ryhl wrote:
> 
> > We don't currently have any way to perform that kind of pointer-math on
> > user pointers, nor do we have any users of it. I imagine that this type
> > checking is only useful if you can actually perform pointer math in the
> > first place?
> 
> What you want is something like
> 	x->field::UserPtr(beta) iff
> 		x::UserPtr(alpha) and
> 		_.field::beta where _::alpha
> 
> Generated code would be "add offset and cast to pointer to type of...",
> but doing that manually would really invite headache.

... except that -> comes with wrong connotations, of course.  Hell knows
what a decent syntax would look like.