linux-kernel - Re: [PATCH 1/2] uaccess: Simplify code pattern for masked user copies

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Message-ID: <20250209211805.5fc2e9e4@pumpkin>
Date: Sun, 9 Feb 2025 21:18:05 +0000
From: David Laight <david.laight.linux@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, Alexander
 Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan
 Kara <jack@...e.cz>, Arnd Bergmann <arnd@...db.de>, Kees Cook
 <kees@...nel.org>
Subject: Re: [PATCH 1/2] uaccess: Simplify code pattern for masked user
 copies

On Sun, 9 Feb 2025 12:40:32 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sun, 9 Feb 2025 at 11:48, David Laight <david.laight.linux@...il.com> wrote:
> >
> > You almost need it to be 'void masked_user_access_begin(&uaddr)'.  
> 
> Maybe we just need to make it a two-stage thing, with
> 
>         if (!user_access_ok(uaddr, size))
>                 return -EFAULT;
>         user_read_access_begin(&uaddr);
>         unsafe_get_user(val1, &uaddr->one, Efault);
>         unsafe_get_user(val2, &uaddr->two, Efault);
>         user_read_access_end();
>         ... all done ..
> 
> Efault:
>         user_read_access_end();
>         return -EFAULT;
> 
> and that would actually simplify some things: right now we have
> separate versions of the user address checking (for
> read/write/either): user_read_access_begin() and friends.
> 
> We still need those three versions, but now we'd only need them for
> the simpler non-conditional case that doesn't have to bother about the
> size.

Except for the ppc? case which needs the size to open a bounded window.
(I'm not sure how that handler r/w access.)
So you either have to pass the size twice or come back to:
	if (!user_read_access_begin(&uaddr, size))
		return -EFAULT;
	unsafe_get_user(...);

   David