linux-kernel - RE: [PATCH] x86: Allow user accesses to the base of the guard page

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Message-ID: <90e813acc14848b1aa0995578f80ffad@AcuMS.aculab.com>
Date: Sun, 24 Nov 2024 10:28:17 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Linus Torvalds' <torvalds@...ux-foundation.org>
CC: Andrew Cooper <andrew.cooper3@...rix.com>, "bp@...en8.de" <bp@...en8.de>,
	Josh Poimboeuf <jpoimboe@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Arnd Bergmann
	<arnd@...nel.org>, Mikel Rychliski <mikel@...elr.com>, Thomas Gleixner
	<tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Dave Hansen
	<dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: RE: [PATCH] x86: Allow user accesses to the base of the guard page

From: Linus Torvalds
> Sent: 23 November 2024 23:45
> 
> On Sat, 23 Nov 2024 at 14:36, David Laight <David.Laight@...lab.com> wrote:
> >
> > The problem is that it is valid to pass a buffer that ends right
> > at the end of valid user memory.
> 
> There's a difference between "valid" and "we care".
> 
> This is way past that case. The only possible reason for that
> zero-byte thing at the end of the address space is somebody actively
> looking for some edge case, not a real use.

Mikel gave the exact test that was failing.

I should have been more clear that the issue is fixing valid transfers
that end at the end of valid memory without breaking zero length
transfers anywhere else.

	David

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