linux-kernel - Re: [PATCHv9 02/16] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <20250728201137.113d0f45@pumpkin>
Date: Mon, 28 Jul 2025 20:11:37 +0100
From: David Laight <david.laight.linux@...il.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
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Subject: Re: [PATCHv9 02/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

On Mon,  7 Jul 2025 11:03:02 +0300
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> From: Sohil Mehta <sohil.mehta@...el.com>
> 
> For patching, the kernel initializes a temporary mm area in the lower
> half of the address range. See commit 4fc19708b165 ("x86/alternatives:
> Initialize temporary mm for patching").
> 
> Disable LASS enforcement during patching to avoid triggering a #GP
> fault.
> 
> The objtool warns due to a call to a non-allowed function that exists
> outside of the stac/clac guard, or references to any function with a
> dynamic function pointer inside the guard. See the Objtool warnings
> section #9 in the document tools/objtool/Documentation/objtool.txt.
> 
> Considering that patching is usually small, replace the memcpy() and
> memset() functions in the text poking functions with their open coded
> versions.
...

Or just write a byte copy loop in C with (eg) barrier() inside it
to stop gcc converting it to memcpy().

	David