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Date: Mon, 29 Jun 2026 05:43:15 +0100
From: Matthew Wilcox <willy@infradead.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Xiang Mei <xmei5@asu.edu>, Pedro Falcato <pfalcato@suse.de>,
	Kees Cook <kees@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	linux-hardening@vger.kernel.org,
	Uladzislau Rezki <urezki@gmail.com>,
	"Gustavo A . R . Silva" <gustavoars@kernel.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Jennifer Miller <jmill@asu.edu>,
	Tiffany Bao <tbao@asu.edu>, Ruoyu Wang <fishw@asu.edu>,
	Adam Doupe <doupe@asu.edu>, Kyle Zeng <zengyhkyle@asu.edu>,
	Yan Shoshitaishvili <yans@asu.edu>
Subject: Re: [PATCH] mm/vmalloc: widen guard region to defeat ENTER-based
 stack pivot
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References: <20260626173444.2252041-1-xmei5@asu.edu>
 <aj7UNceukU9zYeGH@pedro-suse.lan>
 <CAPpSM+RVFDOYJFT2iwgNYdjbwqF-MmrbYizpLYksd1SLb5bT9g@mail.gmail.com>
 <499a4e15-34a4-421b-946a-c855199fdbf9@zytor.com>
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In-Reply-To: <499a4e15-34a4-421b-946a-c855199fdbf9@zytor.com>

On Sun, Jun 28, 2026 at 07:09:37PM -0700, H. Peter Anvin wrote:
> On 2026-06-26 13:05, Xiang Mei wrote:
> > 
> > Thanks for your questions and attention.
> > 
> > 1) `enter` can do big enough stack pivoting (where we can't find many
> > `add/sub/adc/sbb rsp, ` doing so)
> 
> Say what?
> 
> Using add/sub/lea on %rsp is probably more common than enter, because enter is
> considered kind of slow.
> 
> gcc, I believe, will generate "leave" but not "enter" as a result.
> 
> 
> > 2) `enter` is not rare since we can take part in the instruction.
> 
> This sentence doesn't parse.

They mean that we can jump into the middle of an instruction, and it is
not rare to see a 'c8' byte in the instruction stream.