Received-SPF: pass (google.com: domain of linux-kernel+bounces-59310-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Fri, 9 Feb 2024 15:30:37 +0300
From: Alexey Dobriyan <adobriyan@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Florian Weimer <fweimer@redhat.com>, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
	x86@kernel.org, Eric Biederman <ebiederm@xmission.com>,
	linux-mm@kvack.org
Subject: Re: [PATCH v4] ELF: AT_PAGE_SHIFT_MASK -- supply userspace with
 available page shifts
Message-ID: <acd02481-ca2e-412a-8c6b-d9dff1345139@p183>
References: <ecb049aa-bcac-45c7-bbb1-4612d094935a@p183>
 <202402050445.0331B94A73@keescook>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <202402050445.0331B94A73@keescook>

On Mon, Feb 05, 2024 at 04:48:08AM -0800, Kees Cook wrote:
> On Mon, Feb 05, 2024 at 12:51:43PM +0300, Alexey Dobriyan wrote:
> > +#define ARCH_AT_PAGE_SHIFT_MASK					\
> > +	do {							\
> > +		u32 val = 1 << 12;				\
> > +		if (boot_cpu_has(X86_FEATURE_PSE)) {		\
> > +			val |= 1 << 21;				\
> > +		}						\
> > +		if (boot_cpu_has(X86_FEATURE_GBPAGES)) {	\
> > +			val |= 1 << 30;				\
> > +		}						\
> 
> Can we use something besides literal "12", "21", and "30" values here?

Ehh, no, why? Inside x86_64 the page shifts are very specific numbers,
they won't change.