Re: [RFC PATCH 6/7] execmem: add support for cache of large ROX pages

From: Mike Rapoport <rppt@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christoph Hellwig <hch@infradead.org>,
	Helge Deller <deller@gmx.de>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Russell King <linux@armlinux.org.uk>, Song Liu <song@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Uladzislau Rezki <urezki@gmail.com>,
	Will Deacon <will@kernel.org>,
	bpf@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
	linux-modules@vger.kernel.org, linux-parisc@vger.kernel.org,
	linux-riscv@lists.infradead.org,
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	linuxppc-dev@lists.ozlabs.org, x86@kernel.org
Subject: Re: [RFC PATCH 6/7] execmem: add support for cache of large ROX pages
Date: Mon, 15 Apr 2024 20:00:26 +0300	[thread overview]
Message-ID: <Zh1dKkFr6zdBh2Kz@kernel.org> (raw)
In-Reply-To: <20240415104750.GJ40213@noisy.programming.kicks-ass.net>

On Mon, Apr 15, 2024 at 12:47:50PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 11, 2024 at 07:05:25PM +0300, Mike Rapoport wrote:
> 
> > To populate the cache, a writable large page is allocated from vmalloc with
> > VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> > ROX.
> 
> > +static void execmem_invalidate(void *ptr, size_t size, bool writable)
> > +{
> > +	if (execmem_info->invalidate)
> > +		execmem_info->invalidate(ptr, size, writable);
> > +	else
> > +		memset(ptr, 0, size);
> > +}
> 
> +static void execmem_invalidate(void *ptr, size_t size, bool writeable)
> +{
> +       /* fill memory with INT3 instructions */
> +       if (writeable)
> +               memset(ptr, 0xcc, size);
> +       else
> +               text_poke_set(ptr, 0xcc, size);
> +}
> 
> Thing is, 0xcc (aka INT3_INSN_OPCODE) is not an invalid instruction.
> It raises #BP not #UD.

Do you mean that _invalidate is a poor name choice or that it's necessary
to use an instruction that raises #UD?

-- 
Sincerely yours,
Mike.