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Date: Mon, 15 Apr 2024 12:47:50 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mike Rapoport <rppt@kernel.org>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [RFC PATCH 6/7] execmem: add support for cache of large ROX pages
Message-ID: <20240415104750.GJ40213@noisy.programming.kicks-ass.net>
References: <20240411160526.2093408-1-rppt@kernel.org>
 <20240411160526.2093408-7-rppt@kernel.org>
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In-Reply-To: <20240411160526.2093408-7-rppt@kernel.org>

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.