From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v2 23/31] arm64: Debugging support
Date: Mon, 20 Aug 2012 10:07:54 +0100
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To: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <Catalin.Marinas@arm.com>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "ulrich.weigand@linaro.org" <ulrich.weigand@linaro.org>

On Fri, Aug 17, 2012 at 08:06:07AM +0100, Arnd Bergmann wrote:
> Sorry for the dumb question, but why do you even need PTRACE_GET_THREAD_AREA
> for 64 bit tasks? I thought the thread pointer is a GPR, or is this just
> for compat tasks?

The TLS is stored in a co-processor register which is read-only for
userspace. However, it's banked, so accessing the TLS of your child (the guy
you're ptracing) requires a system call.

Will