linux-kernel - Re: [PATCH v11 3/9] arm64: add copy_to/from

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Date:	Fri, 18 Mar 2016 18:59:02 +0530
From:	Pratyush Anand <panand@...hat.com>
To:	James Morse <james.morse@....com>,
	David Long <dave.long@...aro.org>,
	Will Deacon <will.deacon@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Sandeepa Prabhu <sandeepa.s.prabhu@...il.com>,
	William Cohen <wcohen@...hat.com>,
	Steve Capper <steve.capper@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Marc Zyngier <marc.zyngier@....com>,
	Dave P Martin <Dave.Martin@....com>,
	Mark Rutland <mark.rutland@....com>,
	Robin Murphy <Robin.Murphy@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Jens Wiklander <jens.wiklander@...aro.org>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Yang Shi <yang.shi@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Suzuki K. Poulose" <suzuki.poulose@....com>,
	Kees Cook <keescook@...omium.org>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	John Blackwood <john.blackwood@...r.com>,
	Feng Kan <fkan@....com>,
	Balamurugan Shanmugam <bshanmugam@....com>,
	Vladimir Murzin <Vladimir.Murzin@....com>,
	Mark Salyzyn <salyzyn@...roid.com>,
	Petr Mladek <pmladek@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v11 3/9] arm64: add copy_to/from_user to kprobes blacklist

On 17/03/2016:01:27:26 PM, Pratyush Anand wrote:
> @David: This patch was added in v9 and fixup_exception() had been dropped in v9.
> Since, dropping of fixup_exception() also caused to fail some systemtap test
> cases, so it was added back in v10. I wonder if we really need this patch.
> May be you can try to run related test case by dropping this patch. 

Had a closer look to the code, and noticed that fixup_exception() does not have
any role in handling of page fault of copy_to_user(). Then, why do we have the
problem.
Probably, I can see why does not it work. So, when we are single stepping an
instruction and page fault occurs, we will come to el1_da in entry.S. Here, we
do enable_dbg. As soon as we will do this, we will start receiving single step
exception after each instruction (not sure, probably for each alternate
instruction). Since, there will not be any matching single step handler for
these instructions, so we will see warning "Unexpected kernel single-step
exception at EL1". 

So, I think, we should 

(1) may be do not enable debug for el1_da, or
(2) enable_dbg only when single stepping is not enabled, or
(3) or disable single stepping during el1_da execution.

(1) will solve the issue for sure, but not sure if it could be the best choice.

Will, what do you suggest?

~Pratyush