Re: [PATCH 5/5] arm64: Add uprobe support

From: Pratyush Anand <panand@redhat.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	srikar@linux.vnet.ibm.com, Will Deacon <will.deacon@arm.com>,
	open list <linux-kernel@vger.kernel.org>,
	Vladimir Murzin <vladimir.murzin@arm.com>,
	Russell King - ARM Linux <linux@arm.linux.org.uk>,
	vijaya.kumar@caviumnetworks.com, Dave Long <dave.long@linaro.org>,
	Shi Yang <yang.shi@linaro.org>,
	Jungseok Lee <jungseoklee85@gmail.com>,
	Steve Capper <steve.capper@linaro.org>,
	"Suzuki K. Poulose" <suzuki.poulose@arm.com>,
	Andre Przywara <andre.przywara@arm.com>,
	Shaokun Zhang <zhangshaokun@hisilicon.com>,
	Ashok Kumar <ashoks@broadcom.com>,
	Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>,
	Will Cohen <wcohen@redhat.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Oleg Nesterov <oleg@redhat.com>,
	James Morse <james.morse@arm.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 5/5] arm64: Add uprobe support
Date: Tue, 27 Sep 2016 20:33:25 +0530	[thread overview]
Message-ID: <c7675b74-982d-d8b6-935f-a74139820174@redhat.com> (raw)
In-Reply-To: <20160927135133.GF17336@e104818-lin.cambridge.arm.com>

On Tuesday 27 September 2016 07:21 PM, Catalin Marinas wrote:
>>>>> Looking at prepare_uprobe(), we have a weak is_trap_insn() function.
>>>>> > > > > This check is meaningless without knowing which instruction set we
>>>>> > > > > target. A false positive here, however, is not that bad as we wouldn't
>>>>> > > > > end up inserting the wrong breakpoint in the executable. But it looks to
>>>>> > > > > me like the core uprobe code needs to pass some additional information
>>>>> > > > > like the type of task or ELF format to the arch code to make a useful
>>>>> > > > > choice of breakpoint type.
>>>> > > >
>>>> > > > It seems that 'strtle r0, [r0], #160' would have the closest matching
>>>> > > > aarch32 instruction wrt BRK64_OPCODE_UPROBES(0xd42000A0). But that too
>>>> > > > seems a bad instruction. So, may be we can use still weak
>>>> > > > is_trap_insn().
>>> > >
>>> > > Even if the is_trap_insn() check passes, we would reject the probe in
>>> > > arch_uprobe_analyze_insn() immediately after based on the mm type check,
>>> > > so not too bad.
>> >
>> > OK..I will have an always returning false from arm64 is_trap_insn() in v2.
> For the time being, I think the default is_trap_insn() check is still
> useful on arm64.

I have already sent V2 with arm64 is_trap_insn() :(

> The problem gets trickier when we add AArch32 support
> as it may return 'true' on an AArch32 instruction that matches the
> AArch64 BRK (or vice-versa). That's when we need to either pass the mm
> to is_trap_insn() or simply return false and always perform the check in
> the arch_uprobe_analyze_insn() (which should, in addition, check for the
> trap instruction).

Yes, I agree that we will have to modify is_trap_insn() for supporting 
aarch32 task tracing.

>
> There is also the is_trap_at_addr() function which uses is_trap_insn().
> I haven't checked the call paths here, are there any implications if
> is_trap_insn() always returns false?

I had looked into it and also tested that a tracepoint at an application 
having a same instruction as that of "uprobe break instruction" ie "BRK 
#0x5" is rejected. So, I think a false positive return from 
is_tarp_insn() is still OK.

~Pratyush