linux-kernel - Re: [PATCH v2] misc: sram-exec: Use aligned fncpy instead of memcpy

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Date:   Wed, 17 May 2017 09:23:17 -0500
From:   Dave Gerlach <d-gerlach@...com>
To:     Tony Lindgren <tony@...mide.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-omap@...r.kernel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Keerthy J <j-keerthy@...com>
Subject: Re: [PATCH v2] misc: sram-exec: Use aligned fncpy instead of memcpy

On 05/17/2017 08:47 AM, Tony Lindgren wrote:
> * Russell King - ARM Linux <linux@...linux.org.uk> [170517 04:46]:
>> On Wed, May 17, 2017 at 11:13:17AM +0200, Greg Kroah-Hartman wrote:
>>> On Tue, May 16, 2017 at 09:01:27AM -0700, Tony Lindgren wrote:
>>>> * Russell King - ARM Linux <linux@...linux.org.uk> [170503 11:58]:
>>>>> On Mon, Apr 10, 2017 at 09:52:47AM -0500, Dave Gerlach wrote:
>>>>>> Currently the sram-exec functionality, which allows allocation of
>>>>>> executable memory and provides an API to move code to it, is only
>>>>>> selected in configs for the ARM architecture. Based on commit
>>>>>> 5756e9dd0de6 ("ARM: 6640/1: Thumb-2: Symbol manipulation macros for
>>>>>> function body copying") simply copying a C function pointer address
>>>>>> using memcpy without consideration of alignment and Thumb is unsafe on
>>>>>> ARM platforms.
>>>>>>
>>>>>> The aforementioned patch introduces the fncpy macro which is a safe way
>>>>>> to copy executable code on ARM platforms, so let's make use of that here
>>>>>> rather than the unsafe plain memcpy that was previously used by
>>>>>> sram_exec_copy. Now sram_exec_copy will move the code to "dst" and
>>>>>> return an address that is guaranteed to be safely callable.
>>>>>>
>>>>>> In the future, architectures hoping to make use of the sram-exec
>>>>>> functionality must define an fncpy macro just as ARM has done to
>>>>>> guarantee or check for safe copying to executable memory before allowing
>>>>>> the arch to select CONFIG_SRAM_EXEC.
>>>>>>
>>>>>> Signed-off-by: Dave Gerlach <d-gerlach@...com>
>>>>>
>>>>> Looks a lot saner, thanks.  It's just a bit sad that we lose the type
>>>>> checking.
>>>>>
>>>>> Acked-by: Russell King <rmk+kernel@...linux.org.uk>
>>>>
>>>> Looks like this is still pending so I'll add it into
>>>> omap-for-v4.12/fixes so we can get this out of the way.
>>>
>>> It's a "fix"?  Looked to be a 4.13 issue, sorry for the delay, otherwise
>>> I would have queued it up earlier.
>>
>> Technically, it is a fix, but my greps for "sram_exec_copy" indicate
>> that the code does not yet have any in-tree users.  So I don't think
>> there's any urgency to picking this up, and I think no need to back
>> port to stable trees.
>
> OK fine, I'll drop it today from my fixes (and for-next) no problem.
> I did add a fixes tag to it which would then create confusion later
> on too with stable trees.
>
> Dave, probably best to resend the patch to Greg in few days with acks
> added and rebased against v4.12-rc1 because it won't apply without a
> merge because of the header changes.

Yes, there are no users yet. Was planning on resending this anyway, so I'll do 
that before I send my patches that make use of this.

Regards,
Dave

>
> Regards,
>
> Tony
>
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>
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