linux-kernel - Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

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Date:   Mon, 3 Aug 2020 12:00:04 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Mark Rutland' <mark.rutland@....com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor



On 8/3/20 11:57 AM, David Laight wrote:
> From: Madhavan T. Venkataraman
>> Sent: 03 August 2020 17:03
>>
>> On 8/3/20 3:27 AM, David Laight wrote:
>>> From: Mark Rutland
>>>> Sent: 31 July 2020 19:32
>>> ...
>>>>> It requires PC-relative data references. I have not worked on all architectures.
>>>>> So, I need to study this. But do all ISAs support PC-relative data references?
>>>> Not all do, but pretty much any recent ISA will as it's a practical
>>>> necessity for fast position-independent code.
>>> i386 has neither PC-relative addressing nor moves from %pc.
>>> The cpu architecture knows that the sequence:
>>> 	call	1f
>>> 1:	pop	%reg
>>> is used to get the %pc value so is treated specially so that
>>> it doesn't 'trash' the return stack.
>>>
>>> So PIC code isn't too bad, but you have to use the correct
>>> sequence.
>> Is that true only for 32-bit systems only? I thought RIP-relative addressing was
>> introduced in 64-bit mode. Please confirm.
> I said i386 not amd64 or x86-64.

I am sorry. My bad.

>
> So yes, 64bit code has PC-relative addressing.
> But I'm pretty sure it has no other way to get the PC itself
> except using call - certainly nothing in the 'usual' instructions.

OK.

Madhavan