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

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Date:   Mon, 3 Aug 2020 16:57:47 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Madhavan T. Venkataraman'" <madvenka@...ux.microsoft.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

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.

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.

	David

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