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Message-ID: <a3068e3126a942c7a3e7ac115499deb1@AcuMS.aculab.com>
Date: Mon, 3 Aug 2020 08:27:25 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Mark Rutland' <mark.rutland@....com>, "Madhavan T. Venkataraman"
<madvenka@...ux.microsoft.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: 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.
David
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