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Date: Mon, 3 Aug 2020 11:03:21 -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 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. Madhavan
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