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
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