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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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