Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758271AbcDEMPN (ORCPT ); Tue, 5 Apr 2016 08:15:13 -0400 Received: from mailapp01.imgtec.com ([195.59.15.196]:62735 "EHLO mailapp01.imgtec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757963AbcDEMPL (ORCPT ); Tue, 5 Apr 2016 08:15:11 -0400 Date: Tue, 5 Apr 2016 13:14:53 +0100 From: "Maciej W. Rozycki" To: Ralf Baechle CC: Kees Cook , "kernel-hardening@lists.openwall.com" , Linux MIPS Mailing List , Matt Redfearn , Aaro Koskinen , Masahiro Yamada , Alexander Sverdlin , LKML , Thomas Gleixner , David Daney , Jaedon Shin , James Hogan , Jonas Gorski , Paul Burton Subject: Re: [kernel-hardening] [PATCH v2 00/11] MIPS relocatable kernel & KASLR In-Reply-To: <20160404233721.GB26295@linux-mips.org> Message-ID: References: <1459415142-3412-1-git-send-email-matt.redfearn@imgtec.com> <20160404233721.GB26295@linux-mips.org> User-Agent: Alpine 2.00 (DEB 1167 2008-08-23) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" X-Originating-IP: [10.20.78.160] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2642 Lines: 53 On Tue, 5 Apr 2016, Ralf Baechle wrote: > > > * Relocation is supported only by multiples of 64k bytes. This > > > eliminates the need to handle R_MIPS_LO16 relocations as the bottom > > > 16bits will remain the same at the relocated address. > > > > IIUC, that's actually better than x86, which needs to be 2MB aligned. > > On MIPS a key concern was maintaining a reasonable size for the final > kernel image. The R_MIPS_LO16 relocatio records make a significant > portion of the relocations in a relocatable .o file, so we wanted to > get rid of them. This results in a relocation granularity of 64kB. > If we were truely, truely stingy we could come up with a relocation format > to save a few more bits but I doubt that'd make any sense. Additionally, for historical reasons, with 32-bit (o32) ELF images the REL relocation format is used making borrow propagation from R_MIPS_LO16 to its corresponding R_MIPS_HI16 relocation a pain to handle. It is solvable as the static linker does handle it, in particular doing the reasonable thing for orphan relocations, but I think it's a complication worth avoiding if the cost is so little. > > > * In 64 bit kernels, relocation is supported only within the same 4Gb > > > memory segment as the kernel link address (CONFIG_PHYSICAL_START). > > > This eliminates the need to handle R_MIPS_HIGHEST and R_MIPS_HIGHER > > > relocations as the top 32bits will remain the same at the relocated > > > address. > > > > Interesting. Could the relocation code be updated in the future to > > bump the high addresses too? > > It could but yet again, the idea was to keep the size of the final > generated file under control. The R_MIPS_HIGHER and R_MIPS_HIGHEST > relocations can be discarded if we constrain the addresses to be in > a single 4GB segment. Removing this constraint would make a kernel > image much bigger so I suggested to add this restriction at least for > this initial version. For the record, with 64-bit ELF images the RELA relocation format is used, so there's no such concern about borrows as with 32-bit ones, because the whole addend is always readily available and does not have to be calculated from parts coming from different relocations. Consequently the handling of R_MIPS_HIGHER and R_MIPS_HIGHEST (and also R_MIPS_HI16 and R_MIPS_LO16) relocations in 64-bit ELF images is straightforward if we decided to include them. I suspect extending the handling to R_MIPS_HIGHER only will suffice all use cases for the foreseeable future as I don't expect MIPS systems with more than 256TiB of RAM to appear anytime soon. FWIW, Maciej