Re: [RFC 06/22] kvm: Adapt assembly for PIE support
,Chris Metcalf ,"Paul E . McKenney" ,Andrew Morton ,Christopher Li ,Dou Liyang ,Masahiro Yamada ,Daniel Borkmann ,Markus Trippelsdorf ,Peter Foley ,Steven Rostedt ,Tim Chen ,Catalin Marinas ,Matthew Wilcox ,Michal Hocko ,Rob Landley ,Jiri Kosina ,"H . J . Lu" ,Paul Bolle ,Baoquan He ,Daniel Micay ,the arch/x86 maintainers ,"linux-cry...@vger.kernel.org" ,Linux Kernel Mailing List ,xen-de...@lists.xenproject.org,kvm list ,linux-pm ,linux-arch ,Linux-Sparse ,Kernel Hardening From: h...@zytor.com Message-ID: <83ba7600-bc8d-4c91-812c-dd2a0bf44...@zytor.com> On July 19, 2017 3:58:07 PM PDT, Ard Biesheuvel wrote: >On 19 July 2017 at 23:27, H. Peter Anvin wrote: >> On 07/19/17 08:40, Thomas Garnier wrote: This doesn't look right. It's accessing a per-cpu variable. The per-cpu section is an absolute, zero-based section and not subject >to relocation. >>> >>> PIE does not respect the zero-based section, it tries to have >>> everything relative. Patch 16/22 also adapt per-cpu to work with PIE >>> (while keeping the zero absolute design by default). >>> >> >> This is silly. The right thing is for PIE is to be explicitly >absolute, >> without (%rip). The use of (%rip) memory references for percpu is >just >> an optimization. >> > >Sadly, there is an issue in binutils that may prevent us from doing >this as cleanly as we would want. > >For historical reasons, bfd.ld emits special symbols like >__GLOBAL_OFFSET_TABLE__ as absolute symbols with a section index of >SHN_ABS, even though it is quite obvious that they are relative like >any other symbol that points into the image. Unfortunately, this means >that binutils needs to emit R_X86_64_RELATIVE relocations even for >SHN_ABS symbols, which means we lose the ability to use both absolute >and relocatable symbols in the same PIE image (unless the reloc tool >can filter them out) > >More info here: >https://sourceware.org/bugzilla/show_bug.cgi?id=19818 The reloc tool already has the ability to filter symbols. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
Re: [RFC 06/22] kvm: Adapt assembly for PIE support
,Chris Metcalf ,"Paul E . McKenney" ,Andrew Morton ,Christopher Li ,Dou Liyang ,Masahiro Yamada ,Daniel Borkmann ,Markus Trippelsdorf ,Peter Foley ,Steven Rostedt ,Tim Chen ,Catalin Marinas ,Matthew Wilcox ,Michal Hocko ,Rob Landley ,Jiri Kosina ,"H . J . Lu" ,Paul Bolle ,Baoquan He ,Daniel Micay ,the arch/x86 maintainers ,"linux-cry...@vger.kernel.org" ,Linux Kernel Mailing List ,xen-de...@lists.xenproject.org,kvm list ,linux-pm ,linux-arch ,Linux-Sparse ,Kernel Hardening From: h...@zytor.com Message-ID: <83ba7600-bc8d-4c91-812c-dd2a0bf44...@zytor.com> On July 19, 2017 3:58:07 PM PDT, Ard Biesheuvel wrote: >On 19 July 2017 at 23:27, H. Peter Anvin wrote: >> On 07/19/17 08:40, Thomas Garnier wrote: This doesn't look right. It's accessing a per-cpu variable. The per-cpu section is an absolute, zero-based section and not subject >to relocation. >>> >>> PIE does not respect the zero-based section, it tries to have >>> everything relative. Patch 16/22 also adapt per-cpu to work with PIE >>> (while keeping the zero absolute design by default). >>> >> >> This is silly. The right thing is for PIE is to be explicitly >absolute, >> without (%rip). The use of (%rip) memory references for percpu is >just >> an optimization. >> > >Sadly, there is an issue in binutils that may prevent us from doing >this as cleanly as we would want. > >For historical reasons, bfd.ld emits special symbols like >__GLOBAL_OFFSET_TABLE__ as absolute symbols with a section index of >SHN_ABS, even though it is quite obvious that they are relative like >any other symbol that points into the image. Unfortunately, this means >that binutils needs to emit R_X86_64_RELATIVE relocations even for >SHN_ABS symbols, which means we lose the ability to use both absolute >and relocatable symbols in the same PIE image (unless the reloc tool >can filter them out) > >More info here: >https://sourceware.org/bugzilla/show_bug.cgi?id=19818 The reloc tool already has the ability to filter symbols. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.