Re: [RFC 06/22] kvm: Adapt assembly for PIE support

["H. Peter Anvin" <hpa@zytor.com>]

<paul.gortmaker@windriver.com>,Chris Metcalf <cmetcalf@mellanox.com>,"Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,Andrew Morton <akpm@linux-foundation.org>,Christopher Li <sparse@chrisli.org>,Dou Liyang <douly.fnst@cn.fujitsu.com>,Masahiro Yamada <yamada.masahiro@socionext.com>,Daniel Borkmann <daniel@iogearbox.net>,Markus Trippelsdorf <markus@trippelsdorf.de>,Peter Foley <pefoley2@pefoley.com>,Steven Rostedt <rostedt@goodmis.org>,Tim Chen <tim.c.chen@linux.intel.com>,Catalin Marinas <catalin.marinas@arm.com>,Matthew Wilcox <mawilcox@microsoft.com>,Michal Hocko <mhocko@suse.com>,Rob Landley <rob@landley.net>,Jiri Kosina <jkosina@suse.cz>,"H . J . Lu" <hjl.tools@gmail.com>,Paul Bolle <pebolle@tiscali.nl>,Baoquan He <bhe@redhat.com>,Daniel Micay <danielmicay@gmail.com>,the arch/x86 maintainers <x86@kernel.org>,"linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,xen-devel@lists.xenproject.org,kvm list
<kvm@vger.kernel.org>,linux-pm <linux-pm@vger.kernel.org>,linux-arch <linux-arch@vger.kernel.org>,Linux-Sparse <linux-sparse@vger.kernel.org>,Kernel Hardening <kernel-hardening@lists.openwall.com>
From: hpa@zytor.com
Message-ID: <83BA7600-BC8D-4C91-812C-DD2A0BF4474B@zytor.com>

On July 19, 2017 3:58:07 PM PDT, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>On 19 July 2017 at 23:27, H. Peter Anvin <hpa@zytor.com> 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.
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