From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Fri, 22 Sep 2017 12:06:16 -0700
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Cc: Ingo Molnar <mingo@kernel.org>, Thomas Garnier <thgarnie@google.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S . Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        Matthias Kaehlcke <mka@chromium.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross
 <jgross@suse.com>, Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Joerg Roedel <joro@8bytes.org>, Tom Lendacky <thomas.lendacky@amd.com>,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
        Brian Gerst <brgerst@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        "Rafael J . Wys
To: Kees Cook <keescook@chromium.org>
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On 09/22/17 11:57, Kees Cook wrote:
> On Fri, Sep 22, 2017 at 11:38 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>> We lose EBX on 32 bits, but we don't lose RBX on 64 bits - since x86-64
>> has RIP-relative addressing there is no need for a dedicated PIC register.
> 
> FWIW, since gcc 5, the PIC register isn't totally lost. It is now
> reusable, and that seems to have improved performance:
> https://gcc.gnu.org/gcc-5/changes.html

It still talks about a PIC register on x86-64, which confuses me.
Perhaps older gcc's would allocate a PIC register under certain
circumstances, and then lose it for the entire function?

For i386, the PIC register is required by the ABI to be %ebx at the
point any PLT entry is called.  Not an issue with -mno-plt which goes
straight to the GOT, although in most cases there needs to be a PIC
register to find the GOT unless load-time relocation is permitted.

	-hpa