Re: [Xen-devel] x86: PIE support and option to extend KASLR randomization

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From: "H.J. Lu" <hjl.tools@xxxxxxxxx>
Message-ID: <CFFA3E3A-3136-4FAF-80E1-96A515A5C903@xxxxxxxxx>



On September 23, 2017 3:06:16 AM GMT+08:00, "H. Peter Anvin" <hpa@xxxxxxxxx> 
wrote:
>On 09/22/17 11:57, Kees Cook wrote:
>> On Fri, Sep 22, 2017 at 11:38 AM, H. Peter Anvin <hpa@xxxxxxxxx>
>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
We need a static PIE option so that compiler can optimize it
without using hidden visibility.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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