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

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Thu, Sep 21, 2017 at 2:21 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> On Thu, Sep 21, 2017 at 9:10 AM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>>
>> On 21 September 2017 at 08:59, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>> >
>> > ( Sorry about the delay in answering this. I could blame the delay on the 
>> > merge
>> >   window, but in reality I've been procrastinating this is due to the 
>> > permanent,
>> >   non-trivial impact PIE has on generated C code. )
>> >
>> > * Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>> >
>> >> 1) PIE sometime needs two instructions to represent a single
>> >> instruction on mcmodel=kernel.
>> >
>> > What again is the typical frequency of this occurring in an x86-64 
>> > defconfig
>> > kernel, with the very latest GCC?
>> >
>> > Also, to make sure: which unwinder did you use for your measurements,
>> > frame-pointers or ORC? Please use ORC only for future numbers, as
>> > frame-pointers is obsolete from a performance measurement POV.
>> >
>> >> 2) GCC does not optimize switches in PIE in order to reduce relocations:
>> >
>> > Hopefully this can either be fixed in GCC or at least influenced via a 
>> > compiler
>> > switch in the future.
>> >
>>
>> There are somewhat related concerns in the ARM world, so it would be
>> good if we could work with the GCC developers to get a more high level
>> and arch neutral command line option (-mkernel-pie? sounds yummy!)
>> that stops the compiler from making inferences that only hold for
>> shared libraries and/or other hosted executables (GOT indirections,
>> avoiding text relocations etc). That way, we will also be able to drop
>> the 'hidden' visibility override at some point, which we currently
>> need to prevent the compiler from redirecting all global symbol
>> references via entries in the GOT.
>
> My plan was to add a -mtls-reg=<fs|gs> to switch the default segment
> register for stack cookies but I can see great benefits in having a
> more general kernel flag that would allow to get rid of the GOT and
> PLT when you are building position independent code for the kernel. It
> could also include optimizations like folding switch tables etc...
>
> Should we start a separate discussion on that? Anyone that would be
> more experienced than I to push that to gcc & clang upstream?

After separate discussion, opened:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82303

>
>>
>> All we really need is the ability to move the image around in virtual
>> memory, and things like reducing the CoW footprint or enabling ELF
>> symbol preemption are completely irrelevant for us.
>
>
>
>
> --
> Thomas



-- 
Thomas

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