Re: RFC: Audit external function called indirectly via GOT

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Mar 22, 2018 at 6:30 AM, Alan Modra <amodra@gmail.com> wrote:
> On Thu, Mar 22, 2018 at 05:39:18AM -0700, H.J. Lu wrote:
>> On Thu, Mar 22, 2018 at 5:29 AM, Alan Modra <amodra@gmail.com> wrote:
>> > On Wed, Mar 21, 2018 at 10:15:26PM -0700, Cary Coutant wrote:
>> >> If you get rid of the GOT entry, and have the point of call jump
>> >> indirectly through the PLTGOT entry, which is initialized to point to
>> >> part (b) of the PLT entry, everything should work the same as without
>> >> -fno-plt. Essentially, all -fno-plt would do is inline part (a) of the
>> >> PLT entry.
>> >>
>> >> -cary
>> >>
>> >> * I'm using parts (a) and (b) to refer to the two parts of a PLT
>> >> entry: (a) an indirect jump via the PLTGOT entry, and (b) code that
>> >> jumps to the lazy binding routine, passing the JUMP_SLOT index.
>> >
>> > Yes, that essentially is what I've done for -fno-plt on powerpc.
>> > The call stub code is inlined while the rest of the PLT is more or
>> > less unchanged.  So you get all of the usual lazy-binding features
>> > by default, and can use "-z now -z relro" if you want a read-only
>> > PLT.
>> >
>>
>> On x86, PLT is always read-only.  The issue is the writable PLTGOT.
>
> Yes, I do know how the x86 PLT works.  (Or to be more honest, how it
> used to work..)  To be clear, I was using PLT to refer to the whole
> scheme, ie. the code to do an indirect jump (x86 .plt), plus a table
> of addresses (x86 .plt.got), plus code for lazy binding (x86 .plt
> again).  Like x86 the powerpc PLT code to do indirect jumps and lazy
> binding is read-only nowadays.  -fno-plt on powerpc inlines the code
> to do the indirect jump, but leaves the table of addresses and the
> lazy binding code functionally unchanged.
>
>> On x86, -fno-plt removes the writable PLTGOT.
>
> I think that may have been a mistake.  You could have kept .plt.got
> functionally unchanged, giving you a writable .plt.got by default with
> -fno-plt, and read-only when "-z now -z relro".  Just like the usual
> -fplt case.
>

This is done on purpose.  See "Alternate Code Sequences For
Security" chapter on x86-64 psABI version 1.0.

-- 
H.J.