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Re: [PATCH] ARM: Don't apply pointer encryption to the frame pointer


On 12/10/2013 03:05 PM, Will Newton wrote:
> On 10 December 2013 18:57, Carlos O'Donell <carlos@redhat.com> wrote:
>> On 12/10/2013 01:06 PM, Joseph S. Myers wrote:
>>> On Tue, 10 Dec 2013, Will Newton wrote:
>>>
>>>> 2013-12-10  Will Newton  <will.newton@linaro.org>
>>>>
>>>>      * sysdeps/arm/__longjmp.S: Don't apply pointer encryption
>>>>      to fp register.
>>>>      * sysdeps/arm/setjmp.S: Likewise.
>>>>      * sysdeps/arm/include/bits/setjmp.h (JMP_BUF_REGLIST): Add
>>>>      fp to register list, remove a4.
>>>>      * sysdeps/unix/sysv/linux/arm/sysdep.h: (PTR_MANGLE_LOAD):
>>>>      New macro.
>>>
>>> OK, presuming you've tested this with the glibc testsuite.
>>>
>>
>> Is it really true that ruby checks the FP?
>>
>> I don't see such code? Can you please point it out?
> 
> In vm_core.h:
> 
>    474      jmp_buf machine_regs;
> 
> In vm.c:
> 
>   1589          if (GET_THREAD() != th && th->machine_stack_start &&
> th->machine_stack_end) {
>   1590              rb_gc_mark_machine_stack(th);
>   1591              rb_gc_mark_locations((VALUE *)&th->machine_regs,
>   1592                                   (VALUE *)(&th->machine_regs) +
>   1593                                   sizeof(th->machine_regs) /
> sizeof(VALUE));
>   1594          }
> 
> So it looks like a conservative GC that uses the jmp_buf as a data
> array to find potentially reachable objects. If fp contained a pointer
> to an object then the pointer encryption would render it
> undiscoverable and it would not get marked as live and could be
> collected in error.
> 
> There are a number of "ifs" involved and I haven't got the testsuite
> running yet but it looks like a possibility.
> 

Thanks for pointing that out.

I'll get a new glibc to the Fedora ruby maintainer and ask them to
test in parallel.

Cheers,
Carlos.


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