[llvm-commits] [PATCH] improve ARM halt encoding

[Jim Grosbach]

Hi JF,

Assuming there's real security benefits to be had, yes. However, I'm reticent to add complexity to the code on a purely theoretical benefit. Can you elaborate a bit more on why this is worth it?

In particular, I'm skeptical of benefits to an overlapping ARM/Thumb TRAP instruction. Now, I can definitely see benefit if you could find a way to get ARM ISel to more frequently have Thumb2 undefined bitpatterns in the bitstream (as the low-order bits of normal ARM instructions, that is). That would be quite clever and downright sneaky. Also likely pretty hard to do it w/o completely crushing performance…

-Jim

On Jan 16, 2013, at 4:27 PM, JF Bastien <jfb at google.com> wrote:

> As discussed over IRC: it then makes sense to only change the encoding for the NaCl triple (which is effectively its own OS).
> 
> 
> On Wed, Jan 16, 2013 at 4:17 PM, Jim Grosbach <grosbach at apple.com> wrote:
> 
> On Jan 16, 2013, at 3:45 PM, Renato Golin Linaro <renato.golin at linaro.org> wrote:
> 
>> On 16 January 2013 22:39, Jim Grosbach <grosbach at apple.com> wrote:
>> The entire encoding is semantically significant on Darwin. I suspect that's true on other platforms, too, but I don't know for sure.
>> 
>> I'm not sure either. I agree with Bastien that it *should* trap on both ARM and Thumb, but it also depends on what catch routine is installed and other hard-to-know problems.
>> 
> 
> Both instructions will trap; however, how they trap is also important. In this case, it's the difference between the user program terminating with SIGILL vs. SIGTRAP.
> 
> -Jim
> 
> 
>> Bastien, Have you tested in which platforms?
>> 
>> cheers,
>> --renato
>> 
>> 
> 
> 

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