[LLVMdev] mmap and vm_protect on ARM+Apple systems

[Jim Grosbach]

On Nov 16, 2012, at 4:12 PM, "Kaylor, Andrew" <andrew.kaylor at intel.com> wrote:

> “ARM Darwin can't do this. Memory is never allowed to be both writable and executable at the same time.”
>  
> That seems very sensible.  So do the JIT engines just not work there now?

They work by allocating RW memory, then switching the permissions to RX before running any code stored there. That requires special process permissions.
>  
> Currently both MCJIT and JIT are using AllocateRWX which on that platform mmap’s the memory as RX, then vm_protect’s it as RX+VM_PROT_COPY, then vm_protect’s it as RW.
>  
> My best guess is that this is trying to set up something that’s actually backed by two pages where whatever is written to the RW page will be copied to the RX page, though if that’s possible it would seem to defeat the purpose of not allowing RWX.
>  
> The case I’m ultimately interested in is the second case where things get allocated as RW and later set to RX.  I was trying to support the RWX case to provide an implementation that behaved like the old code, but maybe I just shouldn’t bother?

Yep. There was never a working RWX on ARM/Darwin. AFAIK it only existed on X86/Darwin because it was allowed and easier that way than twiddling the permissions. If you get a RW->RX path, it's fine for both.

-j

>  
> -Andy
>  
> From: Jim Grosbach [mailto:grosbach at apple.com] 
> Sent: Friday, November 16, 2012 3:59 PM
> To: Kaylor, Andrew
> Cc: llvmdev at cs.uiuc.edu List
> Subject: Re: [LLVMdev] mmap and vm_protect on ARM+Apple systems
>  
> Hi Andy,
>  
> On Nov 16, 2012, at 1:56 PM, "Kaylor, Andrew" <andrew.kaylor at intel.com> wrote:
> 
> 
> Hi,
>  
> Can anyone tell me something about mmap and vm_protect on ARM+Apple systems?
>  
> Some. :)
> 
> 
>  
> I’m working on a new memory manager implementation for MCJIT and I want to replace calls to Memory::AllocateRWX with calls to Memory::allocateMappedMemory, possibly still with the RWX flags.  However, looking at the Memory::AllocateRWX implementation I see that it’s jumping through some hoops in the case where both ‘__APPLE__’ and ‘__arm__’ are defined.
>  
> I want to handle two cases:
>  
> 1)      Allocate memory as RWX, copy JITed code into it and execute it without ever touching the permissions again.
>  
> ARM Darwin can't do this. Memory is never allowed to be both writable and executable at the same time.
> 
> 
> 2)      Allocate memory as RW, copy JITed code into it and then set the permissions to RX before executing.
>  
> This is the path that can work, if the JIT process has been appropriately blessed by the system. Normal applications can't do this (the vm_protect() will fail transitioning from writable to executable).
> 
> 
>  This seems pretty straight-forward with the functions that use mmap and mprotect, but the AllocateRWX implementation leads me to believe that won’t work in the ARM+Apple case.  I read some vm_protect documentation, but I can’t quite reconcile it with the AllocateRWX implementation.
>  
> My goal is to make sure the Memory::allocateMappedMemory and Memory::protectMappedMemory functions will work everywhere.
>  
> Any suggestions?
>  
> Thanks,
> Andy
>  
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