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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">“</span>ARM Darwin can't do this. Memory is never allowed to be both writable and executable at the same time.<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">That seems very sensible. So do the JIT engines just not work there now?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">-Andy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Jim Grosbach [mailto:grosbach@apple.com]
<br>
<b>Sent:</b> Friday, November 16, 2012 3:59 PM<br>
<b>To:</b> Kaylor, Andrew<br>
<b>Cc:</b> llvmdev@cs.uiuc.edu List<br>
<b>Subject:</b> Re: [LLVMdev] mmap and vm_protect on ARM+Apple systems<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hi Andy,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Nov 16, 2012, at 1:56 PM, "Kaylor, Andrew" <<a href="mailto:andrew.kaylor@intel.com">andrew.kaylor@intel.com</a>> wrote:<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Hi,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Can anyone tell me something about mmap and vm_protect on ARM+Apple systems?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Some. :)<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">I want to handle two cases:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal" style="text-indent:-.25in"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">1)</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Allocate
memory as RWX, copy JITed code into it and execute it without ever touching the permissions again.<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">ARM Darwin can't do this. Memory is never allowed to be both writable and executable at the same time.<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal" style="text-indent:-.25in"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">2)</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Allocate
memory as RW, copy JITed code into it and then set the permissions to RX before executing.<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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).<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> 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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">My goal is to make sure the Memory::allocateMappedMemory and Memory::protectMappedMemory functions will work everywhere.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Any suggestions?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Thanks,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Andy<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:13.5pt;font-family:"Helvetica","sans-serif"">_______________________________________________<br>
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<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev"><span style="color:purple">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</span></a><o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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