[webkit-dev] Stack Alignment error in LLINT

[Michael Saboff]

Rodney,

The “align the stack” code for 32 bit platforms is found in Source/JavaScriptCore/llint/LowLevelInterpreter32_64.asm (http://trac.webkit.org/browser/trunk/Source/JavaScriptCore/llint/LowLevelInterpreter32_64.asm) beginning at line 218.  There isn’t an if / elsif for Linux or QNX.  This stack aligning code is at the top of the code that I think is generating your SIGTRAP.  Unfortunately I don’t think that the offline assembler that “assembles” the LLINT code has an OS specific type for X86 QNX.  You can try locally in your build adding “or X86” to the end of the “if X86_WIN” to see if that takes care of your issue.

It would also be interesting to know if Linux build with exceptions has the problem as well.

- Michael

> On Jul 27, 2015, at 8:50 AM, Rodney Dowdall <rdowdall at cranksoftware.com> wrote:
> 
> Thanks for the response Michael.  Where does the code that does the padding live?  It may be that I need to add something in there.  I can try changing what the stack alignment should be, but from what I understand the QNX OS should align to 16.  Maybe it is the exceptions that are causing the problem.  I could try building a 32 bit Linux build with exceptions on to see if I get the same problem.
> 
> Regards,
> Rodney
> 
> On 07/27/2015 11:24 AM, Michael Saboff wrote:
>> Rodney,
>> 
>> JavaScriptCore assumes that the stack is 16 byte aligned.  This may be more restrictive than some OS / ABIs, but there is code to pad appropriately for some platforms. You didn’t say what OS you are running on, but maybe there needs to be a code added for your OS.  Turning on C++ exceptions could be responsible, as that could be changing stack layout.  I don’t know of any platform that turns on C++ exceptions, so you may be in uncharted waters.
>> 
>> Your build is 2 months old, the current revision is 187434.  You could also try a more recent build.
>> 
>> - Michael
>> 
>>> On Jul 27, 2015, at 8:07 AM, Rodney Dowdall <rdowdall at cranksoftware.com> wrote:
>>> 
>>> Hello
>>> 
>>> I am seeing a SIGTRAP generated in the LLINT code when I try and load up a page.  It happens as soon as the page tries to execute JavaScript.  The target is an 32 bit x86 machine.  The SIGTRAP appears to happen when it is checking the stack alignment.  I have tried compiling the code with the gcc option -mstackrealign and without it.  The SIGTRAP is generated in the same spot with or without the option.  C++ exceptions are turned on (they have to be with this particular compiler.  The compiler is gcc based).  The version of Webkit that I am building from is 184845.
>>> 
>>> 
>>> Here is the assembly execution that causes the SIGTRAP:
>>> 
>>>          vmEntryToJavaScript:
>>> b9a80ef7:   push %ebp
>>> b9a80ef8:   mov %esp,%ebp
>>> b9a80efa:   push %esi
>>> b9a80efb:   push %edi
>>> b9a80efc:   push %ebx
>>> b9a80efd:   mov 0xc(%ebp),%ebx
>>> b9a80f00:   mov 0x8(%ebp),%edi
>>> b9a80f03:   mov %ebp,%esp
>>> b9a80f05:   sub $0x20,%esp
>>> b9a80f08:   mov %ebx,(%esp)
>>> b9a80f0b:   mov 0x1498(%ebx),%edx
>>> b9a80f11:   mov %edx,0x4(%esp)
>>> b9a80f15:   mov 0x1494(%ebx),%edx
>>> b9a80f1b:   mov %edx,0x8(%esp)
>>> b9a80f1f:   mov 0x10(%ebp),%esi
>>> b9a80f22:   mov 0x20(%esi),%edx
>>> b9a80f25:   add $0x4,%edx
>>> b9a80f28:   shl $0x3,%edx
>>> b9a80f2b:   mov %esp,%eax
>>> b9a80f2d:   sub %edx,%eax
>>> b9a80f2f:   cmp 0x2384(%ebx),%eax
>>> b9a80f35:   jae 0xb9a80f71 <vmEntryToJavaScript+122>
>>> 
>>> b9a80f71:   mov %eax,%esp
>>> b9a80f73:   mov $0x4,%eax
>>> b9a80f78:   sub $0x1,%eax
>>> b9a80f7b:   mov 0x4(%esi,%eax,8),%ecx
>>> b9a80f7f:   mov %ecx,0xc(%esp,%eax,8)
>>> b9a80f83:   mov (%esi,%eax,8),%ecx
>>> b9a80f86:   mov %ecx,0x8(%esp,%eax,8)
>>> b9a80f8a:   test %eax,%eax
>>> b9a80f8c:   jne 0xb9a80f78 <vmEntryToJavaScript+129>
>>> 
>>> b9a80f9e:   sub $0x1,%ecx
>>> b9a80fa1:   movl $0xfffffffc,0x2c(%esp,%ecx,8)
>>> b9a80fa9:   movl $0x0,0x28(%esp,%ecx,8)
>>> b9a80fb1:   cmp %ecx,%edx
>>> b9a80fb3:   jne 0xb9a80f9e <vmEntryToJavaScript+167>
>>> b9a80fb5:   mov 0x28(%esi),%eax
>>> b9a80fb8:   test %edx,%edx
>>> b9a80fba:   je 0xb9a80fd0 <vmEntryToJavaScript+217>
>>> 
>>> b9a80f78:   sub $0x1,%eax
>>> b9a80f7b:   mov 0x4(%esi,%eax,8),%ecx
>>> b9a80f7f:   mov %ecx,0xc(%esp,%eax,8)
>>> b9a80f83:   mov (%esi,%eax,8),%ecx
>>> b9a80f86:   mov %ecx,0x8(%esp,%eax,8)
>>> b9a80f8a:   test %eax,%eax
>>> b9a80f8c:   jne 0xb9a80f78 <vmEntryToJavaScript+129>
>>> 
>>> b9a80f78:   sub $0x1,%eax
>>> b9a80f7b:   mov 0x4(%esi,%eax,8),%ecx
>>> b9a80f7f:   mov %ecx,0xc(%esp,%eax,8)
>>> b9a80f83:   mov (%esi,%eax,8),%ecx
>>> b9a80f86:   mov %ecx,0x8(%esp,%eax,8)
>>> b9a80f8a:   test %eax,%eax
>>> b9a80f8c:   jne 0xb9a80f78 <vmEntryToJavaScript+129>
>>> 
>>> b9a80f78:   sub $0x1,%eax
>>> b9a80f7b:   mov 0x4(%esi,%eax,8),%ecx
>>> b9a80f7f:   mov %ecx,0xc(%esp,%eax,8)
>>> b9a80f83:   mov (%esi,%eax,8),%ecx
>>> b9a80f86:   mov %ecx,0x8(%esp,%eax,8)
>>> b9a80f8a:   test %eax,%eax
>>> b9a80f8c:   jne 0xb9a80f78 <vmEntryToJavaScript+129>
>>> b9a80f8e:   mov 0x10(%esi),%edx
>>> b9a80f91:   sub $0x1,%edx
>>> b9a80f94:   mov 0x20(%esi),%ecx
>>> b9a80f97:   sub $0x1,%ecx
>>> b9a80f9a:   cmp %ecx,%edx
>>> b9a80f9c:   je 0xb9a80fb5 <vmEntryToJavaScript+190>
>>> 
>>> b9a80fd0:   mov %esp,0x1498(%ebx)
>>> b9a80fd6:   mov %ebp,0x1494(%ebx)
>>> b9a80fdc:   add $0x8,%esp
>>> b9a80fdf:   mov %esp,%ecx
>>> b9a80fe1:   and $0xf,%ecx
>>> b9a80fe4:   test %ecx,%ecx
>>> b9a80fe6:   je 0xb9a80fee <vmEntryToJavaScript+247>
>>> b9a80fe8:   mov $0xbad0dc02,%ecx
>>> b9a80fed:   int3
>>> 
>>> So using the LLintAssembly.h I tracked this too:
>>> 
>>>    "\tjz " LOCAL_LABEL_STRING(_offlineasm_doVMEntry__makeCall__checkStackPointerAlignment__stackPointerOkay
>>> 
>>> Which leads me to believe that the alignment on my stack is wrong. The value of esp is 0x7db9284.  The value of ecx after the and is 4, so that looks right.
>>> 
>>> I don't have a lot of experience with the LLINT, so I was wondering if there was a specific place I should start to look to see why this error is beign generated.
>>> 
>>> Thanks,
>>> Rodney
>>> 
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>