Re: [webkit-dev] Stack Alignment error in LLINT

Rodney Dowdall Mon, 27 Jul 2015 08:51:50 -0700

Thanks for the response Michael. Where does the code that does thepadding live? It may be that I need to add something in there. I cantry changing what the stack alignment should be, but from what Iunderstand the QNX OS should align to 16. Maybe it is the exceptionsthat are causing the problem. I could try building a 32 bit Linux buildwith 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 <rdowd...@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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Re: [webkit-dev] Stack Alignment error in LLINT

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