[webkit-dev] Stack Alignment error in LLINT

[Rodney Dowdall]

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