Re: [Qemu-devel] Crash due to invalid env->current_tb

[Adam Lackorzynski <adam@os.inf.tu-dresden.de>]

On Wed Apr 30, 2008 at 11:08:46 +0200, Alexander Graf wrote:
>
> On Apr 29, 2008, at 8:40 PM, Adam Lackorzynski wrote:
>
>>
>> On Tue Apr 29, 2008 at 20:09:00 +0300, Blue Swirl wrote:
>>> On 4/29/08, Adam Lackorzynski <adam@os.inf.tu-dresden.de> wrote:
>>>> Hi,
>>>>
>>>> I've been experiencing crashes of latest svn Qemu, host ia32 and  
>>>> target
>>>> arm, host gcc is 'gcc version 3.4.6 (Debian 3.4.6-7)'.
>>>> The segfault happens because of an invalid env->current_tb which  
>>>> seems
>>>> to be caused by generated code. The following code in cpu_exec
>>>>
>>>>   tc_ptr = tb->tc_ptr;
>>>>   env->current_tb = tb;
>>>>   gen_func = (void *)tc_ptr;
>>>>   T0 = gen_func();
>>>>   env->current_tb = NULL;
>>>>
>>>> is being compiled to the following
>>>>
>>>>   mov    0x14(%ecx),%eax
>>>>   mov    %ecx,0x56c(%ebp)
>>>>   xor    %edi,%edi
>>>>   call   *%eax
>>>>   mov    %edi,0x56c(%ebp)
>>>>
>>>> After the call edi isn't 0 anymore and gets the bogus value. As  
>>>> edi is
>>>> callee saved the code itself seems ok.
>>>> When I add a barrier before "env->current_tb = NULL" the xor is  
>>>> placed
>>>> after the call and everything works fine. So might the problem be  
>>>> that
>>>> generated code isn't preserving edi/registers?
>>>
>>> Right. How did you make the barrier? My version (attached) just
>>> crashes, I'm not fluent on i386 assembly. Maybe your version could
>>> serve as a temporary fix.
>>
>> I just added an 'asm volatile("")' to stop reordering of instructions
>> which of course isn't enough. The following works for me:
>>
>> ===================================================================
>> --- cpu-exec.c	(revision 4276)
>> +++ cpu-exec.c	(working copy)
>> @@ -690,6 +691,11 @@
>> 		fp.ip = tc_ptr;
>> 		fp.gp = code_gen_buffer + 2 * (1 << 20);
>> 		(*(void (*)(void)) &fp)();
>> +#elif defined(__i386)
>> +		asm volatile ("call *%1\n"
>> +		              : "=a" (T0)
>> +			      : "r" (gen_func)
>> +			      : "esi", "edi");
>> #else
>>                 T0 = gen_func();
>> #endif
>
> There was a comment from Fabrice on how to do prologues in TCG to save / 
> restore the clobbered values. Btw, ebx gets clobbered as well.

tcg/README says that some registers are clobbered. So something like
this should be safe:

Index: cpu-exec.c
===================================================================
--- cpu-exec.c	(revision 4276)
+++ cpu-exec.c	(working copy)
@@ -690,6 +691,15 @@
 		fp.ip = tc_ptr;
 		fp.gp = code_gen_buffer + 2 * (1 << 20);
 		(*(void (*)(void)) &fp)();
+#elif defined(__i386)
+		asm volatile ("push %%ebp\n"
+		              "push %%ebx\n"
+		              "call *%1\n"
+			      "pop %%ebx\n"
+			      "pop %%ebp\n"
+		              : "=a" (T0)
+			      : "r" (gen_func)
+			      : "esi", "edi", "ecx", "edx");
 #else
                 T0 = gen_func();
 #endif




Adam
-- 
Adam                 adam@os.inf.tu-dresden.de
  Lackorzynski         http://os.inf.tu-dresden.de/~adam/