How to Use Crashlogs to Debug Your Own DLLs

MetaTrader 4 Client Terminal has an integrated means to detect error conditions that occur during the terminal work and generate crashlogs where such errors are reported. The generated report is stored in the logs\crashlog.log file to be sent to the trade server at the next startup of the client terminal. It should be noted that the error condition report does not contain any user's personal details, but only the system data that allow to localize the error in the client terminal. This information is very important for manufacturers since it is used to correct critical errors. Then the software developed will become even more crashproof.

25 to 30% of all crashlogs received from users appear due to errors occurring when functions imported from custom dlls are executed. This information will not help the developers of the client temrinal in any way, but it can help the developers of the corresponding dlls in their trouble-shooting. We will show how the data from the error report can be used. Examples named ExpertSample.dll and ExportFunctions. mq4 that can be found in the experts\samples directory were taken as a basis.

The full text of the error report is given below:

Time        : 2006.07.12 14:43
Program     : Client Terminal
Version     : 4.00 (build: 195, 30 Jun 2006)
Owner       : MetaQuotes Software Corp. (MetaTrader)
OS          : Windows XP Professional 5.1 Service Pack 2 (Build 2600)
Processors  : 2, type 586, level 15
Memory      : 2095848/1727500 kb
Exception   : C0000005
Address     : 77C36FA3
Access Type : read
Access Addr : 00000000

Registers   : EAX=000000FF CS=001b EIP=77C36FA3 EFLGS=00010202
            : EBX=FFFFFFFF SS=0023 ESP=024DFABC EBP=024DFAC4
            : ECX=0000003F DS=0023 ESI=00000000 FS=003b
            : EDX=00000003 ES=0023 EDI=10003250 GS=0000

Stack Trace : 10001079 0045342E 0045D627 004506EC
            : 7C80B50B 00000000 00000000 00000000
            : 00000000 00000000 00000000 00000000
            : 00000000 00000000 00000000 00000000
Modules     :
          1 : 00400000 00292000 C:\Program Files\MetaTrader 4\terminal.exe
          2 : 10000000 00005000 C:\Program Files\MetaTrader 4\experts\libraries\ExpertSample.dll
         ...   .......................................................... 
         35 : 7C9C0000 00819000 C:\WINDOWS\system32\SHELL32.dll

Call stack  :
77C36F70:0033 [77C36FA3] memcpy                           [C:\WINDOWS\system32\msvcrt.dll]
10001051:0028 [10001079] GetStringValue                   [C:\Program Files\MetaTrader 4\experts\libraries\ExpertSample.dll]
00452DD0:065E [0045342E] ?CallDllFunction@CExpertInterior
00459AC0:3B67 [0045D627] ?ExecuteStaticAsm@CExpertInterior
004505E0:010C [004506EC] ?RunExpertInt@CExpertInterior
7C80B357:01B4 [7C80B50B] GetModuleFileNameA               [C:\WINDOWS\system32\kernel32.dll]

So, what has happened?

• Exception : C0000005 means an error condition that has occurred due to Access Violation.
• Access Type : read means that there has been an attempt to read.
• Acess Addr : 00000000 means that the out-of-process memory has a zero address.
  

Now, let us look at the call stack.

The address of 77C36FA3 is the same as that of the top of stack. This means that the error occurred during execution of the memcpy function that copies the content of one memory area to another one. At that, we can judge with much certainty about whether there was an attempt to copy data from the memory area having a zero address.

The second line of the call stack informs us about what function called the memcpy function with wrong parameters. This is the GetStringValue function from the library named ExpertSample.dll.

Let us have a look at the source code of this function:

__declspec(dllexport) char* __stdcall GetStringValue(char *spar)
  {
   static char temp_string[256];
//----
   printf("GetStringValue takes \"%s\"\n",spar);
   memcpy(temp_string,spar,sizeof(temp_string)-1);
   temp_string[sizeof(temp_string)-1]=0;
//----
   return(temp_string);
  }

We can see that the memcpy function is only once called within the above function. Since the first parameter indicates the existing memory area occupied by the temp_string variable, we can conclude that it is the second parameter which is worng. Indeed, there is no checking of the variable for 0 in the given example. Line if(spar==NULL) would protect us against the crash.

So, what should be done if there were more than one call of the memcpy function in the function anayzed? In our project settings, let us set up the output of the most detailed listing of the compilation.

After the project has been rebuilt, we will have a listing file with the extension .cod for every source .cpp file. We are now interested in the ExpertSample. cod, but only the part of code obtained for the GetStringValue function. Here it is:

?GetStringValue@@YGPADPAD@Z PROC NEAR           ; GetStringValue
 
; 70   :   {
 
  00051 55       push    ebp
  00052 8b ec        mov     ebp, esp
 
; 71   :    static char temp_string[256];
; 72   : //----
; 73   :    printf("GetStringValue takes \"%s\"\n",spar);
 
  00054 8b 45 08     mov     eax, DWORD PTR _spar$[ebp]
  00057 50       push    eax
  00058 68 00 00 00 00   push    OFFSET FLAT:$SG19680
  0005d ff 15 00 00 00
    00       call    DWORD PTR __imp__printf
  00063 83 c4 08     add     esp, 8
 
; 74   :    memcpy(temp_string,spar,sizeof(temp_string)-1);
 
  00066 68 ff 00 00 00   push    255            ; 000000ffH
  0006b 8b 4d 08     mov     ecx, DWORD PTR _spar$[ebp]
  0006e 51       push    ecx
  0006f 68 00 00 00 00   push    OFFSET FLAT:_?temp_string@?1??GetStringValue@@YGPADPAD@Z@4PADA
  00074 e8 00 00 00 00   call    _memcpy
  00079 83 c4 0c     add     esp,  12            ; 0000000cH
 
; 75   :    temp_string[sizeof(temp_string)-1]=0;
 
  0007c c6 05 ff 00 00
    00 00        mov     BYTE PTR _?temp_string@?1??GetStringValue@@YGPADPAD@Z@4PADA+255, 0
 
; 76   : //----
; 77   :    return(temp_string);
 
  00083 b8 00 00 00 00   mov     eax, OFFSET FLAT:_?temp_string@?1??GetStringValue@@YGPADPAD@Z@4PADA
 
; 78   :   }
 
  00088 5d       pop     ebp
  00089 c2 04 00     ret     4
?GetStringValue@@YGPADPAD@Z ENDP            ; GetStringValue
 

Digits 10001051:0028 in the second line of the call stack give the address within the GetStringValue function. After the function placed a line above in the call stack is executed, the control will be given to this address. In the object code, the GetStringValue function starts with address 00051 (it should be noted that addresses are presented in hexadecimal notation). Let us add 0028 to this value and so we will get address 00079. At this address, the add esp,12 instruction is situated that follows immediately after the memcpy function calling instruction. We have found this spot.

Let us investigate the case when the error occurs immediately inside of the imported function. Let us modify the code:

__declspec(dllexport) char* __stdcall GetStringValue(char *spar)
  {
   static char temp_string[256];
//----
   printf("GetStringValue takes \"%s\"\n",spar);
   for(int i=0; i<sizeof(temp_string)-1; i++)
     {
      temp_string[i]=spar[i];
      if(spar[i]==0) break;
     }
   temp_string[sizeof(temp_string)-1]=0;
//----
   return(temp_string);
  }

We have replaced the memcpy function call with our own byte-wise data copying loop. But we did not use the checking for 0 in order to create an error condition and the error report. In the new report, the call stack looks a bit different:

Call stack  :
10001051:003A [1000108B] GetStringValue                   [C:\Program Files\MetaTrader 4\experts\libraries\ExpertSample.dll]
00452DD0:065E [0045342E] ?CallDllFunction@CExpertInterior
00459AC0:3B67 [0045D627] ?ExecuteStaticAsm@CExpertInterior
004505E0:010C [004506EC] ?RunExpertInt@CExpertInterior
7C80B357:01B4 [7C80B50B] GetModuleFileNameA               [C:\WINDOWS\system32\kernel32.dll]

The error occurred at address 003A in the GetStringValue function. Let as look at the generated listing.

?GetStringValue@@YGPADPAD@Z PROC NEAR           ; GetStringValue
 
; 70   :   {
 
  00051 55       push    ebp
  00052 8b ec        mov     ebp, esp
  00054 51       push    ecx
 
; 71   :    static char temp_string[256];
; 72   : //----
; 73   :    printf("GetStringValue takes \"%s\"\n",spar);
 
  00055 8b 45 08     mov     eax, DWORD PTR _spar$[ebp]
  00058 50       push    eax
  00059 68 00 00 00 00   push    OFFSET FLAT:$SG19680
  0005e ff 15 00 00 00
    00       call    DWORD PTR __imp__printf
  00064 83 c4 08     add     esp, 8
 
; 74   :    for(int i=0; i<sizeof(temp_string)-1; i++)
 
  00067 c7 45 fc 00 00
    00 00        mov     DWORD PTR _i$[ebp], 0
  0006e eb 09        jmp     SHORT $L19682
$L19683:
  00070 8b 4d fc     mov     ecx, DWORD PTR _i$[ebp]
  00073 83 c1 01     add     ecx, 1
  00076 89 4d fc     mov     DWORD PTR _i$[ebp], ecx
$L19682:
  00079 81 7d fc ff 00
    00 00        cmp     DWORD PTR _i$[ebp], 255    ; 000000ffH
  00080 73 22        jae     SHORT $L19684
 
; 76   :       temp_string[i]=spar[i];
 
  00082 8b 55 08     mov     edx, DWORD PTR _spar$[ebp]
  00085 03 55 fc     add     edx, DWORD PTR _i$[ebp]
  00088 8b 45 fc     mov     eax, DWORD PTR _i$[ebp]
  0008b 8a 0a        mov     cl, BYTE PTR [edx]
  0008d 88 88 00 00 00
    00       mov     BYTE PTR _?temp_string@?1??GetStringValue@@YGPADPAD@Z@4PADA[eax], cl
 
; 77   :       if(spar[i]==0) break;
 
  00093 8b 55 08     mov     edx, DWORD PTR _spar$[ebp]
  00096 03 55 fc     add     edx, DWORD PTR _i$[ebp]
  00099 0f be 02     movsx   eax, BYTE PTR [edx]
  0009c 85 c0        test    eax, eax
  0009e 75 02        jne     SHORT $L19685
  000a0 eb 02        jmp     SHORT $L19684
$L19685:
 
; 78   :      }
 
  000a2 eb cc        jmp     SHORT $L19683
$L19684:
 
; 79   :    temp_string[sizeof(temp_string)-1]=0;
 
  000a4 c6 05 ff 00 00
    00 00        mov     BYTE PTR _?temp_string@?1??GetStringValue@@YGPADPAD@Z@4PADA+255, 0
 
; 80   : //----
; 81   :    return(temp_string);
 
  000ab b8 00 00 00 00   mov     eax, OFFSET FLAT:_?temp_string@?1??GetStringValue@@YGPADPAD@Z@4PADA
 
; 82   :   }
 
  000b0 8b e5        mov     esp, ebp
  000b2 5d       pop     ebp
  000b3 c2 04 00     ret     4
?GetStringValue@@YGPADPAD@Z ENDP            ; GetStringValue

The initial address is the same: 00051. Let us add 003A and obtain address 0008B. At this address, the mov cl, BYTE PTR [edx] instruction is situated. Let us see the registers contents in the report:

Registers   : EAX=00000000 CS=001b EIP=1000108B EFLGS=00010246
            : EBX=FFFFFFFF SS=0023 ESP=0259FAD4 EBP=0259FAD8
            : ECX=77C318BF DS=0023 ESI=018ECD80 FS=003b
            : EDX=00000000 ES=0023 EDI=000000E8 GS=0000

Well, of course, EDX register contains zeros. We accessed to the out-of-process memory and got the crash.

In the end, two lines about how we have passed zero indication to the imported function.

   string null_string;
   string sret=GetStringValue(null_string);

We passed an uninitialized string as a parameter. Be careful with uninitialized strings, always check the received indications for NULL, and let you have as few crashes as possible.