Debugging Software Crashes II
This article continues our discussion on debugging software crashes. Here we focus on memory corruption crash symptoms. We will also look at the special considerations in debugging C++ code crashes. Finally we will look at techniques to simplify crash debugging.
Debugging Memory Corruption
Programs store data in any of the following ways:
|Global||All variables of objects declared as global in a C/C++ program fall into this category. This also includes static variable declarations.|
|Heap||Memory allocated using new or malloc is allocated on the heap. In many systems, stack and heap are allocated from opposite sides of a memory block. (See the figure below)|
|Stack||All local variables and function parameters are passed on the stack. Stack is also used for storing the return address of the calling functions. Stack also keeps the register contents and return address when an interrupt service routine is called.|
Memory corruption in the global area, stack or the heap can have confusing symptoms. These symptoms are explored here.
Global Memory Corruption
If a global data location is found to be corrupted, there is good chance that this is caused by array index overflow from the previous global data declarations. Also the corruption might have been caused by an array index underflow (array accessed with a negative index) from the next variable declarations. The following rules should be helpful in debugging this condition:
- If you have a debugging system which allows you to put breakpoints on data write to a certain location, use that feature to find the offending program corrupting the memory. If you don't have the luxury of such a tool, the following steps might help.
- If the variable is a part of structure, check if overflow/underflow of previous or next variables in the structure could have caused this corruption.
- If other structure member access seems harmless, use the linker generated symbol map to locate other global variables declared in the vicinity of the corrupted structure. Examine the data structures to determine if they could have caused the corruption.
- Sometimes looking at the corrupted memory locations can also give a good idea of the cause of corruption. You might be able to recognize a string or data pattern identifying the culprit. This might be your only hope if the corruption is caused by an un-initialized pointer.
- Extent of corruption might also give a clue of the cause of corruption. Try to determine the starting and ending points of a corruption (only possible if the corrupting program is writing in an identifiable pattern).
Heap Memory Corruption
Corruption on the heap can be very hard to detect. A heap corruption could lead to a crash in heap management primitives that are invoked by memory management functions like malloc and free. It might be very hard to detect the original source of corruption as the buffer that lead to corruption of adjacent buffers might have long been freed. Guidelines for debugging crashes in heap area are:
- If a crash is observed in memory management primitives of the operating system, heap corruption is a possibility. It has been observed that memory buffer corruption sometimes leads to corruption of OS buffer linked list, causing crashes on OS code.
- If a memory corruption is observed in an allocated buffer, check the buffers in the vicinity of this buffer to look for source of corruption.
- Corruption of buffers close to heap boundary might be due to stack overflow or stack overwrite leading to heap corruption (see the above figure)
- Conversely, stack corruption might take place if a write into the heap overflows and corrupts the stack area.
Stack Memory Corruption
Stack corruption by far produces the most varied symptoms. Modern programming languages use the stack for a large number of operations like maintaining local variables, function parameter passing, function return address management. See the article on c to assembly translation for details.
Here are the rules for debugging stack corruption:
- If a crash is observed when a function returns, this might be due to stack corruption. The return address on the stack might have been corrupted by stack operations of called functions.
- Crash after an interrupt service routine returns might also be caused by stack corruption.
- Stack corruption can also be suspected when a passed parameter seems to have a value different from the one passed by the calling function.
- When a stack corruption is detected, one should look at the local variables in the called and calling functions to look for possible sources of memory corruption. Check array and pointer declarations for sources of errors.
- Sometimes stray corruption of a processors registers might also be due to a stack corruption. If a register gets corrupted due to no reason, one possibility is that an offending thread or program corrupted the register context on the stack. When the register is restored as a part of a context switch, the task crashes.
- Corruption in heap can trickle down to the stack.
- Stack overflow takes place when a programs function nesting exceeds the stack allocated to the program. This can cause a stack area or heap area corruption. (Depends upon who attempts to access the corrupted memory first, a heap operation or stack operation).
Crash Debugging in C++
We have been discussing crash debugging techniques that apply equally well to C as well as C++. This section covers crash debugging techniques that are specific to C++.
Invalid Object Pointer
Many C++ developers get confused by crashes that involve method invocation on a corrupted pointer. Developers need to realize that invoking a method for an illegal object pointer is equivalent to passing an illegal pointer to a function. A crash would result when any member variable is accessed in the called method.
In the example given below, when HandleMsg() is invoked for a NULL pX, the crash will result only when an access is attempted to member variables of X. There will be no problem in calling PrepareForMessage() or HandleYMsg() for Y pointer. (For more details on this refer to C and C++ article.
V-Table Pointer Corruption
All classes with virtual functions have a pointer to the V-table corresponding to overrides for that class. The V-table pointer is generally stored just after the elements of the base class. Corruption of the v-table pointer can baffle developers as the real problem often gets hidden by the symptoms of the crash.
The figure above shows the declaration of class A and B. The figure below shows the memory layout for an object of class B. If m_array array is indexed with an index exceeding its size, the first variable to be corrupted will be the v-table pointer. This problem will manifest as a crash on invoking method SendCommand. The reason this happens is that SendCommand is a virtual function, so the real access will be using a virtual table. If the virtual table pointer is corrupted, calling this function will take you to never-never land.
For more details on v-table organization refer to C and C++ Comparison II article.
Dynamic Memory Allocation
Many C++ programs involve a lot of dynamic memory allocation by new. Many C++ crashes can be attributed to not checking for memory allocation failure. In C++ this can be achieved in two ways:
- Handle out of memory exception
- Check for new returning a NULL pointer.
Simplifying Crash Debugging
Here are a few simple techniques for simplifying crash debugging:
Obtaining Stack Dump
Make sure that every embedded processor in the system supports dumping of the stack at the time of crash. The crash dump should be saved in non volatile memory so that it can be retrieved by tools on processor reboot. In fact attempt should be made to save as much as possible of processor state and key data structures at the time of crash.
An ounce of prevention is better than a pound of cure. Detecting crash causing conditions by using assert macro can be a very useful tool in detecting problems much before they lead to a crash. Basically assert macros check for a condition which the function assumes to be true. For example, the code below shows an assertion which checks that the message to be processed is non NULL. During initial debugging of the system this assert condition might help you detect the condition, before it leads to a crash.
Note that asserts do not have any overhead in the shipped system as in the release builds asserts are defined to a NULL macro, effectively removing all the assert conditions.
Defensive Checks and Tracing
Similar to asserts, use of defensive checks can many times save the system from a crash even when an invalid condition is detected. The main difference here is that unlike asserts, defensive checks remain in the shipped system.
Tracing and maintaining event history can also be very useful in debugging crashes in the early phase of development. However tracing of limited use in debugging systems when the system has been shipped