Debugger and process memory
Just a small note about the debugger plugins and events. Many users
who try to develop a plugin for the debugger notice that IDA
behaves slightly differently in the notification callbacks than anywhere else.
For example, IDA might claim that EIP points to an address without a segment,
or none of exported names of a loaded DLL are available.
This happens because of the database synchronization. When you query IDA
about a segment or a name, the information from the database is returned.
The database is not (and can not be) always in sync
with the process memory. The debugged process might allocate or free memory chunks
thousands of times per second. The synchronization operation is very expensive:
it requires enumerating all memory regions and collecting information about their types, permissions, etc.
That’s why IDA tries to perform it as rarely as possible.
We tried to save the end user from this problem: the database
gets synchronized as soon as the process is suspended. This is the
expected behavior, so far so good.
Alas, we can not provide plugin writers with the same virtual reality: there are
two distinct entities, the process memory and the program segmentation. The latter is stored in the database
and it can go out of sync with the former. As a plugin writer, you have the following
- Bypass the database and talk directly to the debugger module using the
dbg->read_memory() and dbg->write_memory() functions.
Advantages: no synchronization issues
Disadvantages: only memory read/write functions are available, names, functions,
and other high level abstractions are not; also, direct calls are not cached.
- Explicitly synchronize the database with the process memory.
Advantages: all high level abstractions are available,
repetitive “read memory” requests
will be cached by the kernel (especially useful for remote debugging);
breakpoints in the process memory are hidden
- Do not synchronize.
Disadvantages: occasionally a call to an IDA function might fail (e.g. getseg()
might return NULL for a valid address); nevertheless, this approach can be used if
you know in advance that the memory config won’t change or the changes are irrelevant to your analysis
Feel free to use the best method depending on your needs. When you need to force
a synchronization, use this:
Or maybe a better approach exists?.. 😉