- Part 1 - Installation, Interface, Symbols, Remote/Local Debugging, Help, Modules, and Registers
- Part 2 - Breakpoints
- Part 3 - Inspecting Memory, Stepping Through Programs, and General Tips and Tricks
SteppingReally the whole reason you're using a debugger is to inspect the state of a process during a specific operation or function. Just about every instruction that gets executed alters the program state in some way which means having the ability to execute an instruction then inspect the state is extremely important. The first part of this is "stepping" - executing instructions then pausing. WinDBG offers a number of different stepping commands depending on where you are in the program and where you want to go.
- Step-Into - When the instruction is a
call, follow the
calland pause at the first instruction in the called function.
- Step-Over - When the instruction is a
call, execute the function and all subfunctions, pausing at the instruction in the current function after the
- Step-Out - Execute all instructions and pause after the current function is complete (
retat the end of the current function)
A note to make here is that both Step-Into and Step-Over will execute a single instruction and pause - behavior only changes when a
callinstruction is reached.
g(Go) command is more of a breakpoint command but its functionality blurs the lines between breakpoints and stepping commands. It's used to resume execution of program but unlike most of the stepping commands, it not really meant to be used on an instruction by instruction basis.
gwill resume the program until a breakpoint or exception occurs. Really, you would use
gto execute all of the instructions up to a breakpoint, whereas with stepping commands you're executing instructions without setting a breakpoint. However, to clarify, debuggers will pause when hitting a breakpoint regardless if you use a stepping command or something like
gis straightforward to use:
While the program is running, WinDBG will give you a message in the command input box:
If you know the address you'd like to execute until then just provide it as an argument to
0:001> g notepad!WinMainCRTStartup
Single SteppingExecuting a single instruction, then pausing is called Single Stepping. This can be achieved by either using the "Step-Into" or "Step-Over" commands since both behave the same on non-
callinstructions. Rather then show them both here, let's look at these commands individually.
To Step-Into with WinDBG, use the
t(Trace) command. Each step will show you the state of the registers and the instructions that will be executed. In this example we'll pause at the program's entry point (
notepad!WinMainCRTStartup) and look at the first few instructions to be executed (
u eip). The first instruction is a
notepad!__security_init_cookiefunction. Let's see how debugger behaves when Stepping-Into with
Here we can see that we were running within
notepad!WinMainCRTStartup, then on the
tto follow the call into the
notepad!__security_init_cookiefunction, where we paused on the first instruction.
WinDBG uses the
pcommand to step over a function call. This means that the
calland all subinstructions within the called function will be executed and the program will pause on the next instruction within the current function (e.g.
notepad!WinMainCRTStartup). Let's look at the same scenario, but this time we'll use
Here we can see that the instruction after the
push 58h. When we Step-Over with
pwe automatically execute everything within the
notepad!__security_init_cookiefunction, then pause at the
Stepping-Out with WinDBG can be achieved with the
gu(Go Up) command. This command scans the current function for a
retthen pauses after it gets executed. This an important behavior, because if, for whatever reason, the function doesn't end in a
retor a code path doesn't lead to one, you could experience unexpected results with
gu. Let's see what it looks like:
Here we've paused on
notepad!WinMainCRTStartup+0x1dwhich is a
notepad!_imp__GetStartupInfoA. We can see (
u eip L2) that the instruction after the
mov dword ptr [ebp-4],0FFFFFFFEh. So we'll single step (
t) into the function and pause at the first instruction. Now we use
guto execute all instructions and function calls in our child function then pause on the next instruction in the parent function, which is our
mov dword ptr [ebp-4],0FFFFFFFEh
Executing until Return
guis good and all, but sometimes you want to look at the stack right before the function returns, in this scenario, you'll need to use either
tt(Trace to Next Return) or
pt. Both are easy to call:
The important thing here to remember is that
ttwill stop at the next return, even if its not in the current function. For instance, consider the following pseudocode, our goal is to pause on the
func: call somefunc ret somefunc: call someotherfunc ret someotherfunc: ret
In this example, if pause at
call somefunc, then use
tt, we'll end up pausing at the
A better approach for this scenario might be to use
pt: Using the same pseudocode, if we pause at
call somefunc, then use
pt, we'd execute all the code in
someotherfunc), then pause at the
func. In all reality for this example we could just use
p, but that doesn't illustrate the point :)
Ultimately it depends on what you, as the person using the debugger, want to do.
Inspecting MemoryNow we can finally get into the most important part of debugging: Inspecting Memory. WinDBG provides the
d(Display Memory) command for this purpose. In its most simple form you can run it like this:
But this is more or less useless. Running
dby itself for the first time will output the memory where
eippoints to. This is useless because
eipshould be pointing to a code segment and, to make sense of that, you'd really need to use the
u(Unassemble) command. So a much better starting out command would be:
0:001> d esp
This will show us the values on the stack. With
d, WinDBG will display data using the format specified by the last
dcommand executed. If this is you're first time running
d, it doesn't have previous command stored, so WinDBG will give you the output of the
db(Display Byte) command.
dbwill output the data in bytes and provide the corresponding ASCII values:
Display WordsWords, or 2 byte values, can be shown with
dw(Display Word). Alternatively, you can use
dWto show Words and ASCII values:
Display DWORDsMy favorite memory viewing command is
dd(Display DWORDs). A DWORD is a double word, so 4 bytes.
ddwill just show you the DWORDS while
dcwill show DWORDs and ASCII values:
Display QuadwordsTo display quadwords (4 words/8 bytes) within WinDBG, use
Display BitsYou can even show binary with
Displaying StringsStrings are displayed with
da, essentially WinDBG will print everything as ASCII until it reaches a null byte. So here, even though
espisn't a string, it'll treat everything as a string until it reaches a null. Just to further illustrate this, I've printed out the 5 bytes at
db esp L5:
AddressingSo far we've just been looking at the memory that
espis pointing to by using
espas the parameter to our memory inspection comman however there are a number of different ways to reference memory that can be useful when starting out.
Registers - As we've seen, you can use any register and WinDBG will use the address in that register as the memory address:
0:001> dd eax
Memory Address - You can also just use the memory address itself by providing it:
0:001> db 0020faa0
Offsets - You can also use offsets with registers or memory addresses by using mathematical expressions:
0:001> db 0020faa0 + 18
0:001> dd ebp - 18
0:001> dq ebp*eax
These expressions can be used wherever an address can be used. Here's what it looks like in WinDBG:
WinDBG will output question marks (
?) for invalid/free memory.
PointersThere are often times where a value on the stack is just a pointer to another location. If you'd like to look at that value you'd need to do two look ups. For instance, say we know that value
ebp+4is a pointer to some assembly code that we want to read. To view that assembly, we'd need two commands. The first command shows us the memory address at
0:001> dd ebp+4
Then the second requires us to manually copy the value at that address then paste it in as an argument to the
ucommand so we can view that assembly:
0:001> u 777be2d1
This is all fine, but there's an easier way with the
poi()we just provide
ebp+4as its parameter and it will automatically take the value at that address and use it, rather then just using the value of
0:001> u poi(ebp+4)
Limiting outputBy default WinDBG will output a set amount of data, however we can limit how of that data is outputted with the
L(Size Range Specifier) attribute.
Lwill work with most commands and just needs to be appended to the end with a value:
0:001> dd esp L1
The number specified with
Lis the size, which is related to the command executed. For instance, with
Lwill mean the number of bytes to print, while with
Lwill mean the number of DWORDS.
That's really it to get you off the ground inspecting memory - I know, three blog posts building up to this functionality, and its just this tiny little section? Yup - there are some more memory inspection commands, but to get started,
dis the core command. Check out the tips below for more info.
Tips and TricksNow that you're off the ground, lets look at some handy tricks and tips that can make your debugging experience much better.
Keyboard ShortcutsChances are you'll be starting and stopping an application hundreds of times while you're debugging so any little shortcut can solve you tons of time in the long run. Keyboard shortcuts are huge, here are the four I use the most:
- F6 - Attach to a process. With the Attach Window open, use the "End" key to drop down to the bottom (where newly launched applications are).
- CTRL + E - Open Executable
- CTRL + Break - "Break" into a running application - used to pause a running program
- F5 - Shortcut for
Converting FormatsIf you haven't figured this out already, WinDBG prints numbers in hex by default. That means
12isn't the same as decimal 12. One quick tip is the
.formatscommand. To use it is straightforward:
0:001> .formats value
Where value is something you want to convert. So
.formatswill take whatever you provide and output it in a variety of formats:
12is actually 18 :). However, you can also provide decimal values using the
MathThere may be times where you need to calculate an offset or just do some basic math. WinDBG will evaluate expressions with the
These expressions can be as simple or as complicated as you'd like can contain all of the standard addressing that WinDBG uses:
calc.exewhen you have WinDBG!
ExtensionsTo make life easier, there are a number of extensions that people have created for WinDBG. These are great little tools that can be used within the debugger to provide functionality. Some are even built by Microsoft. The useful ones are
!peb. I'll cover these and more in another blog post - So stay tuned!
Cheat SheetsThere are a couple of really nice WinDBG command references, cheat sheets, and tutorials out there if you don't like
.hh, here are a couple good ones:
Typo: you posted the wrong pic for the "u poi(ebp+4)"-exampleReplyDelete
Fixed - thank you very much!Delete
Awesome tutorials :)ReplyDelete