I am trying to write a Python script for GDB to trace a function.
The idea is to set a breakpoint on an address location, let the program run and then, when it breaks, log to file registers, vectors and stack and find out what address the next instruction will be, set a breakpoint on that location and rinse and repeat.
I read through the documentation and I'm pretty confident registers, vectors and memory locations can be easily dumped. The actual problem is finding what the next instruction location will be as it requires to analyze the disassembly of the current instruction to determine where the next breakpoint should be placed.
Update
I am doing all this without using stepi or nexti because the target I'm debugging works only with hardware breakpoints and as far as I know those commands use software breakpoints to break at the next instruction
Is there anything like that in GDB?
Yes, you can do this in gdb. Rather than trying to set a breakpoint on the next instruction, you can instead use the si command to single-step to the next instruction.
Related
I am attempting to use this answer to generate an instruction trace between two lines of code.
Unfortunately, the condition in the while loop is a simple count, and I need to keep running the loop until a particular line of code in the source code is reached.
Does there exists a way to check whether we are either on a particular line of code, or at a particular breakpoint, within a pure gdb script?
I am aware of the solution here which uses the Python API. I am also aware of pin-instat, but I want to know whether this can done with pure gdb.
What if do what you want in this way
1) Get information about pc for the line which you would like to reach
Use info line or use disas /m to get information about addresses of the particular line of code.
2) Write the similar loop as in Tracing/profiling instructions
while $pc != ADDRESS-FROM-FIRST-STEP
si
end.
This way you will keep running the loop until a particular line of code in the source code is reached
I'm running through some assembly code in GDB trying to debug my program and I'd like to see what happens if I ignore an instruction entirely, is there a way to do this? (skip past it to the next line without executing it) without having to edit the source code and comment out the function and then recompile?
is there a way to do this
Sure: jump *0x1234 will jump to instruction at address 0x1234.
skip past it to the next line without executing it
"Next line" and assembly debugging rarely go together. As this answer shows, you can skip a line as well.
I am using GDB along with a JTAG device, an Abatron BDI2000, to debug a programs running on a Motorola M68332.
The 68332 does not have any hardware breakpoint registers. It has very primitive debugging features.
The build tools do not generate 'elf' files, so no symbols for GDB to use.
Also the program I'm debugging is running in Flash.
In fact the 68332 has only one debug instruction, ti. ti by itself steps to the next assembly instruction. ti xxx steps until the address xxx is reached. [Yes, this is caveman days, cold hammer and chisel :)]
I am able to use GDB with target remote to connect to the BDI2000 and issue the GDB commands 'nexti'. Due to the limitations of the 68332, 'stepi' is equivalent to 'nexti'.
Single stepping is only command available.
The monitor command 'monitor ti ' states change the program counter to and step.
If one uses a 'monitor' command that changes the registers, then GDB does not know about the command and its register cache become out of sync. I have created GDB functions which have the GDB command 'flushregs' at the end of each of them. This marks the register cache dirty. The GDB command will fetch a new set of registers.
I would like to create a symbol table file for debugging, but have not found any documentation on the GDB symbol file format.
Are there alternatives to what I have setup?
I do have a RAM overlay for the Flash area. Would this allow software breakpoints?
Thanks in advance for any advice.
I found I can use 'convenience' variables as a substitute for symbols, since I'm not using ever symbol in the program all at once.
set $Symbol=(unsigned int*)<address>
Each 'Symbol' is declared a pointer to an unsigned int at an address. One can put these statements in .gdbinit, and add to them over time.
One can then state
break $Symbol
I show a GDB command function that can be passed one of these 'convenience' variables in the question linked below.
How do I write a GDB function to make a comparison to the program counter
Is there a way to set a breakpoint at every line in the code with GDB? Obviously I don't want to hit b *addr for every single line, so I'm wondering if there's a fast way to do this.
Edit
Note that I am running a binary created by someone else and I do not have access to the source code. Unfortunately, that binary has not been compiled with the -g flag. Therefore, I cannot just single step through each line in the code.
Further Edit
As Jason points out below, you can indeed single step through the code so long as you use si or ni, as opposed to just simply s (step) or n (next). n or s work fine, though, if the source code had been compiled with -g, but it steps through lines of source code, as opposed to stepping through every assembly instruction like ni or si do in a binary that was compiled without -g.
Use si (stepi) to instruction step through the code. You can use ni (nexti) to step over library functions you're not interested in. If you accidentally step into one of them, finish should get you back to your original routine. People working at this level typically have gdb set to display the next few instructions that are about to be executed, e.g. disp/3i $pc.
Can't you just place the breakpoint on the first line of execution and then step through each line ? This depends on what are you trying to achieve by setting breakpoints on each line. If you want to evaluate expressions, you can do it by following my logic (step through each line).
PowerPC has hardware support for ranged breakpoints, and GCB offers:
break-range start end
in that arch. So I think you could just break on the entire memory address, or the entire text section (untested).
The command fails on x86.
Doc: https://sourceware.org/gdb/onlinedocs/gdb.html#index-break_002drange-1548
I am writing gdb command scripts to simplify the debugging. One of the problems I have
is that I am setting a breakpoint, and I want to disable it afterwards, and only enable it after another breakpoint is hit.
What I want to do is this
$my_break_number = break SomeFile.cpp:231
disable $my_break_number
but unfortunately gdb doesn't work this way. I have read the manual, but I cannot find any information on how to do this. Hopefully there is some information I have missed.
gdb will automatically set a convenience variable $bpnum with the last set breakpoint number.
You can possibly use that after setting a breakpoint to disable it (I haven't tested when a breakpoint is ambiguous and creates multiple breakpoints, I think it will work and disable all breakpoint locations created.)
see: http://sourceware.org/gdb/current/onlinedocs/gdb/Set-Breaks.html#Set-Breaks
if you need to use the breakpoint number from commands, that is probably not what you want, but it works for the question as specified.
It sounds like you may want to use the Python GDB scripting, which gives you a lot better programmatic access to breakpoints than what is possible with "regular" command scripts.
Also info breakpoints gives useful information such as:
number of breakpoint, how many time the breakpoint was hit, address in memory, what function is it in, file and line number of breakpoint