In gdb, when debugging inside a function, we can use "finish" command to run to the end of a function.
My question is: how does gdb know the ending position of a function, especially when there's no debugging symbol to match source code "{}"?
I guess gdb looks for either "leave" or "mov %rbp, %rsp,pop %rbp" under x86 in order to judge whether it has reached the end of a function.
But the problem is,
(1) There're still some extra registers that needs to push/pop at the begin/end of a function call, depending on source code and ABI structure.
(2)The number of registers needs to be push/pop is decided during compilation phase, and I'm afraid this "number" information is not available throw binary executable file.
So, how does gdb determine, where is the end of a function call, so that "finish" command can jump to it?
Thanks!
gdb doesn't try to analyze the machine code. Instead, it unwinds the stack, finds the caller's PC, and sets a temporary breakpoint there. Then it lets the inferior run until the breakpoint is hit.
Due to the way gdb's unwinder is designed, this automatically handles finish from an inlined function as well (though there are still a few special cases in the code due to this).
Related
Basically I am using ptrace to inject a shell code to a remote process for execution. But I found some weird behavior regarding RIP register.
What I do is I copy my shell code to the start address of where the program is mapped. Then I set the RIP using ptrace to the address where the start address is. And then I resume the target process for executing the code. Once the shell code finishes (by running int3) I will get signal and recover the code that I just modified.
It works fine except when the remote process is blocked inside of a system call like sleep. If the remote process is blocked inside of a system call at the moment I attach the process, after I set the RIP to where I want to execute my shell code and then resume the target process, I will observe that the RIP is actually 2 less than what the address that I put in the ptrace call. For example if I set the RIP to be 0x4000, once I resume it the RIP becomes 0x3ffe. Typically it crashes for my case due to the segment fault, obviously. But if I grab the register right after I set it without resuming the process, the RIP is the value that I just set. Currently I work around it by insert 2 nop instructions ahead of my shell code and always add 2 when I set the RIP. I just want to know is there anything that I miss for setting the RIP or my whole method for injecting code is totally unstable?
My dev box is Ubuntu14.04, kernel is 3.13.0-45-generic.
If I recall correctly, if you interrupt the process while it's blocked in a syscall, the program counter value, upon continuing, will be subtracted by sizeof(syscall instruction) by the kernel. So once you do a PTRACE_DETACH, the process will re-do the syscall it was interrupted from.
I overcome the problem the same way you did (always adding a tiny nop-sled and incrementing RIP).
I found a trick to automate gdb commands "less disturbing", to do it I simply switch to the last thread in non-stop mode and stop that thread, then execute the requested commands and continue that thread and then return back to main thread.
But this trick won't work if the inferior only has only one thread. So I need to create a thread that runs forever at the background, by this way I'll be able to attach to it anytime I want without having to disturb the inferior.
The only solution that came to my mind was this:
1-)Break at syscall
2-)Allocate some memory with gdb to inject codes
3-)Replace syscall with the jmp instruction that points to the allocated memory
4-)pushad and execute the code that "somehow" creates the thread
5-)Replace the jmp with syscall back
6-)popad and jmp back to where syscall located
But this is way too hacky and I still have no idea about the "somehow" part. Is there a more elegant way to do this? Maybe gdb has some tools for it and I'm missing it. If not, how can I do the "somehow" part?
Yes, it is. I found an elegant way to do it, you don't have to do such hacky stuff. You can inject a thread like this:
1-)Write a code that crates a thread in any compilable language
2-)Compile it so a .so file(-g option should be passed, gdb will need debug symbols)
3-)Load it to the inferior with linux system call dlopen: call dlopen(".so path",int)
4-)Call any function you implemented in the .so file you injected by executing call funcname(), gdb will pick it up automatically when you press Tab(that's why you need debug symbols)
This is related to: https://stackoverflow.com/a/13413099/1284631
Now, the question is:
Why the reboot() system call, when called with LINUX_REBOOT_CMD_HALT parameter (see here: http://lxr.linux.no/linux+v3.6.6/kernel/sys.c#L480) is calling do_exit(0) after having already called kernel_halt(), as calling kernel_halt() boils down to calling stop_this_cpu() (see here: http://lxr.linux.no/linux+v3.6.6/arch/x86/kernel/process.c#L519), as part of native_machine_halt() (see here: http://lxr.linux.no/linux+v3.6.6/arch/x86/kernel/reboot.c#L680).
Or, it seems to me that stop_this_cpu() is never returning (it ends with an infinite loop).
So, it is do_exit(0) called just in case that kernel_halt() doesn't do its job and it return? Why not panic() directly instead, then?
Some ideas:
It may be that kernel_halt() refuses to actually halt for a legitimate reason, though I can't think of any.
kernel_halt() may be designed to also be called by a hypervisor or something at a higher or equivalent level than the kernel (custom SMI code maybe?)
Perhaps the kernel_halt() function returns early, "scheduling" the halt, and the actual halt takes place some time later on some hardware. I remember reading about performing an ATX power off in DOS in assembly - you would issue the outb instruction to intiate the power off, but you'd have to have some nops, an endless loop, or a hlt right afterward, as the actual power off could happen some cycles later.
The calling process may wish to handle failure to reboot some other way than a kernel panic.
I am interested in replacing a system call with a custom that I will implement in linux kernel 3.
I read that the sys call table is no longer exposed.
Any ideas?
any reference to this http://www.linuxtopia.org/online_books/linux_kernel/linux_kernel_module_programming_2.6/x978.html example but for kernel 3 will be appreciated :)
Thank you!
I would recommend using kprobes for this kind of job, you can easily break on any kernel address (or symbol...) and alter the execution path, all of this at runtime, with a kernel module if you need to :)
Kprobes work by dynamically replacing an instruction (e.g. first instruction of your syscall entry) by a break (e.g. int3 on x86). Inside the do_int3 handler, a notifier notifies kprobes, which in turn passes the execution to your registered function, from which point you can do almost anything.
A very good documentation is given in Documentation/kprobes.txt so as a tiny example in samples/kprobes/kprobes_example.c (in this example they break on do_fork to log each fork on the system). It has a very simple API and is very portable nowdays.
Warning: If you need to alter the execution path, make sure your kprobes are not optimized (i.e. a jmp instruction to your handler replaces the instruction you break onto instead of an int3) otherwize you won't be able to really alter the execution easily (after the ret of your function, the syscall function will still be executed as usual). If you are only interested in tracing, then this is fine and you can safely ignore this issue.
Write a LKM that would be better optio.What do you mean by replace,do you want to add a new one.
I'm debugging a reference leak in a GObject-based application. GObject has a simple built-in mechanism to help with such matters: you can set the g_trap_object_ref variable in gobject.c to the object that you care about, and then every ref or unref of that object will hit a breakpoint instruction (via G_BREAKPOINT()).
So sure enough, the program does get stopped, with gdb reporting:
Program received signal SIGTRAP, Trace/breakpoint trap.
g_object_ref (_object=0x65f090) at gobject.c:2606
2606 old_val = g_atomic_int_exchange_and_add ((int *)&object->ref_count, 1);
(gdb) _
which is a great start. Now, normally I'd script some commands to be run at a breakpoint I manually set using commands 3 (for breakpoint 3, say). But the equivalent for SIGTRAP, namely handle SIGTRAP, doesn't give me the option of doing anything particularly interesting. Is there a good way to do this?
(I'm aware that there are other ways to debug reference leaks, such as setting watchpoints on the object's ref_count field, refdbg, scripting regular breakpoints on g_object_ref() and g_object_unref(). I'm about to go try of those now. I'm looking specifically for a way to script a response to SIGTRAP. It might come in useful in other situations, too, and I'd be surprised if gdb doesn't support this.)
Do you want to show some values and continue execution of the program? In that case, just define a macro that displays the values you're interested in, continues execution and calls itself recursively:
define c
echo do stuff\n
continue
c
end
GDB doesn't support it.
In general, attaching a command script to signal makes little sense -- your program could be receiving SIGTRAP in any number of places, and the command will not know whether a particular SIGTRAP came in in expected context or not.