<div dir="ltr">Yes, it would fit. I can even insert hooks manually, like:<div><br></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> std::function<void()> instrumentation_hook = []{ /*dear lldb script, please insert your code here*/ }</div><div><br></div><div>instrumentation_hook(); // run instrumentation, by default does nothing.</div></blockquote><div><br></div><div>Is there an easy way to compile some code in lldb and assign to instrumentation_hook ?</div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">2017-02-13 16:33 GMT+03:00 Pavel Labath <span dir="ltr"><<a href="mailto:labath@google.com" target="_blank">labath@google.com</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Making the code persist is easy - that's sort of what expr --top-level does.<br>
<br>
The tricky part is getting your code to execute. When you evaluate<br>
expressions interactively, we manually modify the registers (PC being<br>
the most important one) to point to the code you want to execute. If<br>
you want it to happen automatically at runtime, you would have to<br>
insert a jump instruction somewhere. The problem is, that can't<br>
usually be done without overwriting a couple of instructions of<br>
original code, which means you then have to somehow simulate the<br>
effects of the overwritten instructions. And there's always a danger<br>
that you will overwrite a jump target and things will blow up when<br>
someone tries to jump there. The way this is normally done is that you<br>
have the compiler insert hooks into your code during compilation, that<br>
you can then intercept if necessary. I am not sure if that would fit<br>
your use case.<br>
<span class="HOEnZb"><font color="#888888"><br>
pl<br>
</font></span><div class="HOEnZb"><div class="h5"><br>
<br>
<br>
On 13 February 2017 at 13:13, Roman Popov <<a href="mailto:ripopov@gmail.com">ripopov@gmail.com</a>> wrote:<br>
> Thanks Pavel, you are correct. This was the direction I thought to<br>
> investigate, but I didnt done my homework yet.<br>
><br>
> Yes, dynamic instrumentation is what I want. Looks like both lldb and gdb do<br>
> not allow this directly.<br>
><br>
> As GDB doc says "After execution, the compiled code is removed from gdb and<br>
> any new types or variables you have defined will be deleted."<br>
><br>
> Do you know why it is the case? Why cannot my generated code persist?<br>
><br>
><br>
> Looks like technically it is possible. For example you can allocate memory<br>
> for generated code on heap.<br>
><br>
> GDB does not even allow me to define a new function. But for data dynamic<br>
> allocation works perfectly fine:<br>
><br>
><br>
> Example:<br>
><br>
> #include <stdio.h><br>
><br>
> char message[1000] = "test message\n";<br>
><br>
> int main() {<br>
> char *msg = message;<br>
><br>
> for (int i = 0; i < 5; i++)<br>
> printf("%s\n", msg);<br>
><br>
> return 0;<br>
> }<br>
><br>
><br>
><br>
> gdb session:<br>
><br>
> (gdb) break main.c:8<br>
> Breakpoint 1 at 0x400536: file main.c, line 8.<br>
> (gdb) run<br>
> Starting program: /home/ripopov/work/test_c_<wbr>inject/a.out<br>
><br>
> Breakpoint 1, main () at main.c:8<br>
> 8 for (int i = 0; i < 5; i++)<br>
> (gdb) compile code<br>
>>char *str;<br>
>>str = (char *) malloc(3);<br>
>>str[0] = 'T';<br>
>>str[1] = '\n';<br>
>>str[2] = 0;<br>
>>msg = str;<br>
>>end<br>
> (gdb) c<br>
> Continuing.<br>
> T<br>
><br>
> T<br>
><br>
> T<br>
><br>
> T<br>
><br>
> T<br>
><br>
> [Inferior 1 (process 96445) exited normally]<br>
> (gdb)<br>
><br>
><br>
><br>
><br>
> 2017-02-13 13:56 GMT+03:00 Pavel Labath <<a href="mailto:labath@google.com">labath@google.com</a>>:<br>
>><br>
>> Every time you use the "expr" command, we compile a tiny c++ code<br>
>> snippet inject it into the target process and execute it. If you type<br>
>> "log enable lldb expr" you should be able to follow how exactly that<br>
>> works. You can use pretty much any c++ construct in the expression<br>
>> including declaring variables/types:<br>
>> (lldb) expr -- char s[]="qwerty"; for(int i=0; i < sizeof s; ++i)<br>
>> printf("%d: %c\n", i, s[i]);<br>
>> 0: q<br>
>> 1: w<br>
>> 2: e<br>
>> 3: r<br>
>> 4: t<br>
>> 5: y<br>
>> 6:<br>
>><br>
>><br>
>> So, if your question is "do we support compiling code and running it<br>
>> in the debugged process", then the answer is yes. If you want<br>
>> something that would automatically intercept some function to execute<br>
>> your code while the process is running (some kind of dynamic<br>
>> instrumentation), then the answer is no. (But I don't see any mention<br>
>> of that on the gdb page you quoted either).<br>
>><br>
>> cheers,<br>
>> pavel<br>
>><br>
>> On 12 February 2017 at 18:34, Roman Popov via lldb-dev<br>
>> <<a href="mailto:lldb-dev@lists.llvm.org">lldb-dev@lists.llvm.org</a>> wrote:<br>
>> > Hello Benjamin , all<br>
>> ><br>
>> >>>I recently started using lldb to write a basic instrumentation tool for<br>
>> >>>tracking the values of variables at various code-points in a program.<br>
>> ><br>
>> > I have the same problem of tracing some variables and debugging<br>
>> > application<br>
>> > post-mortem. Without knowing about your experience I've started walking<br>
>> > same<br>
>> > path and encountered same problem. In my case inserting an empty<br>
>> > callback<br>
>> > slows-down application by 100x. This is not acceptable for me, because<br>
>> > instead of minutes I got hours of runtime.<br>
>> ><br>
>> > Did you found any faster solution?<br>
>> ><br>
>> > My current plan is to solve it with code injection: I plan to find<br>
>> > pointers<br>
>> > to interesting values using debugger scripting and then inject code to<br>
>> > trace<br>
>> > them.<br>
>> ><br>
>> > Does LLDB supports code injection ? I've found information only about<br>
>> > gdb<br>
>> ><br>
>> > <a href="https://sourceware.org/gdb/onlinedocs/gdb/Compiling-and-Injecting-Code.html" rel="noreferrer" target="_blank">https://sourceware.org/gdb/<wbr>onlinedocs/gdb/Compiling-and-<wbr>Injecting-Code.html</a><br>
>> > but not about lldb<br>
>> ><br>
>> ><br>
>> > -Roman<br>
>> ><br>
>> ><br>
>> > ______________________________<wbr>_________________<br>
>> > lldb-dev mailing list<br>
>> > <a href="mailto:lldb-dev@lists.llvm.org">lldb-dev@lists.llvm.org</a><br>
>> > <a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev" rel="noreferrer" target="_blank">http://lists.llvm.org/cgi-bin/<wbr>mailman/listinfo/lldb-dev</a><br>
>> ><br>
><br>
><br>
</div></div></blockquote></div><br></div>