Re: [Valgrind-users] Clarification on functions in pub_tool_tooliface.h

[Mike L. <mik...@gm...>]

I did a poor job explaining my inquiry. I'm writing a valgrind tool that's
tracking the IOPS/FLOPS/reads/writes reported by VEX IR during
instrumentation, and doing some minor analysis on the IR with a
instrument_basic_block callback (as documented in
http://www.valgrind.org/docs/manual/writing-tools.html )

I was interested how to also see which data was written to by any syscalls
(not to instrument syscalls), and saw the aforementioned callbacks in
pub_tool_tooliface.h
After reading the comments, my confusion was whether registering a callback
to (*_mem_write) would instrument ALL memory operations (rendering my
instrument_basic_block unnecessary for processing writes).

So my understanding right now is that as long as a syscall wrapper exists,
registering to *_mem_write will give me metadata about userspace memory
modified by supported syscalls, otherwise I use the normal
instrument_basic_block callback. Because I'm not currently interested in
events that happen within the core framework (besides thread swapping,
which I already detect with VG_(get_running_tid)), I wouldn't need to
register to other aforementioned callbacks.



On Thu, Mar 2, 2017 at 5:34 PM Ivo Raisr <iv...@iv...> wrote:

> 2017-03-02 23:07 GMT+01:00 Mike Lui <mik...@gm...>:
> > Hi Ivo,
> >
> > I didn't get a response, but I found a snippet in the original paper:
> > https://www.cs.columbia.edu/~junfeng/09fa-e6998/papers/valgrind.pdf
> >
> > Table 1 says these functions are only for syscalls and the valgrind
> core. Is
> > that still correct?
>
> Hi Mike,
>
> As comment on line 517 in pub_tool_tooliface.h says, these are
> callback functions
> which the Valgrind core (framework) use to pass various events to the
> analysis tool.
> Tool can be one of memcheck, helgrind, drd, cachegrind, callgrind,
> massif, ... [1]
> Various tools are interested in slightly different events, so they do
> or don't register
> event callbacks. Syscall is just one type of such an event.
>
> > callbacks specifically refer to events within the core, right?
> >> > Does this mean that they bear no relevance to profiling the user
> application?
>
> User application usually does not interact with these events.
> What are you trying to achieve? What is your goal? Asking too detailed
> question
> without knowing the context (what are you up to) does not usually lead to
> meaningful answers.
>
> >> > Am I correct that a "mem_write" callback only gets called for memory
> >> > write that happens during translation, internal signal handling, etc,
> along
> >> > with the "new_mmap" calls?
>
> Nope. pre_mem_write and post_mem_write are callbacks for events emited
> by Valgrind core (framework) when it intercepted a syscall on behalf
> of the application.
> Because Valgrind has no idea what is happening during a syscall inside
> kernel,
> this is described by so called "syscall wrappers". They tend to describe
> things
> such as "this particular piece of memory is going to be written to  by
> kernel during syscall (pre_mem_write)"
> and "this particular piece of memory was written to by kernel during
> syscall (post_mem_write)".
>
> >  If I were to instrument the application under test then I'd go through
> the callback given in "basic_tool_funcs", correct?
>
> Instrumentation happens automatically, depending on the analysis tool
> selected at Valgrind startup.
> I recommend you to read Valgrind intro [1] and core [2], at least the
> initial part.
>
> I.
>
> [1] http://valgrind.org/docs/manual/manual-intro.html
> [2] http://valgrind.org/docs/manual/manual-core.html
>