[lldb-dev] [Reproducers] SBReproducer RFC

[Tamas Berghammer via lldb-dev]

Thanks Pavel for looping me in. I haven't looked into the actual
implementation of the prototype yet but reading your description I have
some concern regarding the amount of data you capture as I feel it isn't
sufficient to reproduce a set of usecases.

One problem is when the behavior of LLDB is not deterministic for whatever
reason (e.g. multi threading, unordered maps, etc...). Lets take
SBModule::FindSymbols() what returns an SBSymbolContextList without any
specific order (haven't checked the implementation but I would consider a
random order to be valid). If a user calls this function, then iterates
through the elements to find an index `I`, calls `GetContextAtIndex(I)` and
pass the result into a subsequent function then what will we do. Will we
capture what did `GetContextAtIndex(I)` returned in the trace and use that
value or will we capture the value of `I`, call `GetContextAtIndex(I)`
during reproduction and use that value. Doing the first would be correct in
this case but would mean we don't call `GetContextAtIndex(I)` while doing
the second case would mean we call `GetContextAtIndex(I)` with a wrong
index if the order in SBSymbolContextList is non deterministic. In this
case as we know that GetContextAtIndex is just an accessor into a vector
the first option is the correct one but I can imagine cases where this is
not the case (e.g. if GetContextAtIndex would have some useful side effect).

Other interesting question is what to do with functions taking raw binary
data in the form of a pointer + size (e.g. SBData::SetData). I think we
will have to annotate these APIs to make the reproducer system aware of the
amount of data they have to capture and then allocate these buffers with
the correct lifetime during replay. I am not sure what would be the best
way to attach these annotations but I think we might need a fairly generic
framework because I won't be surprised if there are more situation when we
have to add annotations to the API. I slightly related question is if a
function returns a pointer to a raw buffer (e.g. const char* or void*) then
do we have to capture the content of it or the pointer for it and in either
case what is the lifetime of the buffer returned (e.g.
SBError::GetCString() returns a buffer what goes out of scope when the
SBError goes out of scope).

Additionally I am pretty sure we have at least some functions returning
various indices what require remapping other then the pointers either
because they are just indexing into a data structure with undefined
internal order or they referencing some other resource. Just by randomly
browsing some of the SB APIs I found for example SBHostOS::ThreadCreate
what returns the pid/tid for the newly created thread what will have to be
remapped (it also takes a function as an argument what is a problem as
well). Because of this I am not sure if we can get away with an
automatically generated set of API descriptions instead of wring one with
explicit annotations for the various remapping rules.

If there is interest I can try to take a deeper look into the topic
sometime later but I hope that those initial thoughts are useful.

Tamas

On Mon, Jan 7, 2019 at 9:40 AM Pavel Labath <pavel at labath.sk> wrote:

> On 04/01/2019 22:19, Jonas Devlieghere via lldb-dev wrote:
> > Hi Everyone,
> >
> > In September I sent out an RFC [1] about adding reproducers to LLDB.
> > Over the
> > past few months, I landed the reproducer framework, support for the GDB
> > remote
> > protocol and a bunch of preparatory changes. There's still an open code
> > review
> > [2] for dealing with files, but that one is currently blocked by a
> change to
> > the VFS in LLVM [3].
> >
> > The next big piece of work is supporting user commands (e.g. in the
> > driver) and
> > SB API calls. Originally I expected these two things to be separate, but
> > Pavel
> > made a good case [4] that they're actually very similar.
> >
> > I created a prototype of how I envision this to work. As usual, we can
> > differentiate between capture and replay.
> >
> > ## SB API Capture
> >
> > When capturing a reproducer, every SB function/method is instrumented
> > using a
> > macro at function entry. The added code tracks the function identifier
> > (currently we use its name with __PRETTY_FUNCTION__) and its arguments.
> >
> > It also tracks when a function crosses the boundary between internal and
> > external use. For example, when someone (be it the driver, the python
> > binding
> > or the RPC server) call SBFoo, and in its implementation SBFoo calls
> > SBBar, we
> > don't need to record SBBar. When invoking SBFoo during replay, it will
> > itself
> > call SBBar.
> >
> > When a boundary is crossed, the function name and arguments are
> > serialized to a
> > file. This is trivial for basic types. For objects, we maintain a table
> that
> > maps pointer values to indices and serialize the index.
> >
> > To keep our table consistent, we also need to track return for functions
> > that
> > return an object by value. We have a separate macro that wraps the
> returned
> > object.
> >
> > The index is sufficient because every object that is passed to a
> > function has
> > crossed the boundary and hence was recorded. During replay (see below)
> > we map
> > the index to an address again which ensures consistency.
> >
> > ## SB API Replay
> >
> > To replay the SB function calls we need a way to invoke the corresponding
> > function from its serialized identifier. For every SB function, there's a
> > counterpart that deserializes its arguments and invokes the function.
> These
> > functions are added to the map and are called by the replay logic.
> >
> > Replaying is just a matter looping over the function identifiers in the
> > serialized file, dispatching the right deserialization function, until
> > no more
> > data is available.
> >
> > The deserialization function for constructors or functions that return
> > by value
> > contains additional logic for dealing with the aforementioned indices.
> The
> > resulting objects are added to a table (similar to the one described
> > earlier)
> > that maps indices to pointers. Whenever an object is passed as an
> > argument, the
> > index is used to get the actual object from the table.
> >
> > ## Tool
> >
> > Even when using macros, adding the necessary capturing and replay code is
> > tedious and scales poorly. For the prototype, we did this by hand, but we
> > propose a new clang-based tool to streamline the process.
> >
> > For the capture code, the tool would validate that the macro matches the
> > function signature, suggesting a fixit if the macros are incorrect or
> > missing.
> > Compared to generating the macros altogether, it has the advantage that
> we
> > don't have "configured" files that are harder to debug (without faking
> line
> > numbers etc).
> >
> > The deserialization code would be fully generated. As shown in the
> prototype
> > there are a few different cases, depending on whether we have to account
> for
> > objects or not.
> >
> > ## Prototype Code
> >
> > I created a differential [5] on Phabricator with the prototype. It
> > contains the
> > necessary methods to re-run the gdb remote (reproducer) lit test.
> >
> > ## Feedback
> >
> > Before moving forward I'd like to get the community's input. What do you
> > think
> > about this approach? Do you have concerns or can we be smarter
> > somewhere? Any
> > feedback would be greatly appreciated!
> >
> > Thanks,
> > Jonas
> >
> > [1] http://lists.llvm.org/pipermail/lldb-dev/2018-September/014184.html
> > [2] https://reviews.llvm.org/D54617
> > [3] https://reviews.llvm.org/D54277
> > [4] https://reviews.llvm.org/D55582
> > [5] https://reviews.llvm.org/D56322
> >
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >
>
> [Adding Tamas for his experience with recording and replaying APIs.]
>
>
> Thank you for sharing the prototype Jonas. It looks very interesting,
> but there are a couple of things that worry me about it.
>
> The first one is the usage of __PRETTY_FUNCTION__. That sounds like a
> non-starter even for an initial implementation, as the string that
> expands to is going to differ between compilers (gcc and clang will
> probably agree on it, but I know for a fact it will be different on
> msvc). It that was just an internal property of the serialization
> format, then it might be fine, but it looks like you are hardcoding the
> values in code to connect the methods with their replayers, which is
> going to be a problem.
>
> I've been thinking about how could this be done better, and the best
> (though not ideal) way I came up with is using the functions address as
> the key. That's guaranteed to be unique everywhere. Of course, you
> cannot serialize that to a file, but since you already have a central
> place where you list all intercepted functions (to register their
> replayers), that place can be also used to assign unique integer IDs to
> these functions. So then the idea would be that the SB_RECORD macro
> takes the address of the current function, that gets converted to an ID
> in the lookup table, and the ID gets serialized.
>
> The part that bugs me about this is that taking the address of an
> overloaded function is extremely tedious (you have to write something
> like static_cast<function_prototype>(&SBFoo::Bar)). That would mean all
> of these things would have to be passed to the RECORD macro. OTOH, the
> upshot of this would be that the macro would now have sufficient
> information to perform pretty decent error checking on its invocation.
> Another nice about this could be that once you already have a prototype
> and an address of the function, it should be possible (with sufficient
> template-fu) to synthesize replay code for the function automatically,
> at least in the simple cases, which would avoid the repetitiveness of
> the current replay code. Together, this might obviate the need for any
> clang plugins or other funny build steps.
>
>
> The second thing I noticed is the usage of pointers for identifying
> object. A pointer is good for that but only while the object it points
> to is alive. Once the object is gone, the pointer can (and most likely
> will) be reused. So, it sounds to me like you also need to track the
> lifetime of these objects. That may be as simple as intercepting
> constructor/destructor calls, but I haven't seen anything like that yet
> (though I haven't looked at all details of the patch).
>
> Tying into that is the recording of return values. It looks like the
> current RECORD_RETURN macro will record the address of the temporary
> object in the frame of the current function. However, that address will
> become invalid as soon as the function returns as the result object will
> be copied into a location specified by the caller as a part of the
> return processing. Are you handling this in any way?
>
> The final thing, which I noticed is the lack of any sign of threading
> support. I'm not too worried about that, as that sounds like something
> that could be fitted into the existing framework incrementally, but it
> is something worth keeping in mind, as you're going to run into that
> pretty soon.
>
>
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