[cfe-dev] RFC: Syringe -- A Dynamic Behavior Injection Framework

[Kostya Serebryany via cfe-dev]

[I haven't given this much though, so forgive me if my questions are naive]

* I think you should state the goals and major use cases more clearly.
E.g. if the goal to ever have this in production (looks like it's not) then
the design is scary given all the threats associated with indirect calls.

* What are your performance requirements?
If this is not needed for production, then perhaps 5%-10% is tolerable.

* I'd like to understand more about what's missing for you in XRay.
IIRC, XRay injects ~11 bytes of NOPs into the function prologue, which you
can replace with any code you like, any time you want.
You may for example replace those NOPs with a jump to the payload, which
will achieve your goal. No? Why?

* You can have a simpler implementation that for every instrumented
function does
     if (DivertFuncToPayload) return PayloadForFunc(args); // tail call
and thus instead of an indirect call on main path you have a load/cmp on
the main path and a direct call on the slow path.

--kcc


On Wed, Sep 5, 2018 at 6:10 PM Paul Kirth via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> TLDR; During my internship at Google I developed a proof of concept
> framework for supporting dynamic behavior injection. It allows users to
> specify alternate implementations of functions, and dynamically switch
> between the original and new behavior at runtime. It works by dispatching
> the original call through a function pointer that can either point to the
> original function body or the injected version. We would like feedback
> about our approach, and the communities’ interest in adding our framework
> to LLVM.
>
> -----
>
> Overview
>
> Syringe takes a different approach from other systems interested in
> modifying runtime behavior, such as Detours or XRay, and borrows
> inspiration from JIT compilers to inject new behaviors. JIT compilers often
> use stub functions to dispatch execution to specially optimized versions of
> function bodies. Our approach uses the idea of an indirect call through a
> stub function to instead dispatch control flow to either the original
> behavior or the newly injected behavior. Our runtime component exposes APIs
> to allow the user to modify this behavior during execution.
>
> We achieve this through the following steps:
>
>
>    1.
>
>    Target functions are cloned and renamed
>    2.
>
>    The original function body is replaced with a stub
>    3.
>
>    The new stub makes an indirect call through a pointer controlled by
>    the Syringe runtime
>    4.
>
>    This implementation pointer will either point to the original
>    implementation or the new payload
>    5.
>
>    The payload function is given an alias that can be used to resolve its
>    address at link-time
>    6.
>
>    Callbacks into the runtime are used toggle between the two
>    implementations of a target function
>
>
> Syringe introduces the notion of injection sites (the target function
> whose behavior should be changed), and payloads (the new behavior to inject
> into the target function). These functions must always come in pairs, and
> will cause an error if they do not. However, this is a link time error, as
> Syringe payloads are designed such that they can exist in different
> translation units from their target function. Because these bindings don't
> get resolved until linking, we must tie a payload to its injection site. We
> achieve this by creating an alias for the payload based on its target
> function. The runtime registration functions can then reference this alias,
> and the dynamic linker can patch in the correct address when the Syringe
> runtime is being initialized.
>
> The Syringe runtime is very thin. It currently consists of a registration
> function, and a few APIs for changing the active variant for a target
> function. When calling into the runtime, the target function is looked up
> in the runtime metadata, and the implementation pointer’s value is changed
> to the other variant.
>
> Our current approach involves the following:
>
> 1.
>
> *Clang + LLVM*: Add support for function attributes(
> `[[clang::syringe_injection_site]]`,
> `[[clang::syringe_payload(“target_function_name”)]]` ) to indicate if a
> function should be considered a syringe injection site or a syringe payload
> respectively. The payload attribute requires a parameter to bind the
> syringe site and payload for use in the runtime.
>
> 2.
>
> *Clang*: Add flags (`-fsyringe`,
> `-fsyringe-config-file=”/path/to/config.yml”`) to enable and control
> Syringe instrumentation.
>
> 3.
>
> *LLVM*: Add a Transformation pass that instruments syringe sites and
> payloads, and generates the necessary initialization functions. Function
> cloning, stub creation, and implementation pointer management are all
> implemented using existing code in LLVM’s ORC library.
>
> 4.
>
> *compiler-rt* : Implement a small library called “syringe” that exposes
> the required APIs for toggling between implementations:
>
> `__syringe__toggle_impl(&targetFunction)` uses the function address to
> toggle its implementation
>
> `__syringe__cxx_toggle_impl(&Class::targetMethod, &class_instance)` looks
> up the target address in the class vtable according to the Itanium ABI, and
> uses that to change the runtime value of the target function pointer
>
> `__syringe_registration(&orig_function, &orig_impl, &injected_func,
> &impl_ptr)` used to initialize the runtime data used by Syringe.
>
> Improvements
>
> Our prototype works well in many cases, but has a few shortcomings which
> we would like to address.
>
> First, our implementation is almost completely contained in the LLVM
> backend, and thus has no real understanding of C++. While we can currently
> use the mangled names of functions to achieve our desired result, this is
> cumbersome and error prone. There are additional limitations when
> considering C++ Templates and class hierarchies. Right now, class methods
> can be instrumented and replaced by payloads in the same class hierarchy.
> In this case, an injected method must inherit from the target method’s
> class and override the target function. This has additional challenges if
> the function is virtual, since our runtime uses function addresses to
> resolve which function should be modified. As a result, we do not support
> injection of virtual methods outside of the Itanium ABI, where we can
> reliably index into the vtable and thus perform the correct behavior in the
> runtime. We currently consider C++ templates completely out of scope for
> the current implementation, chiefly because they are too cumbersome to use
> without support from the frontend.
>
> In light of these shortcomings, we believe that our current implementation
> should be extended with more support from the clang frontend to:
>
> 1. Alleviate the need to mangle function names
>
> 2. Directly support C++ class hierarchy
>
> 3. Add support for C++ Templates
>
> 4. Add new intrinsics to directly handle runtime lookups (i.e. directly
> insert real addresses for class methods without (ab)using the Itanium ABI)
>
> I have been exploring how to achieve this in Clang, and believe that it is
> possible to achieve these properties. Clang can correctly resolve the
> unmangled name and  can add a new payload annotation with the mangled name
> if required. Since this is abstracted away from the user, there is little
> downside to directly tagging the functions in this way.
>
> Because Clang understands the class hierarchy, we can add a new annotation
> for class methods that will take the target base class as a parameter.
> Clang, in Sema, can look up the base class and add the correct payload
> annotation to the resulting LLVM function. Similarly for Templates, any
> instantiated template function, or dependent method, can have its payload
> forcibly instantiated, and have the new instantiation correctly tagged.
> This requires that for templates the target and payload definitions must
> appear in the same translation unit, so that their instantiations can be
> correctly resolved. While this forces a change to the actual source code
> (even if it is only an #include directive) it seems to be a reasonable way
> to offer support for a feature a core language feature.
>
> Lastly, calls into the Syringe runtime currently use function addresses as
> keys to manipulate the target function pointer. It should be possible to
> use some new intrinsic(s) that can correctly resolve the address of
> functions and methods without relying on ABI details. Because the compiler
> will be aware of how Syringe works, it should be possible to have the
> compiler directly insert the correct address while providing an intuitive
> API to the user.
>
>
> Other considerations and future work:
>
> Currently Syringe modifies the global definition of a function for the
> entire program. While in some cases this behavior makes sense there are
> several strong use cases for handling behavior injection on a per thread
> basis. One solution here is to use thread local storage to manage these
> pointers on a per-thread basis. It is also possible to manage a global set
> of metadata with per thread information. Suggestions on approaches here are
> most welcome.
>
> Syringe was designed to help automate behavior injection by understanding
> a small set of trigger conditions that could be responsible for enabling
> and disabling the injected behavior. In our initial designs these triggers
> were often based on profiling counters that could be used to toggle the
> behavior after some threshold was exceeded. Currently, this is left up to
> the programmer, but our YAML configuration already supports these sort of
> annotations. In principle there is no reason why these quality of life
> instrumentation should not be implemented as the use and design of Syringe
> solidifies.
>
> Our Syringe prototype currently uses dynamic storage to manage runtime
> metadata. Future versions should transition from this to storing the
> required metadata in RO memory. The indirect call stubs should also have
> additional CFI checks added, because we statically know that only two valid
> targets for any particular Syringe function pointer exist.
>
> Questions
>
>    1.
>
>    Is this something the LLVM community is interested in having?
>    2.
>
>    Do you have feedback on our proposed approach/improvements?
>
>
>
> --
> Paul Kirth
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> cfe-dev at lists.llvm.org
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>
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