FEX/ThunkLibs
Ryan Houdek 0f54369f9b
VDSO: Implements v6.11 vdso getrandom
This is an interesting vdso implementation because it only exists in
x86-64 with kernel v6.11. For Arm64 the implementation is likely to land
in a couple kernel versions from now.

Some differences with vdso_getrandom versus the regular getrandom
syscall
- Has two additional arguments, opaque_state and opaque_len
- Expects the userspace to mmap/munmap this opaque state structure
- Lets userspace query information about this structure upfront
- If the opaque data structure isn't provided then it falls back to
  regular SYS_getrandom

With the "glibc" implementation, we can tell the interface to allocate a
single page that gets unused (otherwise glibc ends up not using the
interface), and fallback to the regular SYS_getrandom.

In the case of the vdso interface, currently the arguments just get
passed through.

Keeping this as a WIP until the ARM64 kernel patches land and I can
actually test the things. Running the "glibc" path works today with the
selftest, but the vdso path is currently completely untested.
2024-09-28 01:06:56 -07:00
..
Generator External: Update fmt from 10.1.1 to 11.0.2 2024-08-18 13:13:41 -07:00
GuestLibs Library Forwarding/GL: Add 32-bit support for most core GL APIs 2024-06-11 17:13:45 +02:00
HostLibs Thunks: Follow up from #3964 2024-08-16 07:39:55 -07:00
include/common Thunks: Shift namespace name 2024-09-27 15:48:31 -07:00
libasound Whole-tree reformat 2024-04-12 16:26:02 +02:00
libdrm Whole-tree reformat 2024-04-12 16:26:02 +02:00
libEGL Whole-tree reformat 2024-04-12 16:26:02 +02:00
libfex_malloc Whole-tree reformat 2024-04-12 16:26:02 +02:00
libfex_malloc_loader Whole-tree reformat 2024-04-12 16:26:02 +02:00
libfex_malloc_symbols Whole-tree reformat 2024-04-12 16:26:02 +02:00
libfex_thunk_test Whole-tree reformat 2024-04-12 16:26:02 +02:00
libGL Library Forwarding/GL: Enable stricter pointer parameter checks 2024-06-11 17:14:23 +02:00
libSDL2 Whole-tree reformat 2024-04-12 16:26:02 +02:00
libVDSO VDSO: Implements v6.11 vdso getrandom 2024-09-28 01:06:56 -07:00
libvulkan Library Forwarding/vulkan: Reload pointers for extension functions more aggressively 2024-05-02 20:02:02 +02:00
libwayland-client Thunks: Shift namespace name 2024-09-27 15:48:31 -07:00
libX11 Library Forwarding: Drop support for libX11 2024-05-02 20:02:02 +02:00
libXext Whole-tree reformat 2024-04-12 16:26:02 +02:00
libXfixes Whole-tree reformat 2024-04-12 16:26:02 +02:00
libXrender Whole-tree reformat 2024-04-12 16:26:02 +02:00
libxshmfence Whole-tree reformat 2024-04-12 16:26:02 +02:00
README.md Thunks/gen: Drop now unneeded callback_unpacks file 2022-08-08 16:08:46 +02:00

FEX Library Thunking (Thunklibs)

FEX supports special guest libraries that call out to host code for speed and compatibility.

We support both guest->host thunks, as well as host->guest callbacks

Building and using

The thunked libraries can be built via the guest-libs and host-libs targets of the main FEX project. The outputs are in $BUILDDIR/Guest and $BUILDDIR/Host

After that, a guest rootfs is needed with the guest-libs installed. Typically this is done with symlinks that replace the native guest libraries. eg

# Unlink original guest lib
unlink $ROOTFS/lib/x86_64-linux-gnu/libX11.so.6
# Make it point to thunked version
ln -s $BUILDDIR/Guest/libX11-guest.so $ROOTFS/lib/x86_64-linux-gnu/libX11.so.6

Finally, FEX needs to be told where to look for the matching host libraries with -t /Host/Libs/Path. eg FEXLoader -c irjit -n 500 -R $ROOTFS -t $BUILDDIR/Host -- /PATH/TO/ELF

We currently don't have any unit tests for the guest libraries, only for OP_THUNK.

Implementation outline

There are several parts that make this possible. This is a rough outline.

In FEX

  • Opcode 0xF 0x3F (IR::OP_THUNK) is used for the Guest -> Host transition. Register RSI (arg0 in guest) is passed as arg0 in host. Thunks are identified by a string in the form library:function that directly follows the Guest opcode.
  • Context::HandleCallback does the Host -> Guest transition, and returns when the Guest function returns.
  • A special thunk, fex:loadlib is used to load and initialize a matching host lib. For more details, look in ThunkHandler_impl::LoadLib
  • ThunkHandler_impl::CallCallback is provided to the host libs, so they can call callbacks. It prepares guest arguments and uses Context::HandleCallback

ThunkLibs, Library loading

  • In Guest code, when a thunking library is loaded it has a constructor that calls the fex:loadlib thunk, with the library name and callback unpackers, if any.
  • In FEX, a matching host library is loaded using dlopen, fexthunks_exports_$libname(CallCallbackPtr, GuestUnpackers) is called to initialize the host library.
  • In Host code, the real host library is loaded using dlopen and dlsym (see ldr generation)

ThunkLibs, Guest -> Host

  • In Guest code (guest packer), a packer takes care of packing the arguments & return value into a struct in Guest stack. The packer is usually exported as a symbol from the Guest library.
  • In Guest code (guest thunk), a thunk does the Guest -> Host transition via OP_THUNK, and passes the struct pointer as an argument
  • FEX handles OP_THUNK and looks up the Host function from the opcode argument
  • In Host code (host unpacker), an unpacker takes the arguments from the struct, and calls a function pointer with the implementation of that function. It also stores the return value, if any, to the struct.
  • In Host code (host unpacker), the unpacker returns, and we do an implicit Host -> Guest transition
  • In Guest code (guest packer), the return value is loaded from the struct and returned, if needed

ThunkLibs, Host -> Guest. This is only possible while handling a Guest -> Host call (ie, callbacks).

  • In Host code (host packer), a packer packs the arguments & return value to a struct in Host stack.
  • In Host code (host packer), ThunkHandler_impl::CallCallback is called with the Guest unpacker, and Guest function as arguments
  • In Guest code (guest unpacker), the arguments are unpacked, the Guest function is called, and the return value is stored to the struct
  • In Guest code (guest unpacker), the unpacker returns and we do an implicit Guest -> Host transition
  • In host code (host packer), the return value is loaded from the struct and returned, if needed

Boilerplate code is automated using a dedicated code generator tool, which parses a C++ source file (libX_interface.cpp) that specializes a templated fex_gen_config struct for each thunked function. The generator will pull all required function signatures from the original library's header files and emit the appropriate boilerplate (guest->host thunks, argument packers/unpackers, host library loader, ...).

In most cases, an empty fex_gen_config specialization is sufficient, but if needed the generator behavior can be customized on a function-by-function basis using an annotation-syntax: Binary properties are toggled by inheriting from a fixed set of tag types (e.g. fexgen::custom_host_impl), whereas complicated properties are customized by defining struct members/aliases with a magic name detected by the generator (e.g. using uniform_va_type = char).

For each thunked library, the generator outputs the following files:

  • thunks.inl: Guest -> Host transition functions that use 0xF 0x3F
  • function_packs.inl: Guest argument packers / rv handling, private to the SO. These are used to solve symbol resolution issues with glxGetProc*, etc.
  • function_packs_public.inl: Guest argument packers / rv handling, exported from the SO. These are identical to the function_packs, but exported from the SO
  • function_unpacks.inl: Host argument unpackers / rv handling
  • ldr.inl: Host loader that dlopens/dlsyms the "real" host library for the implementation functions.
  • ldr_ptrs.inl: Host loader pointer declarations, used by ldr and function_unpacks
  • tab_function_unpacks.inl: Host function unpackers list, passed to FEX after Host library init so it can resolve the Guest Thunks to Host functions

Adding a new library

There are two kinds of libs, simpler ones with no callbacks, and complex ones with callbacks. You can see how libX11 is implemented for a callbacks example, and libasound for a non-callbacks example.

Getting started

  • Create libName/libName_interface.cpp and customize the fex_gen_config template for each thunked function. See some existing lib for details.
  • Create libName/libName_Guest.cpp and libName/libName_Host.cpp. Copy & rename from some existing lib is the way to go.
  • Edit GuestLibs/CMakeLists.txt and HostLibs/CMakeLists.txt to add the new targets, similar to how other libs are done.

Now the host and the guest libs should be built as part of guest-libs and host-libs