Fixes#2754
These panicking fallbacks are at times not ending up in as plt calls
for some reason that I haven't been able to reproduce locally.
So far the only way I can reproduce is building with Canonical's PPA
build system, since rebuilding locally didn't resolve the issue.
This will change the failure mode from these panicking asserts happening
at call time, to dlopen failing during relocation when loading the
thunk. Which LD_DEBUG=all can be used for debugging relocation failure
in that case.
1) The host library needs to be loaded in the global namespace.
2) We need to use `RTLD_DEFAULT` instead of querying the object
directly.
We need to load the host library in the global namespace so the symbols
end up in the global symbol table. This follows how all these symbols
/usually/ get loaded. Either by linking directly to the library or how
loaders will end up loading these.
We need to use RTLD_DEFAULT to follow symbol overriding rules that tend
to occur. For example, MangoHUD will LD_PRELOAD a library that provides
GLX and EGL symbols. Which FEX's thunk libraries need to pick up this
override.
If we are querying the host library directly then we fail to pickup
these overrides, thus breaking MangoHUD and other overlays.
This avoids the need to provide a fallback definition for platform-specific
macros. The definitions are only added host-side, since only Host.h is
included in any interface files.
This changes how host trampolines for guest functions are created. Instead
of doing this purely on the guest-side, it's either the host-side that
creates them in a single step *or* a cooperative two-step initialization
process must be used. In the latter, trampolines are allocated and partially
initialized on the guest and must be finalized on the host before use.
Function pointer arguments given to the guest thunk library aren't
callable on the host, so they need special handling on a case-by-case
basis. Similar problems arise when the guest-side tries to consume a
pointer returned from a native host library. To automate some common
scenarios, this change adds two new annotations:
"callback_guest" indicates the callback parameter is never called on the
host and hence can be marshalled like any other argument. Accidental host
calls to the function pointer are prevented by wrapping it in an opaque
type alias.
"returns_guest_pointer" indicates that the host function returns a pointer
usable in the guest context. This applies e.g. to functions that derive
the returned pointer from input guest pointer arguments.
Previously, the host thunk would seemingly succeed to load even when
the corresponding host library could not be loaded, leading to obscure
crashes later throughout execution.
Ryan Houdek