POSIX fibers crash on macOS arm64: context pointer truncated through makecontext varargs

[Kamayuq]

Hey Schobi encountered an issue with Relacy on macOS.

This is what the agent found:

In relacy/platform.hpp, create_fiber passes the fiber_ctx_t* through a single makecontext argument (makecontext(&fib.fib, (void(*)())fiber_start_fnc, 1, &ctx)). POSIX specifies makecontext arguments as int, so on LP64 targets the pointer is truncated to 32 bits. On macOS arm64 this faults on the first fiber start: thread stacks live at 0x16F_xxxx_xxxx, so fiber_start_fnc receives the pointer with its top bits cut (e.g. 0x16fdfc3f0 → 0x6fdfc3f0) and crashes dereferencing it. Linux x86-64 escapes only by ABI accident (glibc passes the varargs in full 64-bit register slots).

Fix (two-line, the classic idiom): split the pointer into two unsigned halves at the makecontext call and reassemble in the trampoline — static void fiber_start_fnc(unsigned hi, unsigned lo) with (uintptr_t)hi << 32 | lo, called with 2, (unsigned)((uintptr_t)&ctx >> 32), (unsigned)(uintptr_t)&ctx. Verified: with this change (plus building the user TU with -D_XOPEN_SOURCE=700 -D_DARWIN_C_SOURCE, which macOS requires for the full ucontext_t layout — without it the SDK's 56-byte ucontext_t has no inline mcontext storage and getcontext writes ~880 bytes out of bounds), the full relacy suite runs green on an M4 Max.

Edit: And I could use a hug right now.

[Kamayuq]

here is the diff:

diff --git a/relacy/platform.hpp b/relacy/platform.hpp
index a8b2c4d..a8eab8c 100644
--- a/relacy/platform.hpp
+++ b/relacy/platform.hpp
@@ -176,9 +176,9 @@ struct fiber_ctx_t
     ucontext_t* prv;
 };
 
-static void fiber_start_fnc(void* p)
+static void fiber_start_fnc(unsigned hi, unsigned lo)
 {
-    fiber_ctx_t* ctx = (fiber_ctx_t*)p;
+    fiber_ctx_t* ctx = (fiber_ctx_t*)(((uintptr_t)hi << 32) | (uintptr_t)lo);
     void (*volatile ufnc)(void*) = ctx->fnc;
     void* volatile uctx = ctx->ctx;
     if (_setjmp(*ctx->cur) == 0)
@@ -208,7 +208,7 @@ inline void create_fiber(fiber_t& fib, void(*ufnc)(void*), void* uctx)
     fib.fib.uc_link = 0;
     ucontext_t tmp;
     fiber_ctx_t ctx = {ufnc, uctx, &fib.jmp, &tmp};
-    makecontext(&fib.fib, (void(*)())fiber_start_fnc, 1, &ctx);
+    makecontext(&fib.fib, (void(*)())fiber_start_fnc, 2, (unsigned)((uintptr_t)&ctx >> 32), (unsigned)(uintptr_t)&ctx);
     swapcontext(&tmp, &fib.fib);
 }