add linux option for guest memory usage

runtime/include/guest_flat_memory.h

@@ -73,8 +73,12 @@ FaultCounters Counters();
 void LogFaultSummary() noexcept;
 
 // Returns true when the access violation was a guest-space fault this module
-// resolved; the caller must then resume execution. `exceptionPointers` is a
-// Windows EXCEPTION_POINTERS*.
-bool HandleAccessViolation(void* exceptionPointers) noexcept;
+// resolved; the caller must then resume execution. `faultAddress` is the raw
+// host pointer the access violation trapped on (Windows: ExceptionInformation[1];
+// POSIX: siginfo_t::si_addr) and `isWrite` is whether it was a write access
+// (Windows: ExceptionInformation[0] != 0; POSIX: derived from the ucontext).
+// The platform-specific handler that calls this is expected to have already
+// done that extraction - this function only ever works with the parsed pair.
+bool HandleAccessViolation(void* faultAddress, bool isWrite) noexcept;
 
 } // namespace GuestFlat

runtime/include/system_bridge.h

@@ -11,10 +11,23 @@
 
 #include "memory.h"
 
+// Windows' SehLogger longjmps out of a vectored exception handler, where plain setjmp/longjmp is
+// the norm. A POSIX signal handler jumping back to here must use the sig-prefixed pair instead:
+// only sigsetjmp/siglongjmp save and restore the process signal mask, which is what keeps SIGSEGV
+// from staying blocked (and a second fault during the same ctor loop from escalating instead of
+// trapping) after the first recovered fault.
+#if defined(_WIN32)
+using MkwJmpBuf = jmp_buf;
+#define MKW_SETJMP(buf) setjmp(buf)
+#else
+using MkwJmpBuf = sigjmp_buf;
+#define MKW_SETJMP(buf) sigsetjmp(buf, 1)
+#endif
+
 // Global flag to suppress SEH reporting (caught by system_bridge)
 extern bool g_suppressSehReporting;
 // Jump buffer for SEH recovery
-extern thread_local jmp_buf* g_sehJumpTarget;
+extern thread_local MkwJmpBuf* g_sehJumpTarget;
 // SEH details for the most recent trapped exception (used during ctor execution).
 extern thread_local uint32_t g_sehLastExceptionCode;
 extern thread_local uintptr_t g_sehLastExceptionAddress;