unittests/FEXLinuxTests: Ensure that DF is reset inside signal handler

unittests/FEXLinuxTests/tests/signal/signal_df_reset.cpp

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+// SPDX-License-Identifier: MIT
+#include <catch2/catch_test_macros.hpp>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <ucontext.h>
+#include <unistd.h>
+
+int Page {};
+void* signal_page {};
+void* signal_second_page {};
+
+void* backwards_page {};
+void* backwards_second_page {};
+
+__attribute__((naked)) void backwards_set(void* dest, uint8_t value, uint32_t pad, size_t size) {
+  __asm volatile(R"(
+    std;
+    jmp 1f;
+    1:
+    mov rax, rsi;
+    rep stosb;
+    jmp 2f;
+    2:
+    cld;
+    ret;
+  )" ::
+                   : "memory", "cc");
+}
+
+__attribute__((naked)) void forward_set(void* dest, uint8_t value, uint32_t pad, size_t size) {
+  __asm volatile(R"(
+    mov rax, rsi;
+    rep stosb;
+    ret;
+  )" ::
+                   : "memory", "cc");
+}
+
+void sig_handler(int signum, siginfo_t* info, void* context) {
+  // Ensure the fault address isn't in the signal page.
+  auto addr = reinterpret_cast<uint64_t>(info->si_addr);
+  // This REQUIRE will fail if DF isn't reset on signal handler.
+  REQUIRE(!(addr >= (uint64_t)signal_page && addr <= (uint64_t)signal_second_page));
+  forward_set(signal_second_page, 1, 0, 4096);
+
+  // mprotect the page that was originally written to, allowing the code to continue.
+  REQUIRE(mprotect(backwards_page, Page, PROT_READ | PROT_WRITE) == 0);
+}
+
+TEST_CASE("DF flaga reset on signal") {
+  struct sigaction act {};
+
+  act.sa_flags = SA_SIGINFO;
+  act.sa_sigaction = &sig_handler;
+  REQUIRE(sigaction(SIGSEGV, &act, NULL) == 0);
+
+  Page = sysconf(_SC_PAGESIZE);
+
+  // Allocate pages with protections to ensure correct direction.
+  signal_page = ::mmap(nullptr, Page * 3, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  REQUIRE(signal_page != MAP_FAILED);
+  signal_second_page = reinterpret_cast<void*>(reinterpret_cast<uint64_t>(signal_page) + Page);
+
+  // Again for backward direction.
+  backwards_page = ::mmap(nullptr, Page * 3, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  REQUIRE(backwards_page != MAP_FAILED);
+  backwards_second_page = reinterpret_cast<void*>(reinterpret_cast<uint64_t>(backwards_page) + Page);
+
+  // Allow the middle page to read/write.
+  REQUIRE(mprotect(signal_second_page, Page, PROT_READ | PROT_WRITE) == 0);
+
+  // Allow the middle page to read/write.
+  REQUIRE(mprotect(backwards_second_page, Page, PROT_READ | PROT_WRITE) == 0);
+
+  // Copy backwards and cause a signal.
+  backwards_set(backwards_second_page, 1, 0, 4096);
+}