test(libc): guard stpcpy writes and returned terminators

core/libs/prx/libc/CMakeLists.txt

@@ -1175,6 +1175,16 @@ if(BUILD_TESTING)
         set_tests_properties(guest_underflow_error PROPERTIES ENVIRONMENT_MODIFICATION
             "PATH=path_list_prepend:$<TARGET_FILE_DIR:libc>")
     endif()
+    add_test_executable(guest_stpcpy_extent_tests ${LIBS_INCLUDE_DIR}/tests/GuestStpcpyExtent.cpp)
+    target_include_directories(guest_stpcpy_extent_tests PRIVATE ${LIBS_INCLUDE_DIR})
+    target_link_libraries(guest_stpcpy_extent_tests PRIVATE libc)
+    configure_windows_unwind(guest_stpcpy_extent_tests)
+    add_test(NAME guest_stpcpy_extent COMMAND guest_stpcpy_extent_tests)
+    set_tests_properties(guest_stpcpy_extent PROPERTIES TIMEOUT 20)
+    if(WIN32)
+        set_tests_properties(guest_stpcpy_extent PROPERTIES ENVIRONMENT_MODIFICATION
+            "PATH=path_list_prepend:$<TARGET_FILE_DIR:libc>")
+    endif()
     add_test_executable(guest_throw_bad_array_new_length_tests ${LIBS_INCLUDE_DIR}/tests/GuestThrowBadArrayNewLength.cpp)
     target_include_directories(guest_throw_bad_array_new_length_tests PRIVATE ${LIBS_INCLUDE_DIR})
     target_link_libraries(guest_throw_bad_array_new_length_tests PRIVATE libc)

core/libs/tests/GuestStpcpyExtent.cpp

@@ -0,0 +1,86 @@
+#include "prx/libc/include/Api.hpp"
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <string>
+#include <vector>
+
+static void Require(bool condition) {
+    if (!condition) {
+        std::fputs("GuestStpcpyExtent check failed\n", stderr);
+        std::abort();
+    }
+}
+
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <sys/mman.h>
+#include <unistd.h>
+#endif
+
+class Buffer {
+    unsigned char* allocation;
+    std::size_t page;
+public:
+    Buffer() {
+#ifdef _WIN32
+        SYSTEM_INFO info{};
+        GetSystemInfo(&info);
+        page = info.dwPageSize;
+        allocation = static_cast<unsigned char*>(VirtualAlloc(nullptr, page * 3, MEM_RESERVE | MEM_COMMIT, PAGE_NOACCESS));
+        Require(allocation != nullptr);
+        DWORD previous;
+        Require(VirtualProtect(allocation + page, page, PAGE_READWRITE, &previous) != 0);
+#else
+        const long size = sysconf(_SC_PAGESIZE);
+        Require(size > 0);
+        page = static_cast<std::size_t>(size);
+        allocation = static_cast<unsigned char*>(mmap(nullptr, page * 3, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0));
+        Require(allocation != MAP_FAILED);
+        Require(mprotect(allocation + page, page, PROT_READ | PROT_WRITE) == 0);
+#endif
+        std::memset(allocation + page, 0xa5, page);
+    }
+    ~Buffer() {
+#ifdef _WIN32
+        Require(VirtualFree(allocation, 0, MEM_RELEASE) != 0);
+#else
+        Require(munmap(allocation, page * 3) == 0);
+#endif
+    }
+    Buffer(const Buffer&) = delete;
+    Buffer& operator=(const Buffer&) = delete;
+    template<class T = unsigned char>
+    T* At(std::size_t count) {
+        Require(count <= (page - 32) / sizeof(T));
+        return reinterpret_cast<T*>(allocation + page * 2 - (std::max)(count, std::size_t{1}) * sizeof(T));
+    }
+};
+
+template<class T>
+static void Prefix(const T* data) {
+    for (int index = 1; index <= 8; ++index) Require(data[-index] == static_cast<T>(sizeof(T) == 1 ? 0xa5 : 0xa5a5));
+}
+
+int main() {
+    Buffer input, output;
+    for (std::size_t count = 0; count <= 192; ++count) {
+        auto* source = input.At<char>(count + 1);
+        auto* destination = output.At<char>(count + 1);
+        for (std::size_t index = 0; index < count; ++index) source[index] = static_cast<char>(1 + (index * 97 + count) % 255);
+        source[count] = 0;
+        const std::string saved(source, count + 1);
+        std::memset(destination - 16, 0xa5, count + 17);
+        char* end = stpcpy_nid_postfix(destination, source);
+        Require(end == destination + count && *end == 0);
+        Require(std::equal(saved.begin(), saved.end(), destination));
+        Require(std::equal(saved.begin(), saved.end(), source));
+        Require(stpcpy_nid_postfix(end, "") == end);
+        Prefix(reinterpret_cast<unsigned char*>(destination));
+    }
+}