From 0242f056665d3003b59d1fdef446e0a99a1b9f32 Mon Sep 17 00:00:00 2001
From: MarcZX10 <67475268+MarcZX10@users.noreply.github.com>
Date: Sun, 25 Oct 2026 02:27:17 +0000
Subject: [PATCH 1/2] fix(libc): give ctime's unrepresentable-time text a
 buffer per thread

ctime returns a thread-local buffer for every time it formats, but the text for a time localtime cannot represent was written to a function-local static shared by all threads. Two threads calling ctime on such a time raced on it: ThreadSanitizer reports a data race at Time.cpp:608, and the returned pointers were the same. The fallback buffer is now thread-local like the others.
---
 core/libs/prx/libc/src/Time.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/libs/prx/libc/src/Time.cpp b/core/libs/prx/libc/src/Time.cpp
index 14a1b8bc68..1be3104e39 100644
--- a/core/libs/prx/libc/src/Time.cpp
+++ b/core/libs/prx/libc/src/Time.cpp
@@ -604,7 +604,7 @@ char* APS5_VABI asctime_nid_postfix(const GuestTm* timeptr) {
 
 char* APS5_VABI ctime_nid_postfix(const int64_t* timer) {
     if (const GuestTm* local = localtime_nid_postfix(timer)) return asctime_nid_postfix(local);
-    static char unknown[26];
+    static thread_local char unknown[26];
     std::memcpy(unknown, "??? ??? ?? ??:??:?? ????\n", sizeof(unknown));
     errno = EINVAL;
     return unknown;

From 1e5459443d51b6ca3bad8c4aa3a2b523e4e842ad Mon Sep 17 00:00:00 2001
From: MarcZX10 <67475268+MarcZX10@users.noreply.github.com>
Date: Sun, 25 Oct 2026 02:27:32 +0000
Subject: [PATCH 2/2] test(libc): require a separate ctime buffer per thread
 for an unrepresentable time

Two live threads and the caller each call ctime on a time localtime rejects and must receive three distinct buffers. The case fails on current main because they share one static buffer.
---
 core/libs/tests/GuestTimeConversion.cpp | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/core/libs/tests/GuestTimeConversion.cpp b/core/libs/tests/GuestTimeConversion.cpp
index abc96e55f2..d51d617b19 100644
--- a/core/libs/tests/GuestTimeConversion.cpp
+++ b/core/libs/tests/GuestTimeConversion.cpp
@@ -496,6 +496,21 @@ int main() {
     *__error_nid_postfix() = 0;
     Require(std::strcmp(ctime_nid_postfix(&underflow), "??? ??? ?? ??:??:?? ????\n") == 0 && *__error_nid_postfix() == 22,
         "ctime of an unrepresentable time did not report it");
+    {
+        std::barrier finish(3);
+        std::array<const char*, 2> texts{};
+        std::array<std::thread, 2> readers;
+        for (std::size_t index = 0; index < readers.size(); ++index) {
+            readers[index] = std::thread([&, index] {
+                texts[index] = ctime_nid_postfix(&underflow);
+                finish.arrive_and_wait();
+            });
+        }
+        const char* own = ctime_nid_postfix(&underflow);
+        finish.arrive_and_wait();
+        Require(texts[0] != texts[1] && texts[0] != own && texts[1] != own, "ctime of an unrepresentable time shared one buffer between threads");
+        for (auto& reader : readers) reader.join();
+    }
 
     constexpr std::int64_t latest = std::numeric_limits<std::int64_t>::max();
     constexpr std::int64_t earliest = std::numeric_limits<std::int64_t>::min();