From 4c19b94ae948a7f6bcc1704dfc01db553f6d845e Mon Sep 17 00:00:00 2001
From: Timo Kreuzer <timo.kreuzer@reactos.org>
Date: Sun, 11 Jan 2026 15:41:11 +0200
Subject: [PATCH] [VCRUNTIME] Implement _chkesp / _chkesp_failed

---
 sdk/lib/vcruntime/CMakeLists.txt        | 11 +++++++++++
 sdk/lib/vcruntime/i386/_chkesp.s        | 22 ++++++++++++++++++++++
 sdk/lib/vcruntime/i386/_chkesp_failed.c | 25 +++++++++++++++++++++++++
 3 files changed, 58 insertions(+)
 create mode 100644 sdk/lib/vcruntime/i386/_chkesp.s
 create mode 100644 sdk/lib/vcruntime/i386/_chkesp_failed.c

diff --git a/sdk/lib/vcruntime/CMakeLists.txt b/sdk/lib/vcruntime/CMakeLists.txt
index 6f2670bf7fc..2829dd9d30a 100644
--- a/sdk/lib/vcruntime/CMakeLists.txt
+++ b/sdk/lib/vcruntime/CMakeLists.txt
@@ -7,6 +7,10 @@ include_directories(${REACTOS_SOURCE_DIR}/sdk/include/ucrt)
 
 include_directories(inc)
 
+if(DBG)
+    add_compile_definitions(_DEBUG) # TODO: define this globally
+endif()
+
 # Silence GCC/Clang warnings
 if(CMAKE_C_COMPILER_ID STREQUAL "GNU" OR
    CMAKE_C_COMPILER_ID STREQUAL "Clang")
@@ -25,6 +29,7 @@ add_compile_definitions(
 if(${ARCH} STREQUAL "i386")
     list(APPEND VCRT_COMMON_ASM_SOURCES
         i386/__security_check_cookie.s
+        i386/_chkesp.s
     )
 elseif(${ARCH} STREQUAL "amd64")
     list(APPEND VCRT_COMMON_ASM_SOURCES
@@ -96,6 +101,12 @@ list(APPEND VCRT_RUNTIME_SOURCES
     purecall.cpp
 )
 
+if(${ARCH} STREQUAL "i386")
+    list(APPEND VCRT_RUNTIME_SOURCES
+        i386/_chkesp_failed.c
+    )
+endif()
+
 # Runtime library (linked into ucrtbase)
 add_library(vcruntime ${VCRT_RUNTIME_SOURCES} $<TARGET_OBJECTS:vcrt_common> $<TARGET_OBJECTS:setjmp>)
 target_link_libraries(vcruntime ${PSEH_LIB})
diff --git a/sdk/lib/vcruntime/i386/_chkesp.s b/sdk/lib/vcruntime/i386/_chkesp.s
new file mode 100644
index 00000000000..3bb2cba649f
--- /dev/null
+++ b/sdk/lib/vcruntime/i386/_chkesp.s
@@ -0,0 +1,22 @@
+/*
+ * PROJECT:     ReactOS vcruntime library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Implementation of _chkesp
+ * COPYRIGHT:   Copyright 2026 Timo Kreuzer <timo.kreuzer@reactos.org>
+ */
+
+#include <asm.inc>
+
+.code
+
+EXTERN __chkesp_failed:PROC
+
+PUBLIC __chkesp
+__chkesp:
+    jnz _failed
+    ret
+
+_failed:
+    jmp __chkesp_failed
+
+END
diff --git a/sdk/lib/vcruntime/i386/_chkesp_failed.c b/sdk/lib/vcruntime/i386/_chkesp_failed.c
new file mode 100644
index 00000000000..74bba82f2b4
--- /dev/null
+++ b/sdk/lib/vcruntime/i386/_chkesp_failed.c
@@ -0,0 +1,25 @@
+/*
+ * PROJECT:     ReactOS vcruntime library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Implementation of _chkesp_failed
+ * COPYRIGHT:   Copyright 2026 Timo Kreuzer <timo.kreuzer@reactos.org>
+ */
+
+#include <crtdbg.h>
+
+void _chkesp_failed(void)
+{
+#ifdef _DEBUG
+    /* Report the error to the user */
+    _CrtDbgReport(_CRT_ERROR,
+                  __FILE__,
+                  __LINE__,
+                  "",
+                  "The stack pointer was invalid after a function call. "
+                  "This indicates that a function was called with the "
+                  "wrong parmeters or calling convention.\n"
+                  "Click 'Retry' to debug the application.");
+#endif
+
+    __debugbreak();
+}