Merge pull request #18596 from ypsvlq/mingw

mingw-w64: add missing CRT sources

lib/libc/mingw/cfguard/mingw_cfguard_support.c

@@ -0,0 +1,59 @@
+/**
+ * This file has no copyright assigned and is placed in the Public Domain.
+ * This file is part of the mingw-w64 runtime package.
+ * No warranty is given; refer to the file DISCLAIMER.PD within this package.
+ */
+
+// This source file defines the function pointers required to support Control
+// Flow Guard. They shall be included even if CFGuard is disabled when building
+// mingw-w64-crt itself, to allow linking to objects/libraries compiled with
+// CFGuard.
+
+#if defined(__x86_64__)
+
+// The target address is passed as a parameter in an arch-specific manner,
+// however it is not specified how on x86_64 because __guard_dispatch_icall_fptr
+// is used instead. My guess would be that it's passed on %rcx, but it doesn't
+// really matter here because this is a no-op anyway.
+static void __guard_check_icall_dummy(void) {}
+
+// When CFGuard is not active, directly tail-call the target address, which
+// is passed via %rax.
+__asm__(
+    ".globl __guard_dispatch_icall_dummy\n"
+    "__guard_dispatch_icall_dummy:\n"
+    "    jmp *%rax\n"
+);
+
+// This is intentionally declared as _not_ a function pointer, so that the
+// jmp instruction is not included as a valid call target for CFGuard.
+extern void *__guard_dispatch_icall_dummy;
+
+#elif defined(__i386__) || defined(__aarch64__) || defined(__arm__)
+
+// The target address is passed via %ecx (x86), X15 (aarch64) or R0 (arm),
+// but it doesn't really matter here because this is a no-op anyway.
+static void __guard_check_icall_dummy(void) {}
+
+#else
+#   error "CFGuard support is unimplemented for the current architecture."
+#endif
+
+// I am not sure about the `.00cfg` section. This is just an attempt to follow
+// what VC runtime defines -- it places all the guard check function pointers
+// inside this section. The MSVC linker appears to merge this section into
+// `.rdata`, but LLD does not do this at the time of writing.
+// This section should be readonly data. The only thing that modifies these
+// pointers is the PE image loader.
+
+__asm__(".section .00cfg,\"dr\"");
+
+__attribute__(( section (".00cfg") ))
+void *__guard_check_icall_fptr = &__guard_check_icall_dummy;
+
+#if defined(__x86_64__)
+
+__attribute__(( section (".00cfg") ))
+void *__guard_dispatch_icall_fptr = &__guard_dispatch_icall_dummy;
+
+#endif