Missing CAMLparam in win32's Unix.stat (#11737)

The `path` argument is used after a `caml_enter_blocking_section`
when the path does not exists (the path is used to build the
unix error exception).

Unfortunately, during the blocking section we may "yield" to
a thread that can trigger a garbage collection and move the
content of `path` elsewhere, triggering a segfault.

The CAMLparam is therefore needed in that case.

(cherry picked from commit 3d8fb96281c94d31299859e7a72b90ce0976254c)

2 files changed, 12 insertions, 4 deletions

diff --git a/Changes b/Changes
index 57fcf68450..29ab19ecd7 100644
--- a/Changes
+++ b/Changes
@@ -529,6 +529,10 @@ OCaml 5.0
   the bug was only present in development version of OCaml 5.
   (Stephen Dolan, report by Andre Maroneze, review by Gabriel Scherer)
 
+- #11737: Fix segfault condition in Unix.stat under Windows in the presence of
+  multiple threads.
+  (Marc Lasson, Nicolás Ojeda Bär, review by Gabriel Scherer and David Allsopp)
+
 OCaml 4.14.0 (28 March 2022)
 ----------------------------
 
diff --git a/otherlibs/unix/stat_win32.c b/otherlibs/unix/stat_win32.c
index 2e1f6f84de..c1b69c0331 100644
--- a/otherlibs/unix/stat_win32.c
+++ b/otherlibs/unix/stat_win32.c
@@ -348,6 +348,7 @@ static int do_stat(int do_lstat, int use_64, const char* opath, HANDLE fstat, __
 
 CAMLprim value caml_unix_stat(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
@@ -355,11 +356,12 @@ CAMLprim value caml_unix_stat(value path)
   if (!do_stat(0, 0, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("stat", path);
   }
-  return stat_aux(0, st_ino, &buf);
+  CAMLreturn (stat_aux(0, st_ino, &buf));
 }
 
 CAMLprim value caml_unix_stat_64(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
@@ -367,11 +369,12 @@ CAMLprim value caml_unix_stat_64(value path)
   if (!do_stat(0, 1, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("stat", path);
   }
-  return stat_aux(1, st_ino, &buf);
+  CAMLreturn (stat_aux(1, st_ino, &buf));
 }
 
 CAMLprim value caml_unix_lstat(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
@@ -379,11 +382,12 @@ CAMLprim value caml_unix_lstat(value path)
   if (!do_stat(1, 0, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("lstat", path);
   }
-  return stat_aux(0, st_ino, &buf);
+  CAMLreturn (stat_aux(0, st_ino, &buf));
 }
 
 CAMLprim value caml_unix_lstat_64(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
@@ -391,7 +395,7 @@ CAMLprim value caml_unix_lstat_64(value path)
   if (!do_stat(1, 1, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("lstat", path);
   }
-  return stat_aux(1, st_ino, &buf);
+  CAMLreturn (stat_aux(1, st_ino, &buf));
 }
 
 static value do_fstat(value handle, int use_64)