[PATCH for-4.17 v3 14/15] tools/ocaml/xenstored/syslog_stubs.c: avoid potential NULL dereference

[Edwin Török <edvin.torok@xxxxxxxxxx>]

If we are out of memory then strdup may return NULL, and passing NULL to
syslog may cause a crash.

Avoid this by using `caml_stat_strdup` which will raise an OCaml out of
memory exception instead.
This then needs to be paired with caml_stat_free.

Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
---
Reason for inclusion in 4.17:
- fixes a bug in out of memory situations

Changes since v2:
- new in v3
---
 tools/ocaml/xenstored/syslog_stubs.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/ocaml/xenstored/syslog_stubs.c 
b/tools/ocaml/xenstored/syslog_stubs.c
index 4e5e49b557..4ad85c8eb5 100644
--- a/tools/ocaml/xenstored/syslog_stubs.c
+++ b/tools/ocaml/xenstored/syslog_stubs.c
@@ -14,6 +14,7 @@
 
 #include <syslog.h>
 #include <string.h>
+#include <caml/fail.h>
 #include <caml/mlvalues.h>
 #include <caml/memory.h>
 #include <caml/alloc.h>
@@ -35,14 +36,16 @@ static int __syslog_facility_table[] = {
 value stub_syslog(value facility, value level, value msg)
 {
     CAMLparam3(facility, level, msg);
-    const char *c_msg = strdup(String_val(msg));
+    char *c_msg = strdup(String_val(msg));
     int c_facility = __syslog_facility_table[Int_val(facility)]
                    | __syslog_level_table[Int_val(level)];
 
+    if ( !c_msg )
+        caml_raise_out_of_memory();
     caml_enter_blocking_section();
     syslog(c_facility, "%s", c_msg);
     caml_leave_blocking_section();
 
-    free((void*)c_msg);
+    free(c_msg);
     CAMLreturn(Val_unit);
 }
-- 
2.34.1