in_kubernetes_events: reject signed uint64 strings in record_get_field_uint64

strtoull() itself accepts a leading '+'/'-' and skips leading whitespace,
so a resourceVersion string like "-5" silently wrapped around into
18446744073709551611 instead of being rejected. Kubernetes always
serializes resourceVersion as a plain unsigned digits-only decimal
string, so require the first byte to be a digit before calling
strtoull(), on top of the existing errno/ERANGE and full-consumption
checks.

Confirmed via a standalone guard-page harness (same shape as the
existing OOB reproduction in this PR): "-5" and "+5" are now rejected
(previously accepted, wrapping "-5" to UINT64_MAX-4), a valid
digits-only value still round-trips correctly, and overflow beyond
UINT64_MAX still correctly fails via the existing ERANGE check.

Addresses a CodeRabbit review comment on this PR.

Signed-off-by: Raphael Zanarelli <zanarelli.dev@gmail.com>
Signed-off-by: zanarelli <zanarelli.dev@gmail.com>

Author: zanarellidev

plugins/in_kubernetes_events/kubernetes_events.c

@@ -22,6 +22,7 @@
 #include <sys/stat.h>
 #include <inttypes.h>
 #include <errno.h>
+#include <ctype.h>
 
 #include <fluent-bit/flb_input_plugin.h>
 #include <fluent-bit/flb_network.h>
@@ -297,6 +298,18 @@ static int record_get_field_uint64(msgpack_object *obj, const char *fieldname, u
         memcpy(buf, v->via.str.ptr, len);
         buf[len] = '\0';
 
+        /*
+         * strtoull() itself accepts a leading '+'/'-' and skips leading
+         * whitespace, which would let a value like "-5" silently wrap
+         * around into a huge positive number instead of being rejected.
+         * Kubernetes resourceVersion (the only caller) is always a plain,
+         * unsigned, digits-only decimal string, so require that directly
+         * before parsing.
+         */
+        if (!isdigit((unsigned char) buf[0])) {
+            return -1;
+        }
+
         errno = 0;
         *val = strtoull(buf, &end, 10);
         if (errno == ERANGE || end == buf || *end != '\0') {