Fix tp_copy_string_from_user handling of faults

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/probes/lttng-events.h b/probes/lttng-events.h
index 0c8ae0c761b729ef2a6a9ee0a66cc71838b29092..54a451e53388eda82d279d3ec1a98e6a398314f0 100644 (file)
--- a/probes/lttng-events.h
+++ b/probes/lttng-events.h
@@ -340,7 +340,9 @@ static __used struct lttng_probe_desc TP_ID(__probe_desc___, TRACE_SYSTEM) = {
 #define __string(_item, _src)                                                 \
        __event_len += __dynamic_len[__dynamic_len_idx++] = strlen(_src) + 1;
 
-/* strlen_user includes \0 */
+/*
+ * strlen_user includes \0. If returns 0, it faulted.
+ */
 #undef __string_from_user
 #define __string_from_user(_item, _src)                                               \
        __event_len += __dynamic_len[__dynamic_len_idx++] = strlen_user(_src);
@@ -540,17 +542,28 @@ __assign_##dest##_2:                                                      \
        __chan->ops->event_write_from_user(&__ctx, src, len);           \
        goto __end_field_##dest;
 
+/*
+ * If string length is zero, this means reading the string faulted, so
+ * we simply put a \0. If string length is larger than 0, it is the
+ * string length including the final \0.
+ */
 #undef tp_copy_string_from_user
 #define tp_copy_string_from_user(dest, src)                            \
        __assign_##dest:                                                \
-       if (0)                                                          \
-               (void) __typemap.dest;                                  \
-       lib_ring_buffer_align_ctx(&__ctx, ltt_alignof(__typemap.dest)); \
-       __chan->ops->event_write_from_user(&__ctx, src,                 \
-               __get_dynamic_array_len(dest) - 1);                     \
-       __chan->ops->event_memset(&__ctx, 0, 1);                        \
+       {                                                               \
+               size_t __ustrlen;                                       \
+                                                                       \
+               if (0)                                                  \
+                       (void) __typemap.dest;                          \
+               lib_ring_buffer_align_ctx(&__ctx, ltt_alignof(__typemap.dest));\
+               __ustrlen = __get_dynamic_array_len(dest);              \
+               if (likely(__ustrlen) > 1) {                            \
+                       __chan->ops->event_write_from_user(&__ctx, src, \
+                               __ustrlen - 1);                         \
+               }                                                       \
+               __chan->ops->event_memset(&__ctx, 0, 1);                \
+       }                                                               \
        goto __end_field_##dest;
-
 #undef tp_strcpy
 #define tp_strcpy(dest, src)                                           \
        tp_memcpy(dest, src, __get_dynamic_array_len(dest))