logd: Don't embed a flexible array member within another struct

C (but not C++) has a concept of a flexible array member, which
is documented at https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html .
Using a flexible array member indicates that the structure is
really a header for a variable length object.

In logd's case, the variable length structure android_event_string_t
was embedded within another structure called
android_log_event_string_t. This makes gcc's __builtin_object_size()
function really confused. When compiling with C++,
__builtin_object_size(android_log_event_string_t.payload.data, 1)
would return 0, whereas if you compiled the code with C, the same
call would (properly) return -1.

Code which does automatic bounds checking, such as the proposed
patch at https://android-review.googlesource.com/145411 , will
cause problems for logd if this syntax is used.

Don't try to embed a variable length structure within another
structure. This doesn't appear to be valid C nor C++, and
while it's worked, it seems problematic.

Instead, inline the structure so it's one big happy structure.

Change-Id: I8ac02b7142a4f6560f5f80df2effcf720f9896fc

1 files changed, 14 insertions, 2 deletions

diff --git a/include/private/android_logger.h b/include/private/android_logger.h
index 724ca51..04238a6 100644
--- a/include/private/android_logger.h
+++ b/include/private/android_logger.h
@@ -70,7 +70,17 @@ typedef struct __attribute__((__packed__)) {
     android_event_long_t payload;
 } android_log_event_long_t;
 
-/* Event payload EVENT_TYPE_STRING */
+/*
+ * Event payload EVENT_TYPE_STRING
+ *
+ * Danger: do not embed this structure into another structure.
+ * This structure uses a flexible array member, and when
+ * compiled using g++, __builtin_object_size(data, 1) returns
+ * a bad value. This is possibly a g++ bug, or a bug due to
+ * the fact that flexible array members are not supported
+ * in C++.
+ * http://stackoverflow.com/questions/4412749/are-flexible-array-members-valid-in-c
+ */
 typedef struct __attribute__((__packed__)) {
     int8_t type;    // EVENT_TYPE_STRING;
     int32_t length; // Little Endian Order
@@ -80,7 +90,9 @@ typedef struct __attribute__((__packed__)) {
 /* Event with single EVENT_TYPE_STRING */
 typedef struct __attribute__((__packed__)) {
     android_event_header_t header;
-    android_event_string_t payload;
+    int8_t type;    // EVENT_TYPE_STRING;
+    int32_t length; // Little Endian Order
+    char data[];
 } android_log_event_string_t;
 
 #endif