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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
This commit is contained in:
Nick Kralevich 2015-04-07 01:25:43 -07:00
parent 02f1d80ae2
commit 58ba58a97c
2 changed files with 17 additions and 5 deletions
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16
include/private/android_logger.h
View File

@ -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

6
logd/LogAudit.cpp
View File

@ -150,9 +150,9 @@ int LogAudit::logPrint(const char *fmt, ...) {
rc = -ENOMEM;
} else {
event->header.tag = htole32(AUDITD_LOG_TAG);
event->payload.type = EVENT_TYPE_STRING;
event->payload.length = htole32(l);
memcpy(event->payload.data, str, l);
event->type = EVENT_TYPE_STRING;
event->length = htole32(l);
memcpy(event->data, str, l);
logbuf->log(LOG_ID_EVENTS, now, uid, pid, tid,
reinterpret_cast<char *>(event),