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Commit 58ba58a9 authored by Nick Kralevich's avatar Nick Kralevich
Browse files

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
parent 02f1d80a

include/private/android_logger.h

+14 −2
Original line number Diff line number Diff line
@@ -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

logd/LogAudit.cpp

+3 −3
Original line number Diff line number Diff line
@@ -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),