LTTng » LTTng-tools

The lttcomm_session_msg is sent over a unix socket to send messages to the session daemon using the lttng-ctl API.

The LTTNG_PACKED macro is used to ensure the structures in "u" are packed. However this attribute does not apply to inner structures, which must be annotated explicitly; LTTNG_PACKED is not recursive.

#define LTTNG_PACKED __attribute__((__packed__))

For reference, the lttcomm_session_msg structure follows. Note how the snapshot_output member in the "u" union contains a struct lttng_snapshot_output which is not packed. There are multiple instances of this in the structure and the same problem may affect the other IPC structures.

struct lttcomm_session_msg {
        uint32_t cmd_type;      /* enum lttcomm_sessiond_command */
        struct lttng_session session;
        struct lttng_domain domain;
        union {
                struct {
                        char channel_name[LTTNG_SYMBOL_NAME_LEN];
                        char name[NAME_MAX];
                } LTTNG_PACKED disable;
                /* Event data */
                struct {
                        char channel_name[LTTNG_SYMBOL_NAME_LEN];
                        struct lttng_event event;
                        /* Length of following bytecode for filter. */
                        uint32_t bytecode_len;
                        /* exclusion data */
                        uint32_t exclusion_count;
                        /*
                         * After this structure, the following variable-length
                         * items are transmitted:
                         * - char exclusion_names[LTTNG_SYMBOL_NAME_LEN][exclusion_count]
                         * - unsigned char filter_bytecode[bytecode_len]
                         */
                } LTTNG_PACKED enable;
                /* Create channel */
                struct {
                        struct lttng_channel chan;
                } LTTNG_PACKED channel;
                /* Context */
                struct {
                        char channel_name[LTTNG_SYMBOL_NAME_LEN];
                        struct lttng_event_context ctx;
                } LTTNG_PACKED context;
                /* Use by register_consumer */
                struct {
                        char path[PATH_MAX];
                } LTTNG_PACKED reg;
                /* List */
                struct {
                        char channel_name[LTTNG_SYMBOL_NAME_LEN];
                } LTTNG_PACKED list;
                struct lttng_calibrate calibrate;
                /* Used by the set_consumer_url and used by create_session also call */
                struct {
                        /* Number of lttng_uri following */
                        uint32_t size;
                } LTTNG_PACKED uri;
                struct {
                        struct lttng_snapshot_output output;
                } LTTNG_PACKED snapshot_output;
                struct {
                        uint32_t wait;
                        struct lttng_snapshot_output output;
                } LTTNG_PACKED snapshot_record;
                struct {
                        uint32_t nb_uri;
                        unsigned int timer_interval;    /* usec */
                } LTTNG_PACKED session_live;
                struct {
                        struct lttng_save_session_attr;
                } LTTNG_PACKED save_session;
        } u;
} LTTNG_PACKED;

Nice write-up that explains the problem in more details:
http://axelio.wordpress.com/2007/07/24/be-careful-with-packed-structures/

Hopefully I missed something and this is a false alarm...