From 7a962c33a9c457db2da39bd9834e45e523d78881 Mon Sep 17 00:00:00 2001 From: =?utf8?q?J=C3=A9r=C3=A9mie=20Galarneau?= Date: Mon, 1 Nov 2021 15:43:55 -0400 Subject: [PATCH] Fix: relayd: live: mishandled initial null trace chunk MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Observed issue ============== As reported in #1323 (https://bugs.lttng.org/issues/1323), crashes of the relay daemon are observed when running the user space clear tests. The crash occurs with the following stack trace: #0 0x000055fbb861d6ae in urcu_ref_get_unless_zero (ref=0x28) at /usr/local/include/urcu/ref.h:85 #1 lttng_trace_chunk_get (chunk=0x0) at trace-chunk.c:1836 #2 0x000055fbb86051e2 in make_viewer_streams (relay_session=relay_session@entry=0x7f6ea002d540, viewer_session=, seek_t=seek_t@entry=LTTNG_VIEWER_SEEK_BEGINNING, nb_total=nb_total@entry=0x7f6ea9607b00, nb_unsent=nb_unsent@entry=0x7f6ea9607aec, nb_created=nb_created@entry=0x7f6ea9607ae8, closed=) at live.c:405 #3 0x000055fbb86061d9 in viewer_get_new_streams (conn=0x7f6e94000fc0) at live.c:1155 #4 process_control (conn=0x7f6e94000fc0, recv_hdr=0x7f6ea9607af0) at live.c:2353 #5 thread_worker (data=) at live.c:2515 #6 0x00007f6eae86a609 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0 #7 0x00007f6eae78f293 in clone () from /lib/x86_64-linux-gnu/libc.so.6 The race window during which this occurs seems very small as it can take hours to reproduce this crash. However, a minimal reproducer could be identified, as stated in the bug report. Essentially, the same crash can be reproduced by attaching a live viewer to a session that has seen events being produced, been stopped and been cleared. Cause ===== The crash occurs as an attempt is made to take a reference to a viewer session’s trace chunk as viewer streams are created. The crux of the problem is that the code doesn’t expect a viewer session’s trace chunk to be NULL. The viewer session’s current trace chunk is initially set, when a viewer attaches to the viewer session, to a copy the corresponding relay_session’s current trace chunk. A live session always attempts to "catch-up" to the newest available trace chunk. This means that when a viewer reaches the end of a trace chunk, the viewer session may not transition to the "next" one: it jumps to the most recent trace chunk available (the one being produced by the relay_session). Hence, if the producer performs multiple rotations before a viewer completes the consumption of a trace chunk, it will skip over those "intermediary" trace chunks. A viewer session updates its current trace chunk when: 1) new viewer streams are created, 2) a new index is requested, 3) metadata is requested. Hence, as a general principle, the viewer session will reference the most recent trace chunk available _even if its streams do not point to it_. It indicates which trace chunk viewer streams should transition to when the end of their current trace chunk is reached. The live code properly handles transitions to a null chunk. This can be verified by attaching a viewer to a live session, stopping the session, clearing it (thus entering a null trace chunk), and resuming tracing. The only issue is that the case where the first trace chunk of a viewer session is "null" (no active trace chunk) is mishandled in two places: 1) in make_viewer_streams(), where the crash is observed, 2) in viewer_get_metadata(). Solution ======== In make_viewer_streams(), it is assumed that a viewer session will have a non-null trace chunk whenever a rotation is not ongoing. This is reflected by the fact that a reference is always acquired on the viewer session’s trace chunk. That code is one of the three places that can cause a viewer session’s trace chunk to be updated. We still want to update the viewer session to the most recently seen trace chunk (null, in this case). However, there is no reference to acquire and the trace chunk to use for the creation of the viewer stream is NULL. This is properly handled by viewer_stream_create(). The second site to change is viewer_get_metadata() which doesn’t handle a viewer metadata stream not having an active trace chunk at all. Thankfully, the protocol allows us to express this condition by returning the LTTNG_VIEWER_NO_NEW_METADATA status code when a viewer metadata stream doesn’t have an open file and doesn’t have a current trace chunk. Surprisingly, this bug didn’t trigger in the case where a transition to a null chunk occurred _after_ attaching to a viewer session. This is because viewers will typically ask for metadata as a result of an LTTNG_VIEWER_FLAG_NEW_METADATA reply to the GET_NEXT_INDEX command. When a session is stopped and all data was consumed, this command returns that no new data is available, causing the viewers to wait and ask again later. However, when attaching, babeltrace2 (at least, and probably babeltrace 1.x) always asks for an initial segment of metadata before asking for an index. Known drawbacks =============== None. Fixes: #1323 Signed-off-by: Jérémie Galarneau Change-Id: I516fca60755e6897f6b7170c12d706ef57ad61a5 --- src/bin/lttng-relayd/live.c | 47 ++++++++++++++++++++++++----------- src/bin/lttng-relayd/stream.h | 5 ++++ 2 files changed, 38 insertions(+), 14 deletions(-) diff --git a/src/bin/lttng-relayd/live.c b/src/bin/lttng-relayd/live.c index 1a3036660..fe7c26d72 100644 --- a/src/bin/lttng-relayd/live.c +++ b/src/bin/lttng-relayd/live.c @@ -384,8 +384,6 @@ static int make_viewer_streams(struct relay_session *relay_session, goto error_unlock; } } else { - bool reference_acquired; - /* * Transition the viewer session into the newest trace chunk available. */ @@ -402,11 +400,26 @@ static int make_viewer_streams(struct relay_session *relay_session, } } - reference_acquired = lttng_trace_chunk_get( - viewer_session->current_trace_chunk); - assert(reference_acquired); - viewer_stream_trace_chunk = - viewer_session->current_trace_chunk; + if (relay_stream->trace_chunk) { + /* + * If the corresponding relay + * stream's trace chunk is set, + * the viewer stream will be + * created under it. + * + * Note that a relay stream can + * have a NULL output trace + * chunk (for instance, after a + * clear against a stopped + * session). + */ + const bool reference_acquired = lttng_trace_chunk_get( + viewer_session->current_trace_chunk); + + assert(reference_acquired); + viewer_stream_trace_chunk = + viewer_session->current_trace_chunk; + } } viewer_stream = viewer_stream_create( @@ -2016,8 +2029,9 @@ int viewer_get_metadata(struct relay_connection *conn) } } - if (conn->viewer_session->current_trace_chunk != - vstream->stream_file.trace_chunk) { + if (conn->viewer_session->current_trace_chunk && + conn->viewer_session->current_trace_chunk != + vstream->stream_file.trace_chunk) { bool acquired_reference; DBG("Viewer session and viewer stream chunk differ: " @@ -2034,11 +2048,16 @@ int viewer_get_metadata(struct relay_connection *conn) len = vstream->stream->metadata_received - vstream->metadata_sent; - /* - * Either this is the first time the metadata file is read, or a - * rotation of the corresponding relay stream has occurred. - */ - if (!vstream->stream_file.handle && len > 0) { + if (!vstream->stream_file.trace_chunk) { + reply.status = htobe32(LTTNG_VIEWER_NO_NEW_METADATA); + len = 0; + goto send_reply; + } else if (vstream->stream_file.trace_chunk && + !vstream->stream_file.handle && len > 0) { + /* + * Either this is the first time the metadata file is read, or a + * rotation of the corresponding relay stream has occurred. + */ struct fs_handle *fs_handle; char file_path[LTTNG_PATH_MAX]; enum lttng_trace_chunk_status status; diff --git a/src/bin/lttng-relayd/stream.h b/src/bin/lttng-relayd/stream.h index 1639d2fb9..c64b18045 100644 --- a/src/bin/lttng-relayd/stream.h +++ b/src/bin/lttng-relayd/stream.h @@ -174,6 +174,11 @@ struct relay_stream { /* * The trace chunk to which the file currently being produced (if any) * belongs. + * + * Note that a relay stream can have no output trace chunk. For + * instance, after a session stop followed by a session clear, + * streams will not have an output trace chunk until the session + * is resumed. */ struct lttng_trace_chunk *trace_chunk; LTTNG_OPTIONAL(struct relay_stream_rotation) ongoing_rotation; -- 2.34.1