[PATCH 3.12 49/86] tracing: Allocate the snapshot buffer before enabling probe

From: Jiri Slaby
Date: Thu May 04 2017 - 05:21:17 EST

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


commit df62db5be2e5f070ecd1a5ece5945b590ee112e0 upstream.

Currently the snapshot trigger enables the probe and then allocates the
snapshot. If the probe triggers before the allocation, it could cause the
snapshot to fail and turn tracing off. It's best to allocate the snapshot
buffer first, and then enable the trigger. If something goes wrong in the
enabling of the trigger, the snapshot buffer is still allocated, but it can
also be freed by the user by writting zero into the snapshot buffer file.

Also add a check of the return status of alloc_snapshot().

Fixes: 77fd5c15e3 ("tracing: Add snapshot trigger to function probes")
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
kernel/trace/trace.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 174b9a6feea3..dbd488daec33 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5479,11 +5479,13 @@ ftrace_trace_snapshot_callback(struct ftrace_hash *hash,
return ret;

- ret = register_ftrace_function_probe(glob, ops, count);
+ ret = alloc_snapshot(&global_trace);
+ if (ret < 0)
+ goto out;

- if (ret >= 0)
- alloc_snapshot(&global_trace);
+ ret = register_ftrace_function_probe(glob, ops, count);

+ out:
return ret < 0 ? ret : 0;