tracing: Allocate the snapshot buffer before enabling probe

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().

Cc: stable@vger.kernel.org
Fixes: 77fd5c15e3 ("tracing: Add snapshot trigger to function probes")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d484452ae6486e24e3f3cd5bd727f137fb0a6e6b..0ad75e9698f6b0f918df83af90da1204e99d8308 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6733,11 +6733,13 @@ ftrace_trace_snapshot_callback(struct ftrace_hash *hash,
                return ret;
 
  out_reg:
-       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;
 }