tracing: Allocate the snapshot buffer before enabling probe

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@goodmis.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d6e72522fc4e080659dc567f97c1df1ca1bc9e8f..edffb6781c0edc895021839c4e38b553c3f670b7 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5468,11 +5468,13 @@ 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;
 }