[PATCH] function_graph: Use the saved entry's size when reprinting it

[Donggeun Yoo]

When a graph entry does not fit in the trace_seq, print_graph_entry()
saves it in the iterator's fgraph_data and reprints it on the next read.
The entry has already been consumed from the ring buffer by then, so the
copy is all that is left of it.

The copy is sized with iter->ent_size, which no longer describes the
saved entry but whatever entry the iterator has moved on to. The
argument count is derived from the same field, so a 72 byte entry saved
and then reprinted ahead of a 48 byte return entry loses its arguments.

Record the size next to the failure flag, so that the two are always set
together, and restore it before reprinting.

Fixes: ff5c9c576e75 ("ftrace: Add support for function argument to graph tracer")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@xxxxxxxxx>
---
Reproduced under QEMU (x86_64 defconfig plus FTRACE, FUNCTION_GRAPH_TRACER,
DEBUG_INFO_BTF, 2 vCPUs). function_graph with funcgraph-args on, trace_pipe
read in 64K chunks 300 times from a static init:

  before: 271 reprints, 190 of them with a size that did not match the saved
          entry, 184 of those deciding the argument count the wrong way.
          _raw_spin_unlock() takes one argument and printed as
          "_raw_spin_unlock()" 19 times out of 2209.
  after:  same workload, 0 times.

Toggling funcgraph-args and funcgraph-retaddr during the run produces all four
entry sizes (24, 32, 72, 80 here) and they are all reprinted with their own.

No change in tools/testing/selftests/ftrace: 80 passed, 3 failed, 13 unresolved,
81 unsupported both with and without the patch, identical per test. The
in-kernel selftests pass, including "Testing tracer function_graph". A KASAN
build reports nothing on the same workload.

 kernel/trace/trace_functions_graph.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index ff7cb1a76b95..c5befd0c5b9a 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -52,6 +52,7 @@ struct fgraph_data {
 	};
 	struct ftrace_graph_ret_entry	ret;
 	int				failed;
+	int				ent_size;
 	int				cpu;
 };
 
@@ -1274,6 +1275,7 @@ print_graph_entry(struct ftrace_graph_ent_entry *field, struct trace_seq *s,
 		if (s->full) {
 			data->failed = 1;
 			data->cpu = cpu;
+			data->ent_size = iter->ent_size;
 		} else
 			data->failed = 0;
 	}
@@ -1457,6 +1459,7 @@ print_graph_function_flags(struct trace_iterator *iter, u32 flags)
 	if (data && data->failed) {
 		field = &data->ent.ent;
 		iter->cpu = data->cpu;
+		iter->ent_size = data->ent_size;
 		ret = print_graph_entry(field, s, iter, flags);
 		if (ret == TRACE_TYPE_HANDLED && iter->cpu != cpu) {
 			per_cpu_ptr(data->cpu_data, iter->cpu)->ignore = 1;
-- 
2.53.0