fgraph: Copy args in intermediate storage with entry

The output of the function graph tracer has two ways to display its
entries. One way for leaf functions with no events recorded within them,
and the other is for functions with events recorded inside it. As function
graph has an entry and exit event, to simplify the output of leaf
functions it combines the two, where as non leaf functions are separate:

 2)               |              invoke_rcu_core() {
 2)               |                raise_softirq() {
 2)   0.391 us    |                  __raise_softirq_irqoff();
 2)   1.191 us    |                }
 2)   2.086 us    |              }

The __raise_softirq_irqoff() function above is really two events that were
merged into one. Otherwise it would have looked like:

 2)               |              invoke_rcu_core() {
 2)               |                raise_softirq() {
 2)               |                  __raise_softirq_irqoff() {
 2)   0.391 us    |                  }
 2)   1.191 us    |                }
 2)   2.086 us    |              }

In order to do this merge, the reading of the trace output file needs to
look at the next event before printing. But since the pointer to the event
is on the ring buffer, it needs to save the entry event before it looks at
the next event as the next event goes out of focus as soon as a new event
is read from the ring buffer. After it reads the next event, it will print
the entry event with either the '{' (non leaf) or ';' and timestamps (leaf).

The iterator used to read the trace file has storage for this event. The
problem happens when the function graph tracer has arguments attached to
the entry event as the entry now has a variable length "args" field. This
field only gets set when funcargs option is used. But the args are not
recorded in this temp data and garbage could be printed. The entry field
is copied via:

  data->ent = *curr;

Where "curr" is the entry field. But this method only saves the non
variable length fields from the structure.

Add a helper structure to the iterator data that adds the max args size to
the data storage in the iterator. Then simply copy the entire entry into
this storage (with size protection).

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/20250820195522.51d4a268@gandalf.local.home


Reported-by: Sasha Levin <sashal@kernel.org>
Tested-by: Sasha Levin <sashal@kernel.org>
Closes: https://lore.kernel.org/all/aJaxRVKverIjF4a6@lappy/


Fixes: ff5c9c57 ("ftrace: Add support for function argument to graph tracer")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>