linux-kernel - Re: [PATCH v11 2/5] ring-buffer: Introducing ring-buffer mapping functions

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Date: Mon, 15 Jan 2024 11:23:59 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
 kernel-team@...roid.com
Subject: Re: [PATCH v11 2/5] ring-buffer: Introducing ring-buffer mapping
 functions

On Mon, 15 Jan 2024 11:09:38 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> No. The ring buffer logic should not care if the user of it is swapping
> the entire ring buffer or not. It only cares if parts of the ring
> buffer is being swapped or not. That's not the level of scope it should
> care about. If we do not want a swap to happen in update_max_tr()
> that's not ring_buffer.c's problem. The code to prevent that from
> happening should be 100% in trace.c.

What needs to be done, and feel free to add this as a separate patch,
is to have checks where snapshot is used.

  (All errors return -EBUSY)

Before allowing mapping, check to see if:

 1) the current tracer has "use_max_tr" set.
 2) any event has a "snapshot" trigger set
 3) Any function has a "snapshot" command set

Fail if any of the above is true.

Also in reverse, if the buffer is mapped, then fail:

 1) a tracer being set that has "use_max_tr" set.
 2) a "snapshot" command being set on a function
 3) a "snapshot" trigger being set on an event.

For the last two, we may be able to get away with just a below as well.
Adding the tr->flags bit. We could also add a tr->snapshot count to
keep track of everything that is using a snapshot, and if that count is
non-zero, mapping fails.

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2a7c6fd934e9..f534f74ae80f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1175,6 +1175,12 @@ static void tracing_snapshot_instance_cond(struct trace_array *tr,
 		return;
 	}

+	if (tr->flags & TRACE_ARRAY_FL_MAPPED) {
+		trace_array_puts(tr, "*** BUFFER IS MEMORY MAPPED ***\n");
+		trace_array_puts(tr, "*** Can not use snapshot (sorry) ***\n");
+		return;
+	}
+
 	local_irq_save(flags);
 	update_max_tr(tr, current, smp_processor_id(), cond_data);
 	local_irq_restore(flags);

-- Steve