linux-kernel - [for-next][PATCH 2/4] tracing: Initialize scratch

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Message-ID: <20250323050933.602569205@goodmis.org>
Date: Sun, 23 Mar 2025 01:09:16 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: linux-kernel@...r.kernel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Dan Carpenter <dan.carpenter@...aro.org>
Subject: [for-next][PATCH 2/4] tracing: Initialize scratch_size to zero to prevent UB

From: Steven Rostedt <rostedt@...dmis.org>

In allocate_trace_buffer() the following code:

  buf->buffer = ring_buffer_alloc_range(size, rb_flags, 0,
				      tr->range_addr_start,
				      tr->range_addr_size,
				      struct_size(tscratch, entries, 128));

  tscratch = ring_buffer_meta_scratch(buf->buffer, &scratch_size);
  setup_trace_scratch(tr, tscratch, scratch_size);

Has undefined behavior if ring_buffer_alloc_range() fails because
"scratch_size" is not initialize. If the allocation fails, then
buf->buffer will be NULL. The ring_buffer_meta_scratch() will return
NULL immediately if it is passed a NULL buffer and it will not update
scratch_size. Then setup_trace_scratch() will return immediately if
tscratch is NULL.

Although there's no real issue here, but it is considered undefined
behavior to pass an uninitialized variable to a function as input, and
UBSan may complain about it.

Just initialize scratch_size to zero to make the code defined behavior and
a little more robust.

Link: https://lore.kernel.org/all/44c5deaa-b094-4852-90f9-52f3fb10e67a@stanley.mountain/

Reported-by: Dan Carpenter <dan.carpenter@...aro.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 68abea81e51a..be97465485b3 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9394,7 +9394,7 @@ allocate_trace_buffer(struct trace_array *tr, struct array_buffer *buf, int size
 {
 	enum ring_buffer_flags rb_flags;
 	struct trace_scratch *tscratch;
-	unsigned int scratch_size;
+	unsigned int scratch_size = 0;
 
 	rb_flags = tr->trace_flags & TRACE_ITER_OVERWRITE ? RB_FL_OVERWRITE : 0;
 
-- 
2.47.2