ring-buffer: check for swapped buffers in start of committing

Because the irqsoff tracer can swap an internal CPU buffer, it is possible
that a swap happens between the start of the write and before the committing
bit is set (the committing bit will disable swapping).

This patch adds a check for this and will fail the write if it detects it.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 17 insertions, 3 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index f83a42a79ee8..1766c0e8db5a 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2073,7 +2073,8 @@ static void rb_end_commit(struct ring_buffer_per_cpu *cpu_buffer)
 }
 
 static struct ring_buffer_event *
-rb_reserve_next_event(struct ring_buffer_per_cpu *cpu_buffer,
+rb_reserve_next_event(struct ring_buffer *buffer,
+                      struct ring_buffer_per_cpu *cpu_buffer,
                       unsigned long length)
 {
         struct ring_buffer_event *event;
@@ -2083,6 +2084,19 @@ rb_reserve_next_event(struct ring_buffer_per_cpu *cpu_buffer,
 
         rb_start_commit(cpu_buffer);
 
+        /*
+         * Due to the ability to swap a cpu buffer from a buffer
+         * it is possible it was swapped before we committed.
+         * (committing stops a swap). We check for it here and
+         * if it happened, we have to fail the write.
+         */
+        barrier();
+        if (unlikely(ACCESS_ONCE(cpu_buffer->buffer) != buffer)) {
+                local_dec(&cpu_buffer->committing);
+                local_dec(&cpu_buffer->commits);
+                return NULL;
+        }
+
         length = rb_calculate_event_length(length);
  again:
         /*
@@ -2243,7 +2257,7 @@ ring_buffer_lock_reserve(struct ring_buffer *buffer, unsigned long length)
         if (length > BUF_MAX_DATA_SIZE)
                 goto out;
 
-        event = rb_reserve_next_event(cpu_buffer, length);
+        event = rb_reserve_next_event(buffer, cpu_buffer, length);
         if (!event)
                 goto out;
 
@@ -2476,7 +2490,7 @@ int ring_buffer_write(struct ring_buffer *buffer,
         if (length > BUF_MAX_DATA_SIZE)
                 goto out;
 
-        event = rb_reserve_next_event(cpu_buffer, length);
+        event = rb_reserve_next_event(buffer, cpu_buffer, length);
         if (!event)
                 goto out;