ring-buffer: Add output of ring buffer meta page

Add a buffer_meta per-cpu file for the trace instance that is mapped to
boot memory. This shows the current meta-data and can be used by user
space tools to record off the current mappings to help reconstruct the
ring buffer after a reboot.

It does not expose any virtual addresses, just indexes into the sub-buffer
pages.

Link: https://lkml.kernel.org/r/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Vincent Donnefort <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Daniel Bristot de Oliveira <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vineeth Pillai <[email protected]>
Cc: Youssef Esmat <[email protected]>
Cc: Beau Belgrave <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: David Howells <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Kees Cook <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

kernel/trace/ring_buffer.c

@@ -32,6 +32,8 @@
 #include <asm/local64.h>
 #include <asm/local.h>
 
+#include "trace.h"
+
 /*
  * The "absolute" timestamp in the buffer is only 59 bits.
  * If a clock has the 5 MSBs set, it needs to be saved and
@@ -1647,6 +1649,81 @@ static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages)
 	}
 }
 
+static void *rbm_start(struct seq_file *m, loff_t *pos)
+{
+	struct ring_buffer_per_cpu *cpu_buffer = m->private;
+	struct ring_buffer_meta *meta = cpu_buffer->ring_meta;
+	unsigned long val;
+
+	if (!meta)
+		return NULL;
+
+	if (*pos > meta->nr_subbufs)
+		return NULL;
+
+	val = *pos;
+	val++;
+
+	return (void *)val;
+}
+
+static void *rbm_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	(*pos)++;
+
+	return rbm_start(m, pos);
+}
+
+static int rb_meta_subbuf_idx(struct ring_buffer_meta *meta, void *subbuf);
+
+static int rbm_show(struct seq_file *m, void *v)
+{
+	struct ring_buffer_per_cpu *cpu_buffer = m->private;
+	struct ring_buffer_meta *meta = cpu_buffer->ring_meta;
+	unsigned long val = (unsigned long)v;
+
+	if (val == 1) {
+		seq_printf(m, "head_buffer:   %d\n",
+			   rb_meta_subbuf_idx(meta, (void *)meta->head_buffer));
+		seq_printf(m, "commit_buffer: %d\n",
+			   rb_meta_subbuf_idx(meta, (void *)meta->commit_buffer));
+		seq_printf(m, "subbuf_size:   %d\n", meta->subbuf_size);
+		seq_printf(m, "nr_subbufs:    %d\n", meta->nr_subbufs);
+		return 0;
+	}
+
+	val -= 2;
+	seq_printf(m, "buffer[%ld]:    %d\n", val, meta->buffers[val]);
+
+	return 0;
+}
+
+static void rbm_stop(struct seq_file *m, void *p)
+{
+}
+
+static const struct seq_operations rb_meta_seq_ops = {
+	.start		= rbm_start,
+	.next		= rbm_next,
+	.show		= rbm_show,
+	.stop		= rbm_stop,
+};
+
+int ring_buffer_meta_seq_init(struct file *file, struct trace_buffer *buffer, int cpu)
+{
+	struct seq_file *m;
+	int ret;
+
+	ret = seq_open(file, &rb_meta_seq_ops);
+	if (ret)
+		return ret;
+
+	m = file->private_data;
+	m->private = buffer->buffers[cpu];
+
+	return 0;
+}
+
 static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
 		long nr_pages, struct list_head *pages)
 {

kernel/trace/trace.c

@@ -5018,7 +5018,7 @@ static int show_traces_open(struct inode *inode, struct file *file)
 	return 0;
 }
 
-static int show_traces_release(struct inode *inode, struct file *file)
+static int tracing_seq_release(struct inode *inode, struct file *file)
 {
 	struct trace_array *tr = inode->i_private;
 
@@ -5059,7 +5059,7 @@ static const struct file_operations show_traces_fops = {
 	.open		= show_traces_open,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
-	.release	= show_traces_release,
+	.release	= tracing_seq_release,
 };
 
 static ssize_t
@@ -6860,6 +6860,22 @@ tracing_total_entries_read(struct file *filp, char __user *ubuf,
 	return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
 }
 
+static int tracing_buffer_meta_open(struct inode *inode, struct file *filp)
+{
+	struct trace_array *tr = inode->i_private;
+	int cpu = tracing_get_cpu(inode);
+	int ret;
+
+	ret = tracing_check_open_get_tr(tr);
+	if (ret)
+		return ret;
+
+	ret = ring_buffer_meta_seq_init(filp, tr->array_buffer.buffer, cpu);
+	if (ret < 0)
+		__trace_array_put(tr);
+	return ret;
+}
+
 static ssize_t
 tracing_free_buffer_write(struct file *filp, const char __user *ubuf,
 			  size_t cnt, loff_t *ppos)
@@ -7436,6 +7452,13 @@ static const struct file_operations tracing_entries_fops = {
 	.release	= tracing_release_generic_tr,
 };
 
+static const struct file_operations tracing_buffer_meta_fops = {
+	.open		= tracing_buffer_meta_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= tracing_seq_release,
+};
+
 static const struct file_operations tracing_total_entries_fops = {
 	.open		= tracing_open_generic_tr,
 	.read		= tracing_total_entries_read,
@@ -8668,6 +8691,9 @@ tracing_init_tracefs_percpu(struct trace_array *tr, long cpu)
 	trace_create_cpu_file("buffer_size_kb", TRACE_MODE_READ, d_cpu,
 				tr, cpu, &tracing_entries_fops);
 
+	if (tr->range_addr_start)
+		trace_create_cpu_file("buffer_meta", TRACE_MODE_READ, d_cpu,
+				      tr, cpu, &tracing_buffer_meta_fops);
 #ifdef CONFIG_TRACER_SNAPSHOT
 	if (!tr->range_addr_start) {
 		trace_create_cpu_file("snapshot", TRACE_MODE_WRITE, d_cpu,

kernel/trace/trace.h

@@ -645,6 +645,8 @@ trace_buffer_lock_reserve(struct trace_buffer *buffer,
 			  unsigned long len,
 			  unsigned int trace_ctx);
 
+int ring_buffer_meta_seq_init(struct file *file, struct trace_buffer *buffer, int cpu);
+
 struct trace_entry *tracing_get_trace_entry(struct trace_array *tr,
 						struct trace_array_cpu *data);