ring-buffer: Use kaslr address instead of text delta

Instead of saving off the text and data pointers and using them to compare
with the current boot's text and data pointers, just save off the KASLR
offset. Then that can be used to figure out how to read the previous boots
buffer.

The last_boot_info will now show this offset, but only if it is for a
previous boot:

  ~# cat instances/boot_mapped/last_boot_info
  39000000	[kernel]

  ~# echo function > instances/boot_mapped/current_tracer
  ~# cat instances/boot_mapped/last_boot_info
  # Current

If the KASLR offset saved is for the current boot, the last_boot_info will
show the value of "current".

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/20250305164608.274956504@goodmis.org
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 12 insertions, 19 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 510409f97992..9aa2e818eced 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -31,6 +31,7 @@
 
 #include <asm/local64.h>
 #include <asm/local.h>
+#include <asm/setup.h>
 
 #include "trace.h"
 
@@ -49,8 +50,7 @@ static void update_pages_handler(struct work_struct *work);
 struct ring_buffer_meta {
 	int		magic;
 	int		struct_size;
-	unsigned long	text_addr;
-	unsigned long	data_addr;
+	unsigned long	kaslr_addr;
 	unsigned long	first_buffer;
 	unsigned long	head_buffer;
 	unsigned long	commit_buffer;
@@ -550,8 +550,7 @@ struct trace_buffer {
 	unsigned long			range_addr_start;
 	unsigned long			range_addr_end;
 
-	long				last_text_delta;
-	long				last_data_delta;
+	unsigned long			kaslr_addr;
 
 	unsigned int			subbuf_size;
 	unsigned int			subbuf_order;
@@ -1891,16 +1890,13 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
 	}
 }
 
-/* Used to calculate data delta */
-static char rb_data_ptr[] = "";
-
-#define THIS_TEXT_PTR		((unsigned long)rb_meta_init_text_addr)
-#define THIS_DATA_PTR		((unsigned long)rb_data_ptr)
-
 static void rb_meta_init_text_addr(struct ring_buffer_meta *meta)
 {
-	meta->text_addr = THIS_TEXT_PTR;
-	meta->data_addr = THIS_DATA_PTR;
+#ifdef CONFIG_RANDOMIZE_BASE
+	meta->kaslr_addr = kaslr_offset();
+#else
+	meta->kaslr_addr = 0;
+#endif
 }
 
 static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages)
@@ -1928,8 +1924,7 @@ static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages)
 			meta->first_buffer += delta;
 			meta->head_buffer += delta;
 			meta->commit_buffer += delta;
-			buffer->last_text_delta = THIS_TEXT_PTR - meta->text_addr;
-			buffer->last_data_delta = THIS_DATA_PTR - meta->data_addr;
+			buffer->kaslr_addr = meta->kaslr_addr;
 			continue;
 		}
 
@@ -2482,17 +2477,15 @@ struct trace_buffer *__ring_buffer_alloc_range(unsigned long size, unsigned flag
  *
  * Returns: The true if the delta is non zero
  */
-bool ring_buffer_last_boot_delta(struct trace_buffer *buffer, long *text,
-				 long *data)
+bool ring_buffer_last_boot_delta(struct trace_buffer *buffer, unsigned long *kaslr_addr)
 {
 	if (!buffer)
 		return false;
 
-	if (!buffer->last_text_delta)
+	if (!buffer->kaslr_addr)
 		return false;
 
-	*text = buffer->last_text_delta;
-	*data = buffer->last_data_delta;
+	*kaslr_addr = buffer->kaslr_addr;
 
 	return true;
 }