ring-buffer: Make reading page consistent with the code logic

In the loop of __rb_map_vma(), the 's' variable is calculated from the
same logic that nr_pages is and they both come from nr_subbufs. But the
relationship is not obvious and there's a WARN_ON_ONCE() around the 's'
variable to make sure it never becomes equal to nr_subbufs within the
loop. If that happens, then the code is buggy and needs to be fixed.

The 'page' variable is calculated from cpu_buffer->subbuf_ids[s] which is
an array of 'nr_subbufs' entries. If the code becomes buggy and 's'
becomes equal to or greater than 'nr_subbufs' then this will be an out of
bounds hit before the WARN_ON() is triggered and the code exiting safely.

Make the 'page' initialization consistent with the code logic and assign
it after the out of bounds check.

Link: https://lore.kernel.org/20250110162612.13983-1-aha310510@gmail.com
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
[ sdr: rewrote change log ]
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 479bbbcbf61b187adc1dbbe490c420d9b95a95ea..6d61ff78926b5e18ef73281cd19610e43199af68 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7028,7 +7028,7 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
        }
 
        while (p < nr_pages) {
-               struct page *page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+               struct page *page;
                int off = 0;
 
                if (WARN_ON_ONCE(s >= nr_subbufs)) {
@@ -7036,6 +7036,8 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
                        goto out;
                }
 
+               page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+
                for (; off < (1 << (subbuf_order)); off++, page++) {
                        if (p >= nr_pages)
                                break;