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

JIRA: https://issues.redhat.com/browse/RHEL-86682

commit 6e31b75
Author: Jeongjun Park <aha310510@gmail.com>
Date:   Sat Jan 11 01:26:12 2025 +0900

    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>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/trace/ring_buffer.c

@@ -7059,14 +7059,16 @@ 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)) {
 			err = -EINVAL;
 			goto out;
 		}
 
+		page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+
 		for (; off < (1 << (subbuf_order)); off++, page++) {
 			if (p >= nr_pages)
 				break;