Commit 442ffa742daa for kernel

commit 442ffa742daa65a0e8fe003abe9fbe472366e4de
Author: Vincent Donnefort <vdonnefort@google.com>
Date:   Fri Sep 11 20:39:35 2026 +0100

    tracing/remotes: Account for ring buffer page header in size calculation

    trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount the
    required pages because every ring buffer page contains a header
    (BUF_PAGE_HDR_SIZE). Account for that header to ensure allocated remote
    ring buffers aren't smaller than requested by the user.

    The newly introduced helper __calc_nr_pages_ring_buffer_desc() can
    return a value that overflows the descriptor nr_pages field (32 bits).

    Link: https://patch.msgid.link/20260911193937.602202-2-vdonnefort@google.com
    Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
    Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index afc7daa6ee7d..11bffb6a142d 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -3,8 +3,9 @@
 #define _LINUX_RING_BUFFER_H

 #include <linux/mm.h>
-#include <linux/seq_file.h>
 #include <linux/poll.h>
+#include <linux/ring_buffer_types.h>
+#include <linux/seq_file.h>

 #include <uapi/linux/trace_mmap.h>

@@ -279,9 +280,19 @@ static inline struct ring_buffer_desc *__first_ring_buffer_desc(struct trace_buf
 	return (struct ring_buffer_desc *)(&desc->__data[0]);
 }

+/*
+ * Returns the number of pages for a ring_buffer_desc. The caller must ensure it
+ * does not overflow ring_buffer_desc::nr_page_va.
+ */
+static inline unsigned long __calc_nr_pages_ring_buffer_desc(size_t size)
+{
+	/* Takes into account the reader page */
+	return max(DIV_ROUND_UP(size, PAGE_SIZE - BUF_PAGE_HDR_SIZE), 2UL) + 1;
+}
+
 static inline size_t trace_buffer_desc_size(size_t buffer_size, unsigned int nr_cpus)
 {
-	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
+	unsigned long nr_pages = __calc_nr_pages_ring_buffer_desc(buffer_size);
 	struct ring_buffer_desc *rbdesc;

 	return size_add(offsetof(struct trace_buffer_desc, __data),
diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 75fa1ffc4c96..c17902e42ef1 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -980,7 +980,7 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
 			      const struct cpumask *cpumask)
 {
 	size_t min_desc_size = trace_buffer_desc_size(buffer_size, cpumask_weight(cpumask));
-	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
+	unsigned int nr_pages = __calc_nr_pages_ring_buffer_desc(buffer_size);
 	struct ring_buffer_desc *rb_desc;
 	int cpu, ret = -ENOMEM;