[for-linus][PATCH 3/5] tracing/remotes: Account for ring buffer page header in size calculation

[Steven Rostedt <rostedt@kernel.org>]

From: Vincent Donnefort <vdonnefort@google.com>

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>
---
 include/linux/ring_buffer.h | 15 +++++++++++++--
 kernel/trace/trace_remote.c |  2 +-
 2 files changed, 14 insertions(+), 3 deletions(-)

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;
 
-- 
2.53.0