[PATCH v8 0/3] ring-buffer: Fixes for subbuf resizing and persistent buffers

[PATCH v8 0/3] ring-buffer: Fixes for subbuf resizing and persistent buffers

[Vincent Donnefort]

This series addresses multiple issues discovered with the dynamic ring
buffer resizing.

I have managed to reproduce a ring_buffer_read_page() race with

 $ while true; do for i in 8 16 32; do echo $i > /sys/kernel/tracing/buffer_subbuf_size_kb; sleep 0.1; done; done

Changelog:

v8:

  - ring_buffer_read_page() handles gracefully read_page/reader_page
    order mismatch (Steven)

v7 (https://lore.kernel.org/linux-trace-kernel/20260817101533.1558223-1-vdonnefort@google.com/):

  - Match the "static" rb limit with bpage::id bitwidth
  - Cover another 32-bit truncation in rb_range_buffer (Sashiko)
  - Fix uninitialized spare_size (Sashiko)

v6 (https://lore.kernel.org/all/20260814154823.755406-1-vdonnefort@google.com/):

  - New prototype for ring_buffer_alloc_read_page() (Steven)
  - ring_buffer_read_page() to return -EAGAIN (Steven)
  - Keep nr_pages "unsigned long" (Steven)
  - Repase on ring-buffer/next (Drop most of the patches)

v5 (https://lore.kernel.org/all/20260813131152.3589632-1-vdonnefort@google.com/):

  - Reset info->spare_read only when data is in the ring-buffer (Sashiko)
  - Use `unsigned long` for subbuf_size declaration to avoid 32-bit
    truncation. (Sashiko)
  - Make cpu_buffer::free_page a buffer_read_data_page
  - Update kerneldoc for ring_buffer_alloc_read_page()

v4 (https://lore.kernel.org/all/20260812153311.2328812-1-vdonnefort@google.com/):

  - Add rb_subbuf_start() helper (Steven)
  - kerneldoc additions
  - Fix races in trace_pipe_raw readers
  - Use rb_subbuf_capacity() in ring_buffer_subbuf_order_set()
  - use rb_page_capacity() in ring_buffer_map_get_reader (Sashiko)
  - Fix 32-bit overflow in ring_buffer_subbuf_order_set() (Sashiko)
  - Hold cpu_buffer::lock when modifying cpu_buffer->free_page in
    ring_buffer_subbuf_order_set (Sashiko)

v3 (https://lore.kernel.org/all/20260810125633.3344684-1-vdonnefort@google.com/):

  - Drop first 3 patches (Rebased on 7.2-rc7)
  - Add a patch to align "nr_pages" to unsigned int
  - Add a patch to remove useless trace_buffer::cpus
  - Add unsigned long cast for rb_subbuf_size()
  - subbuf_order fix for rb_free_cpu_buffer() (Sashiko)
  - Use __always_inline just like the other accessors for the hot-path.

v2 (https://lore.kernel.org/all/20260806211306.3704194-1-vdonnefort@google.com/):

  - Prevent resizing of the persistent ring buffer
  - Add missing bpage::order init
  - Rework subbuf_size/subbuf_order (Sashiko)
  - Remove ring_buffer_per_cpu::mapped
  - Dynamically calculate trace_buffer::max_data_size

v1 (https://lore.kernel.org/all/20260805153225.2096152-1-vdonnefort@google.com/)

Vincent Donnefort (3):
  tracing: Fix subbuf resize races with trace_pipe_raw readers
  ring-buffer: Cap static ring buffer nr_pages
  ring-buffer: Prevent truncation of nr_pages / nr_subbufs

 include/linux/ring_buffer.h          |   5 +-
 kernel/trace/ring_buffer.c           | 198 ++++++++++++++++++---------
 kernel/trace/ring_buffer_benchmark.c |   6 +-
 kernel/trace/trace.c                 |  94 ++++++-------
 kernel/trace/trace.h                 |   9 +-
 5 files changed, 185 insertions(+), 127 deletions(-)


base-commit: 66498c75b4f8017f62d720d9b59675bdf3abce91
-- 
2.55.0.860.g4b6b3295ed-goog

[PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Vincent Donnefort]

Concurrent subbuffer resizes may crash trace_pipe_raw readers or leak
uninitialized memory to userspace due to stale size values.

Modify ring_buffer_alloc_read_page() to handle the resizing of an
existing buffer_data_read_page if necessary and add a new
ring_buffer_read_page_size(). This new function enables ring-buffer
buffer_data_read_page users to not call the racy
ring_buffer_subbuf_size_get(). This makes the spare_size member of
ftrace_buffer_info redundant.

Finally, handle buffer_data_read_page/reader_page order discrepancy in
ring_buffer_read_page(). On a mismatch simply copy manually the data to
the buffer_data_read_page.

Fixes: bce761d75745 ("ring-buffer: Read and write to ring buffers with custom sub buffer size")
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 0670742b2d60..afc7daa6ee7d 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -218,14 +218,15 @@ bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer);
 size_t ring_buffer_nr_dirty_pages(struct trace_buffer *buffer, int cpu);
 
 struct buffer_data_read_page;
-struct buffer_data_read_page *
-ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu);
+int ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu,
+				struct buffer_data_read_page **rpage);
 void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu,
 				struct buffer_data_read_page *page);
 int ring_buffer_read_page(struct trace_buffer *buffer,
 			  struct buffer_data_read_page *data_page,
 			  size_t len, int cpu, int full);
 void *ring_buffer_read_page_data(struct buffer_data_read_page *page);
+unsigned int ring_buffer_read_page_size(struct buffer_data_read_page *rpage);
 
 struct trace_seq;
 
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 3c3ed639923d..b8e6bd309707 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -330,6 +330,11 @@ struct buffer_data_read_page {
 	struct buffer_data_page	*data;	/* actual data, stored in this page */
 };
 
+static __always_inline unsigned int rb_read_page_capacity(struct buffer_data_read_page *rpage)
+{
+	return (PAGE_SIZE << rpage->order) - BUF_PAGE_HDR_SIZE;
+}
+
 /*
  * Note, the buffer_page list must be first. The buffer pages
  * are allocated in cache lines, which means that each buffer
@@ -6990,56 +6995,78 @@ EXPORT_SYMBOL_GPL(ring_buffer_swap_cpu);
  * ring_buffer_alloc_read_page - allocate a page to read from buffer
  * @buffer: the buffer to allocate for.
  * @cpu: the cpu buffer to allocate.
+ * @rpage: pointer to pass in an already allocated page (can be NULL)
+ *         and returns the allocated page.
  *
- * This function is used in conjunction with ring_buffer_read_page.
+ * This function is used in conjunction with ring_buffer_read_page().
  * When reading a full page from the ring buffer, these functions
  * can be used to speed up the process. The calling function should
  * allocate a few pages first with this function. Then when it
  * needs to get pages from the ring buffer, it passes the result
- * of this function into ring_buffer_read_page, which will swap
+ * of this function into ring_buffer_read_page(), which will swap
  * the page that was allocated, with the read page of the buffer.
  *
+ * If @rpage is provided, and it has a different order than the current
+ * subbuffer order, its payload will be freed and re-allocated. If it
+ * already matches the order, it is simply returned.
+ *
  * Returns:
- *  The page allocated, or ERR_PTR
+ *  0 on success, < 0 on error
  */
-struct buffer_data_read_page *
-ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu)
+int ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu,
+				struct buffer_data_read_page **rpage)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
-	struct buffer_data_read_page *bpage = NULL;
 	unsigned long flags;
+	unsigned int order;
 
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
-		return ERR_PTR(-ENODEV);
+		return -ENODEV;
 
-	bpage = kzalloc_obj(*bpage);
-	if (!bpage)
-		return ERR_PTR(-ENOMEM);
+	if (!rpage)
+		return -EINVAL;
 
-	bpage->order = buffer->subbuf_order;
+	order = buffer->subbuf_order;
+
+	if (*rpage) {
+		if ((*rpage)->order == order)
+			return 0;
+
+		/* We can reuse rpage, but we discard the payload */
+		free_pages((unsigned long)(*rpage)->data, (*rpage)->order);
+		(*rpage)->data = NULL;
+	} else {
+		*rpage = kzalloc_obj(**rpage);
+		if (!*rpage)
+			return -ENOMEM;
+	}
+
+	(*rpage)->order = order;
 	cpu_buffer = buffer->buffers[cpu];
+
 	local_irq_save(flags);
 	arch_spin_lock(&cpu_buffer->lock);
 
 	if (cpu_buffer->free_page.data) {
-		*bpage = cpu_buffer->free_page;
+		**rpage = cpu_buffer->free_page;
 		cpu_buffer->free_page.data = NULL;
 	}
 
 	arch_spin_unlock(&cpu_buffer->lock);
 	local_irq_restore(flags);
 
-	if (bpage->data) {
-		rb_init_data_page(bpage->data);
+	if ((*rpage)->data) {
+		rb_init_data_page((*rpage)->data);
 	} else {
-		bpage->data = alloc_cpu_data(cpu, bpage->order);
-		if (!bpage->data) {
-			kfree(bpage);
-			return ERR_PTR(-ENOMEM);
+		(*rpage)->data = alloc_cpu_data(cpu, (*rpage)->order);
+		if (!(*rpage)->data) {
+			kfree(*rpage);
+			*rpage = NULL;
+			return -ENOMEM;
 		}
 	}
 
-	return bpage;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_alloc_read_page);
 
@@ -7047,21 +7074,30 @@ EXPORT_SYMBOL_GPL(ring_buffer_alloc_read_page);
  * ring_buffer_free_read_page - free an allocated read page
  * @buffer: the buffer the page was allocate for
  * @cpu: the cpu buffer the page came from
- * @data_page: the page to free
+ * @rpage: the buffer_dat_read_page to free
  *
  * Free a page allocated from ring_buffer_alloc_read_page.
  */
 void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu,
-				struct buffer_data_read_page *data_page)
+				struct buffer_data_read_page *rpage)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
-	struct buffer_data_page *dpage = data_page->data;
-	struct page *page = virt_to_page(dpage);
+	struct buffer_data_page *dpage;
 	unsigned long flags;
+	struct page *page;
 
 	if (!buffer || !buffer->buffers || !buffer->buffers[cpu])
 		return;
 
+	if (!rpage)
+		return;
+
+	dpage = rpage->data;
+	if (!dpage)
+		goto out;
+
+	page = virt_to_page(dpage);
+
 	cpu_buffer = buffer->buffers[cpu];
 
 	/*
@@ -7069,14 +7105,14 @@ void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu,
 	 * is different from the subbuffer order of the buffer -
 	 * we can't reuse it
 	 */
-	if (page_ref_count(page) > 1 || data_page->order != buffer->subbuf_order)
+	if (page_ref_count(page) > 1 || rpage->order != buffer->subbuf_order)
 		goto out;
 
 	local_irq_save(flags);
 	arch_spin_lock(&cpu_buffer->lock);
 
 	if (!cpu_buffer->free_page.data) {
-		cpu_buffer->free_page = *data_page;
+		cpu_buffer->free_page = *rpage;
 		dpage = NULL;
 	}
 
@@ -7084,8 +7120,8 @@ void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu,
 	local_irq_restore(flags);
 
  out:
-	free_pages((unsigned long)dpage, data_page->order);
-	kfree(data_page);
+	free_pages((unsigned long)dpage, rpage->order);
+	kfree(rpage);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_free_read_page);
 
@@ -7156,10 +7192,9 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 	if (!dpage)
 		return -1;
 
-	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
+	len = min_t(size_t, len, rb_read_page_capacity(data_page));
 
-	if (data_page->order != cpu_buffer->reader_page->order)
-		return -1;
+	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
 
 	reader = rb_get_reader_page(cpu_buffer);
 	if (!reader)
@@ -7183,7 +7218,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 	 */
 	if (read || (len < (size - read)) ||
 	    cpu_buffer->reader_page == cpu_buffer->commit_page ||
-	    rb_is_static(cpu_buffer)) {
+	    rb_is_static(cpu_buffer) ||
+	    data_page->order != reader->order) {
 		struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
 		unsigned int rpos = read;
 		unsigned int pos = 0;
@@ -7284,7 +7320,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 		 * missed events, then record it there.
 		 */
 		if (missed_events > 0 &&
-		    rb_page_capacity(reader) - size >= sizeof(missed_events)) {
+		    rb_read_page_capacity(data_page) - size >= sizeof(missed_events)) {
 			memcpy(&dpage->data[size], &missed_events,
 			       sizeof(missed_events));
 			local_add(RB_MISSED_STORED, &dpage->commit);
@@ -7304,8 +7340,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 	/*
 	 * This page may be off to user land. Zero it out here.
 	 */
-	if (size < rb_page_capacity(reader))
-		memset(&dpage->data[size], 0, rb_page_capacity(reader) - size);
+	if (size < rb_read_page_capacity(data_page))
+		memset(&dpage->data[size], 0, rb_read_page_capacity(data_page) - size);
 
 	return read;
 }
@@ -7323,6 +7359,18 @@ void *ring_buffer_read_page_data(struct buffer_data_read_page *page)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_page_data);
 
+/**
+ * ring_buffer_read_page_size - get size of the read page.
+ * @page:  the page to get the size from
+ *
+ * Returns size of the page in bytes.
+ */
+unsigned int ring_buffer_read_page_size(struct buffer_data_read_page *rpage)
+{
+	return PAGE_SIZE << rpage->order;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_read_page_size);
+
 /**
  * ring_buffer_subbuf_size_get - get size of the sub buffer.
  * @buffer: the buffer to get the sub buffer size from
diff --git a/kernel/trace/ring_buffer_benchmark.c b/kernel/trace/ring_buffer_benchmark.c
index 593e3b59e42e..c3d34c0e64e2 100644
--- a/kernel/trace/ring_buffer_benchmark.c
+++ b/kernel/trace/ring_buffer_benchmark.c
@@ -104,7 +104,7 @@ static enum event_status read_event(int cpu)
 
 static enum event_status read_page(int cpu)
 {
-	struct buffer_data_read_page *bpage;
+	struct buffer_data_read_page *bpage = NULL;
 	struct ring_buffer_event *event;
 	struct rb_page *rpage;
 	unsigned long commit;
@@ -114,8 +114,8 @@ static enum event_status read_page(int cpu)
 	int inc;
 	int i;
 
-	bpage = ring_buffer_alloc_read_page(buffer, cpu);
-	if (IS_ERR(bpage))
+	ret = ring_buffer_alloc_read_page(buffer, cpu, &bpage);
+	if (ret < 0)
 		return EVENT_DROPPED;
 
 	page_size = ring_buffer_subbuf_size_get(buffer);
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 3e0907aef172..a9cf76a0a3d8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7082,8 +7082,8 @@ ssize_t tracing_buffers_read(struct file *filp, char __user *ubuf,
 {
 	struct ftrace_buffer_info *info = filp->private_data;
 	struct trace_iterator *iter = &info->iter;
+	unsigned int spare_size;
 	void *trace_data;
-	int page_size;
 	ssize_t ret = 0;
 	ssize_t size;
 
@@ -7093,36 +7093,24 @@ ssize_t tracing_buffers_read(struct file *filp, char __user *ubuf,
 	if (iter->snapshot && tracer_uses_snapshot(iter->tr->current_trace))
 		return -EBUSY;
 
-	page_size = ring_buffer_subbuf_size_get(iter->array_buffer->buffer);
-
-	/* Make sure the spare matches the current sub buffer size */
+again:
 	if (info->spare) {
-		if (page_size != info->spare_size) {
-			ring_buffer_free_read_page(iter->array_buffer->buffer,
-						   info->spare_cpu, info->spare);
-			info->spare = NULL;
-		}
+		spare_size = ring_buffer_read_page_size(info->spare);
+		/* Do we have previous read data to read? */
+		if (info->read < spare_size)
+			goto read;
 	}
 
-	if (!info->spare) {
-		info->spare = ring_buffer_alloc_read_page(iter->array_buffer->buffer,
-							  iter->cpu_file);
-		if (IS_ERR(info->spare)) {
-			ret = PTR_ERR(info->spare);
-			info->spare = NULL;
-		} else {
-			info->spare_cpu = iter->cpu_file;
-			info->spare_size = page_size;
-		}
-	}
-	if (!info->spare)
+	/* Make sure the read page order is aligned with the current subbuf order */
+	ret = ring_buffer_alloc_read_page(iter->array_buffer->buffer, iter->cpu_file,
+					  &info->spare);
+	if (ret)
 		return ret;
 
-	/* Do we have previous read data to read? */
-	if (info->read < page_size)
-		goto read;
+	spare_size = ring_buffer_read_page_size(info->spare);
+	info->read = spare_size;
+	info->spare_cpu = iter->cpu_file;
 
- again:
 	trace_access_lock(iter->cpu_file);
 	ret = ring_buffer_read_page(iter->array_buffer->buffer,
 				    info->spare,
@@ -7148,8 +7136,9 @@ ssize_t tracing_buffers_read(struct file *filp, char __user *ubuf,
 	}
 
 	info->read = 0;
+
  read:
-	size = page_size - info->read;
+	size = spare_size - info->read;
 	if (size > count)
 		size = count;
 	trace_data = ring_buffer_read_page_data(info->spare);
@@ -7199,17 +7188,17 @@ int tracing_buffers_release(struct inode *inode, struct file *file)
 }
 
 struct buffer_ref {
-	struct trace_buffer	*buffer;
-	void			*page;
-	int			cpu;
-	refcount_t		refcount;
+	struct trace_buffer		*buffer;
+	struct buffer_data_read_page	*rpage;
+	int				cpu;
+	refcount_t			refcount;
 };
 
 static void buffer_ref_release(struct buffer_ref *ref)
 {
 	if (!refcount_dec_and_test(&ref->refcount))
 		return;
-	ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->page);
+	ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->rpage);
 	kfree(ref);
 }
 
@@ -7270,23 +7259,12 @@ ssize_t tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 	};
 	struct buffer_ref *ref;
 	bool woken = false;
-	int page_size;
 	int entries, i;
 	ssize_t ret = 0;
 
 	if (iter->snapshot && tracer_uses_snapshot(iter->tr->current_trace))
 		return -EBUSY;
 
-	page_size = ring_buffer_subbuf_size_get(iter->array_buffer->buffer);
-	if (*ppos & (page_size - 1))
-		return -EINVAL;
-
-	if (len & (page_size - 1)) {
-		if (len < page_size)
-			return -EINVAL;
-		len &= (~(page_size - 1));
-	}
-
 	if (splice_grow_spd(pipe, &spd))
 		return -ENOMEM;
 
@@ -7294,7 +7272,8 @@ ssize_t tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 	trace_access_lock(iter->cpu_file);
 	entries = ring_buffer_entries_cpu(iter->array_buffer->buffer, iter->cpu_file);
 
-	for (i = 0; i < spd.nr_pages_max && len && entries; i++, len -= page_size) {
+	for (i = 0; i < spd.nr_pages_max && len && entries; i++) {
+		unsigned int page_size;
 		struct page *page;
 		int r;
 
@@ -7306,25 +7285,35 @@ ssize_t tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 
 		refcount_set(&ref->refcount, 1);
 		ref->buffer = iter->array_buffer->buffer;
-		ref->page = ring_buffer_alloc_read_page(ref->buffer, iter->cpu_file);
-		if (IS_ERR(ref->page)) {
-			ret = PTR_ERR(ref->page);
-			ref->page = NULL;
+
+		ret = ring_buffer_alloc_read_page(ref->buffer, iter->cpu_file, &ref->rpage);
+		if (ret) {
 			kfree(ref);
 			break;
 		}
 		ref->cpu = iter->cpu_file;
 
-		r = ring_buffer_read_page(ref->buffer, ref->page,
-					  len, iter->cpu_file, 1);
+		page_size = ring_buffer_read_page_size(ref->rpage);
+
+		r = -EINVAL;
+		if (IS_ALIGNED(*ppos, page_size) && len >= page_size) {
+			r = ring_buffer_read_page(ref->buffer, ref->rpage, len, iter->cpu_file, 1);
+		} else if (!i) {
+			/*
+			 * If the first iteration fails this is an invalid userspace input.
+			 * Otherwise, this is because the subbuf order has been modified. Do not
+			 * report an error and finish the read.
+			 */
+			ret = -EINVAL;
+		}
+
 		if (r < 0) {
-			ring_buffer_free_read_page(ref->buffer, ref->cpu,
-						   ref->page);
+			ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->rpage);
 			kfree(ref);
 			break;
 		}
 
-		page = virt_to_page(ring_buffer_read_page_data(ref->page));
+		page = virt_to_page(ring_buffer_read_page_data(ref->rpage));
 
 		spd.pages[i] = page;
 		spd.partial[i].len = page_size;
@@ -7332,6 +7321,7 @@ ssize_t tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 		spd.partial[i].private = (unsigned long)ref;
 		spd.nr_pages++;
 		*ppos += page_size;
+		len -= page_size;
 
 		entries = ring_buffer_entries_cpu(iter->array_buffer->buffer, iter->cpu_file);
 	}
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 74a7a50d1e78..203d098ee14e 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -745,11 +745,10 @@ static inline int tracing_get_cpu(struct inode *inode)
 void tracing_reset_cpu(struct array_buffer *buf, int cpu);
 
 struct ftrace_buffer_info {
-	struct trace_iterator	iter;
-	void			*spare;
-	unsigned int		spare_cpu;
-	unsigned int		spare_size;
-	unsigned int		read;
+	struct trace_iterator		iter;
+	struct buffer_data_read_page	*spare;
+	unsigned int			spare_cpu;
+	unsigned int			read;
 };
 
 /**
-- 
2.55.0.860.g4b6b3295ed-goog

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] ring_buffer_read_page() incorrectly returns -1 for splice_read requests on subbuf order mismatch
--

commit c4d6ee6f55f23c8ec6da6f812ec3b722b2b7f6b6
Author: Vincent Donnefort <vdonnefort@google.com>

tracing: Fix subbuf resize races with trace_pipe_raw readers

This commit modifies ring_buffer_alloc_read_page() and
ring_buffer_read_page() to handle dynamic resizing of trace ring
buffers, allowing readers to avoid stale size values. It introduces
a manual data copy fallback in ring_buffer_read_page() when the
subbuffer orders do not match.

> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
[ ... ]
> @@ -7183,7 +7218,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  	 */
>  	if (read || (len < (size - read)) ||
>  	    cpu_buffer->reader_page == cpu_buffer->commit_page ||
> -	    rb_is_static(cpu_buffer)) {
> +	    rb_is_static(cpu_buffer) ||
> +	    data_page->order != reader->order) {
>  		struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
>  		unsigned int rpos = read;
>  		unsigned int pos = 0;

[Severity: High]
Does this new condition break splice reads when the buffer order changes?

When tracing_buffers_splice_read() allocates a read page with the new order
and calls ring_buffer_read_page() with full=1, it will now enter this manual
copy block because the subbuffer orders no longer match.

However, just a few lines down inside this block in ring_buffer_read_page(),
there is the following check:

        if (full &&
            (!read || (len < (size - read)) ||
             cpu_buffer->reader_page == cpu_buffer->commit_page))
                return -1;

Since the reader page typically hasn't been partially read yet (read == 0),
and full == 1 from splice, this check evaluates to true and immediately
returns -1 instead of copying the data.

Because tracing_buffers_splice_read() loops and ultimately returns -EAGAIN
on failure, will this cause an infinite loop of -EAGAIN for userspace tools
trying to read across buffer resizes?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260826094528.3738023-1-vdonnefort@google.com?part=1

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Steven Rostedt]

On Wed, 26 Aug 2026 09:59:46 +0000
sashiko-bot@kernel.org wrote:

> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c  
> [ ... ]
> > @@ -7183,7 +7218,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
> >  	 */
> >  	if (read || (len < (size - read)) ||
> >  	    cpu_buffer->reader_page == cpu_buffer->commit_page ||
> > -	    rb_is_static(cpu_buffer)) {
> > +	    rb_is_static(cpu_buffer) ||
> > +	    data_page->order != reader->order) {
> >  		struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
> >  		unsigned int rpos = read;
> >  		unsigned int pos = 0;  
> 
> [Severity: High]
> Does this new condition break splice reads when the buffer order changes?
> 
> When tracing_buffers_splice_read() allocates a read page with the new order
> and calls ring_buffer_read_page() with full=1, it will now enter this manual
> copy block because the subbuffer orders no longer match.
> 
> However, just a few lines down inside this block in ring_buffer_read_page(),
> there is the following check:
> 
>         if (full &&
>             (!read || (len < (size - read)) ||
>              cpu_buffer->reader_page == cpu_buffer->commit_page))
>                 return -1;
> 
> Since the reader page typically hasn't been partially read yet (read == 0),
> and full == 1 from splice, this check evaluates to true and immediately
> returns -1 instead of copying the data.
> 
> Because tracing_buffers_splice_read() loops and ultimately returns -EAGAIN
> on failure, will this cause an infinite loop of -EAGAIN for userspace tools
> trying to read across buffer resizes?
> 


This is a legitimate concern. But with an easy solution. Perhaps just add:

	if (full &&
	    (!read || (len < (size - read)) ||
-	     cpu_buffer->reader_page == cpu_buffer->commit_page))
+	     cpu_buffer->reader_page == cpu_buffer->commit_page) &&
+	    data_page->order == buffer->subbuf_order)
		return -1;

If the user is changing the buffer size at the same time as reading raw
pages, they get what they deserve! I just don't want to let the kernel go
into an infinite loop.

-- Steve

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Vincent Donnefort]

On Wed, Aug 26, 2026 at 10:37:36AM -0400, Steven Rostedt wrote:
> On Wed, 26 Aug 2026 09:59:46 +0000
> sashiko-bot@kernel.org wrote:
> 
> > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > > --- a/kernel/trace/ring_buffer.c
> > > +++ b/kernel/trace/ring_buffer.c  
> > [ ... ]
> > > @@ -7183,7 +7218,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
> > >  	 */
> > >  	if (read || (len < (size - read)) ||
> > >  	    cpu_buffer->reader_page == cpu_buffer->commit_page ||
> > > -	    rb_is_static(cpu_buffer)) {
> > > +	    rb_is_static(cpu_buffer) ||
> > > +	    data_page->order != reader->order) {
> > >  		struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
> > >  		unsigned int rpos = read;
> > >  		unsigned int pos = 0;  
> > 
> > [Severity: High]
> > Does this new condition break splice reads when the buffer order changes?
> > 
> > When tracing_buffers_splice_read() allocates a read page with the new order
> > and calls ring_buffer_read_page() with full=1, it will now enter this manual
> > copy block because the subbuffer orders no longer match.
> > 
> > However, just a few lines down inside this block in ring_buffer_read_page(),
> > there is the following check:
> > 
> >         if (full &&
> >             (!read || (len < (size - read)) ||
> >              cpu_buffer->reader_page == cpu_buffer->commit_page))
> >                 return -1;
> > 
> > Since the reader page typically hasn't been partially read yet (read == 0),
> > and full == 1 from splice, this check evaluates to true and immediately
> > returns -1 instead of copying the data.
> > 
> > Because tracing_buffers_splice_read() loops and ultimately returns -EAGAIN
> > on failure, will this cause an infinite loop of -EAGAIN for userspace tools
> > trying to read across buffer resizes?
> > 
> 
> 
> This is a legitimate concern. But with an easy solution. Perhaps just add:
> 
> 	if (full &&
> 	    (!read || (len < (size - read)) ||
> -	     cpu_buffer->reader_page == cpu_buffer->commit_page))
> +	     cpu_buffer->reader_page == cpu_buffer->commit_page) &&
> +	    data_page->order == buffer->subbuf_order)
> 		return -1;
> 
> If the user is changing the buffer size at the same time as reading raw
> pages, they get what they deserve! I just don't want to let the kernel go
> into an infinite loop.
> 
> -- Steve
> 

Sounds good. Do you prefer to get a v9 or you fold this into the existing commit?

-- 
Vincent

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Steven Rostedt]

On Wed, 26 Aug 2026 17:24:35 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> > This is a legitimate concern. But with an easy solution. Perhaps just add:
> > 
> > 	if (full &&
> > 	    (!read || (len < (size - read)) ||
> > -	     cpu_buffer->reader_page == cpu_buffer->commit_page))
> > +	     cpu_buffer->reader_page == cpu_buffer->commit_page) &&
> > +	    data_page->order == buffer->subbuf_order)
> > 		return -1;
> > 
> > If the user is changing the buffer size at the same time as reading raw
> > pages, they get what they deserve! I just don't want to let the kernel go
> > into an infinite loop.
> > 
> > -- Steve
> >   
> 
> Sounds good. Do you prefer to get a v9 or you fold this into the existing commit?
> 

Please send a v9. It makes the patchwork flow much better. If I modify a
patch, it makes patchwork not be able to automate the status updates as it
moves around git trees.

Thanks,

-- Steve

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Vincent Donnefort]

On Wed, Aug 26, 2026 at 02:31:23PM -0400, Steven Rostedt wrote:
> On Wed, 26 Aug 2026 17:24:35 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > > This is a legitimate concern. But with an easy solution. Perhaps just add:
> > > 
> > > 	if (full &&
> > > 	    (!read || (len < (size - read)) ||
> > > -	     cpu_buffer->reader_page == cpu_buffer->commit_page))
> > > +	     cpu_buffer->reader_page == cpu_buffer->commit_page) &&
> > > +	    data_page->order == buffer->subbuf_order)
> > > 		return -1;
> > > 
> > > If the user is changing the buffer size at the same time as reading raw
> > > pages, they get what they deserve! I just don't want to let the kernel go
> > > into an infinite loop.
> > > 
> > > -- Steve
> > >   
> > 
> > Sounds good. Do you prefer to get a v9 or you fold this into the existing commit?
> > 
> 
> Please send a v9. It makes the patchwork flow much better. If I modify a
> patch, it makes patchwork not be able to automate the status updates as it
> moves around git trees.
> 
> Thanks,
> 
> -- Steve
> 

Looking at the function again, shouldn't we add !rb_is_static(cpu_buffer) as
well to the full page test?

-- 
Vincent

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Steven Rostedt]

On Thu, 27 Aug 2026 07:31:23 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> Looking at the function again, shouldn't we add !rb_is_static(cpu_buffer) as
> well to the full page test?

Why? 

-- Steve

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Vincent Donnefort]

On Thu, Aug 27, 2026 at 09:15:41AM -0400, Steven Rostedt wrote:
> On Thu, 27 Aug 2026 07:31:23 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > Looking at the function again, shouldn't we add !rb_is_static(cpu_buffer) as
> > well to the full page test?
> 
> Why? 
> 
> -- Steve

So that the splice can make progress when the ring-buffer is presistent or
user-mapped. Or do we say that's expected behaviour? 

-- 
Vincent

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Steven Rostedt]

On Thu, 27 Aug 2026 17:21:39 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> On Thu, Aug 27, 2026 at 09:15:41AM -0400, Steven Rostedt wrote:
> > On Thu, 27 Aug 2026 07:31:23 +0100
> > Vincent Donnefort <vdonnefort@google.com> wrote:
> >   
> > > Looking at the function again, shouldn't we add !rb_is_static(cpu_buffer) as
> > > well to the full page test?  
> > 
> > Why? 
> > 
> > -- Steve  
> 
> So that the splice can make progress when the ring-buffer is presistent or
> user-mapped. Or do we say that's expected behaviour? 
> 

Ah yeah, I see. Hmm, looking at the code to see how this would work, I
think I found another bug. Let me look deeper into it.

-- Steve

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Vincent Donnefort]

On Thu, Aug 27, 2026 at 03:33:14PM -0400, Steven Rostedt wrote:
> On Thu, 27 Aug 2026 17:21:39 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > On Thu, Aug 27, 2026 at 09:15:41AM -0400, Steven Rostedt wrote:
> > > On Thu, 27 Aug 2026 07:31:23 +0100
> > > Vincent Donnefort <vdonnefort@google.com> wrote:
> > >   
> > > > Looking at the function again, shouldn't we add !rb_is_static(cpu_buffer) as
> > > > well to the full page test?  
> > > 
> > > Why? 
> > > 
> > > -- Steve  
> > 
> > So that the splice can make progress when the ring-buffer is presistent or
> > user-mapped. Or do we say that's expected behaviour? 
> > 
> 
> Ah yeah, I see. Hmm, looking at the code to see how this would work, I
> think I found another bug. Let me look deeper into it.
> 
> -- Steve

A bug if we allow rb_is_static()?

If not, shall I add it to my series?

-- 
Vincent

Re: [PATCH v8 1/3] tracing: Fix subbuf resize races with trace_pipe_raw readers

[Steven Rostedt]

On August 28, 2026 4:24:36 AM EDT, Vincent Donnefort <vdonnefort@google.com> wrote:
>On Thu, Aug 27, 2026 at 03:33:14PM -0400, Steven Rostedt wrote:
>> On Thu, 27 Aug 2026 17:21:39 +0100
>> Vincent Donnefort <vdonnefort@google.com> wrote:
>> 
>> > On Thu, Aug 27, 2026 at 09:15:41AM -0400, Steven Rostedt wrote:
>> > > On Thu, 27 Aug 2026 07:31:23 +0100
>> > > Vincent Donnefort <vdonnefort@google.com> wrote:
>> > >   
>> > > > Looking at the function again, shouldn't we add !rb_is_static(cpu_buffer) as
>> > > > well to the full page test?  
>> > > 
>> > > Why? 
>> > > 
>> > > -- Steve  
>> > 
>> > So that the splice can make progress when the ring-buffer is presistent or
>> > user-mapped. Or do we say that's expected behaviour? 
>> > 
>> 
>> Ah yeah, I see. Hmm, looking at the code to see how this would work, I
>> think I found another bug. Let me look deeper into it.
>> 
>> -- Steve
>
>A bug if we allow rb_is_static()?

No, something else.

>
>If not, shall I add it to my series?

Yeah, go ahead and update it.

Thanks,

-- Steve 

[PATCH v8 2/3] ring-buffer: Cap static ring buffer nr_pages

[Vincent Donnefort]

Static ring buffers (i.e. persistent, user-mapped and remote) rely on
the bpage::id field. The number of pages for those ring buffers must fit
into that variable. Enforce this limit on ring buffer creation or
user-mapping.

While at it, make buffer_page::id 31 bits. This does not change the
struct buffer_page size.

Fixes: be68d63a139b ("ring-buffer: Add ring_buffer_alloc_range()")
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b8e6bd309707..6089fcc67e2b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -350,7 +350,7 @@ struct buffer_page {
 	local_t		 entries;	/* entries on this page */
 	unsigned long	 real_end;	/* real end of data */
 	unsigned	 order;		/* order of the page */
-	u32		 id:30;		/* ID for external mapping */
+	u32		 id:31;		/* ID for external mapping */
 	u32		 range:1;	/* Mapped via a range */
 	struct buffer_data_page *page;	/* Actual data page */
 };
@@ -657,6 +657,15 @@ static bool rb_is_static(struct ring_buffer_per_cpu *cpu_buffer)
 	return cpu_buffer->user_mapped || cpu_buffer->remote || cpu_buffer->ring_meta;
 }
 
+static unsigned long rb_static_max_pages(void)
+{
+	/*
+	 * Static ring buffers are using bpage::id and must account for the
+	 * reader page.
+	 */
+	return (1UL << 31) - 1;
+}
+
 struct ring_buffer_iter {
 	struct ring_buffer_per_cpu	*cpu_buffer;
 	unsigned long			head;
@@ -2842,6 +2851,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		 */
 		nr_pages = (size - sizeof(struct ring_buffer_cpu_meta)) /
 			(subbuf_size + sizeof(int));
+
+		if (nr_pages > rb_static_max_pages())
+			goto fail_free_buffers;
+
 		/* Need at least two pages plus the reader page */
 		if (nr_pages < 3)
 			goto fail_free_buffers;
@@ -2874,6 +2887,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		/* The writer is remote. This ring-buffer is read-only */
 		atomic_inc(&buffer->record_disabled);
 		nr_pages = desc->nr_page_va - 1;
+
+		if (nr_pages > rb_static_max_pages())
+			goto fail_free_buffers;
+
 		if (nr_pages < 2)
 			goto fail_free_buffers;
 	} else {
@@ -7839,6 +7856,9 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 	/* prevent another thread from changing buffer/sub-buffer sizes */
 	guard(mutex)(&buffer->mutex);
 
+	if (cpu_buffer->nr_pages > rb_static_max_pages())
+		return -E2BIG;
+
 	err = rb_alloc_meta_page(cpu_buffer);
 	if (err)
 		return err;
-- 
2.55.0.860.g4b6b3295ed-goog

[PATCH v8 3/3] ring-buffer: Prevent truncation of nr_pages / nr_subbufs

[Vincent Donnefort]

Although ring_buffer_per_cpu::nr_pages is defined as unsigned long, it
is capped to 32-bits in a few places, limiting the operations possible
on a very large buffer. Use `unsigned long` where appropriate and
prevent truncation of values using nr_pages (or nr_subbufs).

While at it, subbuf_size must be at least `unsigned int`.

Note that persistent, remote and user-mapped ring buffers are capping
the number of pages to 31 bits already, making "int" safe in many
places.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 6089fcc67e2b..1fcf55e2ca81 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1683,7 +1683,7 @@ static void rb_check_pages(struct ring_buffer_per_cpu *cpu_buffer)
  * This is used to help find the next per cpu subbuffer within a mapped range.
  */
 static unsigned long
-rb_range_align_subbuf(unsigned long addr, int subbuf_size, int nr_subbufs)
+rb_range_align_subbuf(unsigned long addr, unsigned int subbuf_size, unsigned long nr_subbufs)
 {
 	addr += sizeof(struct ring_buffer_cpu_meta) +
 		sizeof(int) * nr_subbufs;
@@ -1693,13 +1693,12 @@ rb_range_align_subbuf(unsigned long addr, int subbuf_size, int nr_subbufs)
 /*
  * Return the ring_buffer_meta for a given @cpu.
  */
-static void *rb_range_meta(struct trace_buffer *buffer, int nr_pages, int cpu)
+static void *rb_range_meta(struct trace_buffer *buffer, unsigned long nr_pages, int cpu)
 {
-	int subbuf_size = rb_subbuf_size(buffer);
+	unsigned int subbuf_size = rb_subbuf_size(buffer);
 	struct ring_buffer_cpu_meta *meta;
 	struct ring_buffer_meta *bmeta;
-	unsigned long ptr;
-	int nr_subbufs;
+	unsigned long ptr, nr_subbufs;
 
 	bmeta = buffer->meta;
 	if (!bmeta)
@@ -1745,7 +1744,7 @@ static void *rb_range_meta(struct trace_buffer *buffer, int nr_pages, int cpu)
 /* Return the start of subbufs given the meta pointer */
 static void *rb_subbufs_from_meta(struct ring_buffer_cpu_meta *meta)
 {
-	int subbuf_size = meta->subbuf_size;
+	unsigned int subbuf_size = meta->subbuf_size;
 	unsigned long ptr;
 
 	ptr = (unsigned long)meta;
@@ -1757,11 +1756,11 @@ static void *rb_subbufs_from_meta(struct ring_buffer_cpu_meta *meta)
 /*
  * Return a specific sub-buffer for a given @cpu defined by @idx.
  */
-static void *rb_range_buffer(struct ring_buffer_per_cpu *cpu_buffer, int idx)
+static void *rb_range_buffer(struct ring_buffer_per_cpu *cpu_buffer, unsigned int idx)
 {
 	struct ring_buffer_cpu_meta *meta;
+	unsigned int subbuf_size;
 	unsigned long ptr;
-	int subbuf_size;
 
 	meta = rb_range_meta(cpu_buffer->buffer, 0, cpu_buffer->cpu);
 	if (!meta)
@@ -1777,7 +1776,7 @@ static void *rb_range_buffer(struct ring_buffer_per_cpu *cpu_buffer, int idx)
 
 	ptr = (unsigned long)rb_subbufs_from_meta(meta);
 
-	ptr += subbuf_size * idx;
+	ptr += (unsigned long)subbuf_size * idx;
 	if (ptr + subbuf_size > cpu_buffer->buffer->range_addr_end)
 		return NULL;
 
@@ -1854,13 +1853,13 @@ static bool rb_meta_init(struct trace_buffer *buffer, int scratch_size)
  * must be the same.
  */
 static bool rb_cpu_meta_valid(struct ring_buffer_cpu_meta *meta, int cpu,
-			      struct trace_buffer *buffer, int nr_pages,
+			      struct trace_buffer *buffer, unsigned long nr_pages,
 			      unsigned long *subbuf_mask)
 {
-	int subbuf_size = PAGE_SIZE;
+	unsigned long subbuf_size = PAGE_SIZE;
 	unsigned long buffers_start;
 	unsigned long buffers_end;
-	int i;
+	unsigned long i;
 
 	if (!subbuf_mask)
 		return false;
@@ -2109,8 +2108,8 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
 	struct buffer_page *head_page, *orig_head, *orig_reader;
 	struct rb_validation_state state = { 0 };
 	bool skip = false;
+	unsigned long i;
 	int ret;
-	int i;
 
 	if (!meta || !meta->head_buffer)
 		return;
@@ -2161,7 +2160,7 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
 		rb_validate_buffer(head_page, cpu_buffer, meta, &state, 0, state.ts);
 	}
 	if (i)
-		pr_info("Ring buffer [%d] rewound %d pages\n", cpu_buffer->cpu, i);
+		pr_info("Ring buffer [%d] rewound %lu pages\n", cpu_buffer->cpu, i);
 
 	/* The last rewound page must be skipped. */
 	if (head_page != orig_head)
@@ -2245,7 +2244,8 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
 	}
 }
 
-static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages, int scratch_size)
+static void rb_range_meta_init(struct trace_buffer *buffer,
+			       unsigned long nr_pages, int scratch_size)
 {
 	struct ring_buffer_cpu_meta *meta;
 	unsigned long *subbuf_mask;
@@ -2345,8 +2345,8 @@ static int rbm_show(struct seq_file *m, void *v)
 			   rb_meta_subbuf_idx(meta, (void *)meta->head_buffer));
 		seq_printf(m, "commit_buffer: %d\n",
 			   rb_meta_subbuf_idx(meta, (void *)meta->commit_buffer));
-		seq_printf(m, "subbuf_size:   %d\n", meta->subbuf_size);
-		seq_printf(m, "nr_subbufs:    %d\n", meta->nr_subbufs);
+		seq_printf(m, "subbuf_size:   %u\n", meta->subbuf_size);
+		seq_printf(m, "nr_subbufs:    %u\n", meta->nr_subbufs);
 		return 0;
 	}
 
@@ -2431,7 +2431,7 @@ static void *ring_buffer_desc_page(struct ring_buffer_desc *desc, unsigned int p
 }
 
 static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
-		long nr_pages, struct list_head *pages)
+			       unsigned long nr_pages, struct list_head *pages)
 {
 	struct trace_buffer *buffer = cpu_buffer->buffer;
 	struct ring_buffer_cpu_meta *meta = NULL;
@@ -2559,7 +2559,7 @@ static int rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
 }
 
 static struct ring_buffer_per_cpu *
-rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
+rb_allocate_cpu_buffer(struct trace_buffer *buffer, unsigned long nr_pages, int cpu)
 {
 	struct ring_buffer_per_cpu *cpu_buffer __free(kfree) =
 		alloc_cpu_buffer(cpu);
@@ -2716,8 +2716,8 @@ static void rb_test_inject_invalid_pages(struct trace_buffer *buffer)
 	struct ring_buffer_cpu_meta *meta;
 	struct buffer_data_page *dpage;
 	unsigned long entry_bytes = 0;
+	unsigned int subbuf_size;
 	unsigned long ptr;
-	int subbuf_size;
 	int invalid = 0;
 	int cpu;
 	int i;
@@ -2787,8 +2787,8 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 					 struct ring_buffer_remote *remote)
 {
 	struct trace_buffer *buffer __free(kfree) = NULL;
-	long nr_pages;
-	int subbuf_size;
+	unsigned int subbuf_size;
+	unsigned long nr_pages;
 	int bsize;
 	int cpu;
 	int ret;
@@ -5872,12 +5872,12 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
 static struct buffer_page *
 __rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	int max_loops = cpu_buffer->ring_meta ? cpu_buffer->nr_pages : 3;
+	unsigned long max_loops = cpu_buffer->ring_meta ? cpu_buffer->nr_pages : 3;
 	struct buffer_page *reader = NULL;
+	unsigned long nr_loops = 0;
 	unsigned long overwrite;
 	unsigned long flags;
 	int missed_events = 0;
-	int nr_loops = 0;
 	bool ret;
 
 	local_irq_save(flags);
@@ -6195,8 +6195,8 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
 	struct trace_buffer *buffer;
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct ring_buffer_event *event;
-	int nr_loops = 0;
-	int max_loops;
+	unsigned long nr_loops = 0;
+	unsigned long max_loops;
 
 	if (ts)
 		*ts = 0;
@@ -7442,8 +7442,8 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct buffer_page *bpage, *tmp;
 	unsigned int old_capacity;
+	unsigned long nr_pages;
 	int old_order;
-	int nr_pages;
 	int psize;
 	int err;
 	int cpu;
@@ -7625,10 +7625,10 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
 				   struct buffer_page **subbuf_ids)
 {
 	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
-	unsigned int nr_subbufs = cpu_buffer->nr_pages + 1;
+	unsigned long nr_subbufs = cpu_buffer->nr_pages + 1;
 	struct buffer_page *first_subbuf, *subbuf;
-	int cnt = 0;
-	int id = 0;
+	unsigned int cnt = 0;
+	unsigned int id = 0;
 
 	id = rb_page_id(cpu_buffer, cpu_buffer->reader_page, id);
 	subbuf_ids[id++] = cpu_buffer->reader_page;
-- 
2.55.0.860.g4b6b3295ed-goog

Re: [PATCH v8 3/3] ring-buffer: Prevent truncation of nr_pages / nr_subbufs

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The kernel fails to validate the untrusted `meta->nr_subbufs` value from persistent memory against the expected `nr_pages + 1`, leading to arbitrary heap out-of-bounds writes and potential arbitrary kernel memory corruption.
--

commit 5ed0ab549782b2ff87e1bd2394abb78b10c4b602
Author: Vincent Donnefort <vdonnefort@google.com>

ring-buffer: Prevent truncation of nr_pages / nr_subbufs

Although ring_buffer_per_cpu::nr_pages is defined as unsigned long, it
is capped to 32-bits in a few places, limiting the operations possible
on a very large buffer. Use unsigned long where appropriate and
prevent truncation of values using nr_pages (or nr_subbufs).

While at it, subbuf_size must be at least unsigned int.

Note that persistent, remote and user-mapped ring buffers are capping
the number of pages to 31 bits already, making "int" safe in many
places.

> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 6089fcc67e2b6..1fcf55e2ca81f 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1745,7 +1744,7 @@ static void *rb_range_meta(struct trace_buffer *buffer, int nr_pages, int cpu)
>  /* Return the start of subbufs given the meta pointer */
>  static void *rb_subbufs_from_meta(struct ring_buffer_cpu_meta *meta)
>  {
> -	int subbuf_size = meta->subbuf_size;
> +	unsigned int subbuf_size = meta->subbuf_size;
>  	unsigned long ptr;
>  
>  	ptr = (unsigned long)meta;

[Severity: High]
This isn't a bug introduced by this patch, but does the next line in this
function perform pointer arithmetic using an unvalidated meta->nr_subbufs?

	ptr = rb_range_align_subbuf(ptr, subbuf_size, meta->nr_subbufs);

Could this yield a corrupted or attacker-controlled subbuf base pointer if
meta->nr_subbufs is read from persistent memory without being checked against
expected limits?

[ ... ]

> @@ -1854,13 +1853,13 @@ static bool rb_meta_init(struct trace_buffer *buffer, int scratch_size)
>   * must be the same.
>   */
>  static bool rb_cpu_meta_valid(struct ring_buffer_cpu_meta *meta, int cpu,
> -			      struct trace_buffer *buffer, int nr_pages,
> +			      struct trace_buffer *buffer, unsigned long nr_pages,
>  			      unsigned long *subbuf_mask)
>  {
> -	int subbuf_size = PAGE_SIZE;
> +	unsigned long subbuf_size = PAGE_SIZE;
>  	unsigned long buffers_start;
>  	unsigned long buffers_end;
> -	int i;
> +	unsigned long i;
>  
>  	if (!subbuf_mask)
>  		return false;

[Severity: High]
This isn't a bug introduced by this patch, but is there a risk of an
out-of-bounds heap write further down in this function?

The subbuf_mask is allocated with nr_pages + 1 bits in rb_range_meta_init(),
but later in rb_cpu_meta_valid() operations use the unvalidated
meta->nr_subbufs value read from persistent memory:

	bitmap_clear(subbuf_mask, 0, meta->nr_subbufs);

	/*
	 * Ensure the meta::buffers array has correct data. The data in each subbufs
	 * are checked later in rb_meta_validate_events().
	 */
	for (i = 0; i < meta->nr_subbufs; i++) {
		if (meta->buffers[i] < 0 ||
		    meta->buffers[i] >= meta->nr_subbufs) {
			pr_info("Ring buffer boot meta [%d] array out of range\n", cpu);
			return false;
		}

		if (test_bit(meta->buffers[i], subbuf_mask)) {
			pr_info("Ring buffer boot meta [%d] array has duplicates\n", cpu);
			return false;
		}

		set_bit(meta->buffers[i], subbuf_mask);
	}

If meta->nr_subbufs is larger than nr_pages + 1, could this write out of
bounds and corrupt kernel memory?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260826094528.3738023-1-vdonnefort@google.com?part=3