[PATCH v6 0/2] ring-buffer: Fixes for subbuf resizing and persistent buffers

[PATCH v6 0/2] ring-buffer: Fixes for subbuf resizing and persistent buffers

[Vincent Donnefort]

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

Changelog:

v6

  - 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 (2):
  tracing: Fix subbuf resize races with trace_pipe_raw readers
  ring-buffer: Improve nr_pages type

 include/linux/ring_buffer.h          |   5 +-
 kernel/trace/ring_buffer.c           | 146 ++++++++++++++++++---------
 kernel/trace/ring_buffer_benchmark.c |   6 +-
 kernel/trace/trace.c                 | 104 +++++++++----------
 kernel/trace/trace.h                 |   9 +-
 5 files changed, 159 insertions(+), 111 deletions(-)


base-commit: 5f0775c9f72fd284028d955abb7098ecf212a604
-- 
2.55.0.691.gc56d675ccc-goog

[PATCH v6 1/2] 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 let it handle the resizing of
a previous buffer_data_read_page if necessary and add a new
ring_buffer_read_page_size() which enable ring-buffer users to not use
the racy ring_buffer_subbuf_size_get(). This makes the spare_size member
of ftrace_buffer_info redundant.

Use those functions in trace_pipe_raw readers and handle in both the
case where the subbuf order is modified in the middle of the read.

Fixes: bce761d75745 ("ring-buffer: Read and write to ring buffers with custom sub buffer size")
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 include/linux/ring_buffer.h          |   5 +-
 kernel/trace/ring_buffer.c           | 100 ++++++++++++++++++--------
 kernel/trace/ring_buffer_benchmark.c |   6 +-
 kernel/trace/trace.c                 | 104 +++++++++++++--------------
 kernel/trace/trace.h                 |   9 ++-
 5 files changed, 130 insertions(+), 94 deletions(-)

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 5fc009edc1ec..ec13779922ff 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6990,56 +6990,78 @@ 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 +7069,30 @@ 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 +7100,14 @@ 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 +7115,8 @@ 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);
 
@@ -7122,6 +7153,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_free_read_page);
  * Returns:
  *  >=0 if data has been transferred, returns the offset of consumed data.
  *  <0 if no data has been transferred.
+ *  -EAGAIN if the subbuf size has changed and @data_page must be reallocated.
  */
 int ring_buffer_read_page(struct trace_buffer *buffer,
 			  struct buffer_data_read_page *data_page,
@@ -7159,7 +7191,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
 
 	if (data_page->order != cpu_buffer->reader_page->order)
-		return -1;
+		return -EAGAIN;
 
 	reader = rb_get_reader_page(cpu_buffer);
 	if (!reader)
@@ -7323,6 +7355,18 @@ 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 395238b2b715..355278083b3f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7080,8 +7080,8 @@ 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;
 
@@ -7091,36 +7091,21 @@ 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 */
-	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;
-		}
-	}
-
-	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)
-		return ret;
-
+again:
 	/* Do we have previous read data to read? */
-	if (info->read < page_size)
+	if (info->spare && (info->read < ring_buffer_read_page_size(info->spare)))
 		goto read;
 
- again:
+	/* Make sure the read page order is aligned with the current buffer subbuf order */
+	ret = ring_buffer_alloc_read_page(iter->array_buffer->buffer, iter->cpu_file,
+					  &info->spare);
+	if (ret < 0)
+		return ret;
+
+	spare_size = ring_buffer_read_page_size(info->spare);
+	info->read = spare_size;
+	info->spare_cpu = iter->cpu_file;
+
 	trace_access_lock(iter->cpu_file);
 	ret = ring_buffer_read_page(iter->array_buffer->buffer,
 				    info->spare,
@@ -7128,7 +7113,9 @@ ssize_t tracing_buffers_read(struct file *filp, char __user *ubuf,
 				    iter->cpu_file, 0);
 	trace_access_unlock(iter->cpu_file);
 
-	if (ret < 0) {
+	if (ret == -EAGAIN) {
+		goto again;
+	} else if (ret < 0) {
 		if (trace_empty(iter) && !iter->closed) {
 			if (update_last_data_if_empty(iter->tr))
 				return 0;
@@ -7146,8 +7133,9 @@ 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);
@@ -7197,17 +7185,17 @@ 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);
 }
 
@@ -7268,23 +7256,12 @@ 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;
 
@@ -7292,7 +7269,8 @@ 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;
 
@@ -7304,25 +7282,38 @@ 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;
+
+new_read_page:
+		ret = ring_buffer_alloc_read_page(ref->buffer, iter->cpu_file, &ref->rpage);
+		if (ret < 0) {
 			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);
+			if (r == -EAGAIN)
+				goto new_read_page;
+		} 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;
@@ -7330,6 +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 bf77331f56a4..cbf54eded447 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;
 };
 
 /**
-- 
2.55.0.691.gc56d675ccc-goog

Re: [PATCH v6 1/2] 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] The local variable `spare_size` is left uninitialized via `goto read;` in `tracing_buffers_read()`, resulting in a kernel memory leak to userspace.
--

--- Patch [1]: [PATCH v6 1/2] tracing: Fix subbuf resize races with trace_pipe_raw readers ---
commit bdc760fe7b1aa6eb8c8a8b8c38e9fd02d25051b6
Author: Vincent Donnefort <vdonnefort@google.com>

tracing: Fix subbuf resize races with trace_pipe_raw readers

Concurrent subbuffer resizes may crash trace_pipe_raw readers or leak
uninitialized memory to userspace due to stale size values. This commit
modifies ring_buffer_alloc_read_page() to handle resizing and introduces
ring_buffer_read_page_size().

> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 395238b2b715c..355278083b3fb 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
[ ... ]
> @@ -7091,36 +7091,21 @@ 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 */
> -	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;
> -		}
> -	}
> +again:
> +	/* Do we have previous read data to read? */
> +	if (info->spare && (info->read < ring_buffer_read_page_size(info->spare)))
> +		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 buffer subbuf order */
> +	ret = ring_buffer_alloc_read_page(iter->array_buffer->buffer, iter->cpu_file,
> +					  &info->spare);
> +	if (ret < 0)
>  		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,
[ ... ]
> @@ -7146,8 +7133,9 @@ 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;

[Severity: High]
Can this regression leak uninitialized stack memory to userspace?

If a userspace process does a partial read (consuming fewer bytes than a full
subbuffer page), the next read syscall will take the goto read; branch at the
top of tracing_buffers_read().

When that branch is taken, spare_size is completely uninitialized stack memory.

Could size end up being a large garbage value (or underflow if spare_size <
info->read), causing copy_to_user() below to read beyond the bounds of
info->spare and leak arbitrary kernel memory?

>  	if (size > count)
>  		size = count;
>  	trace_data = ring_buffer_read_page_data(info->spare);
>  	ret = copy_to_user(ubuf, trace_data + info->read, size);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260814154823.755406-1-vdonnefort@google.com?part=1

[PATCH v6 2/2] ring-buffer: Improve nr_pages type

[Vincent Donnefort]

If 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.

Make sure nr_pages is never capped to 32-bits (that includes nr_subbufs)
and reject a value over 32-bits for the user-mapped, persistent buffer
and remote buffer cases where the limiting factor is the shared
meta-data member for the number of pages/subbufs.

While at it, make sure subbuf_size is 'unsigned int'.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 kernel/trace/ring_buffer.c | 46 ++++++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index ec13779922ff..ec127e2ad052 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1669,7 +1669,7 @@ 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;
@@ -1679,13 +1679,12 @@ 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)
@@ -1731,7 +1730,7 @@ 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;
@@ -1746,8 +1745,8 @@ static void *rb_subbufs_from_meta(struct ring_buffer_cpu_meta *meta)
 static void *rb_range_buffer(struct ring_buffer_per_cpu *cpu_buffer, 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)
@@ -1840,10 +1839,10 @@ 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 int subbuf_size = PAGE_SIZE;
 	unsigned long buffers_start;
 	unsigned long buffers_end;
 	int i;
@@ -2231,7 +2230,8 @@ 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;
@@ -2417,7 +2417,7 @@ 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;
@@ -2545,7 +2545,7 @@ 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);
@@ -2702,8 +2702,8 @@ 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;
@@ -2773,8 +2773,8 @@ 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;
@@ -2837,6 +2837,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
 		 */
 		nr_pages = (size - sizeof(struct ring_buffer_cpu_meta)) /
 			(subbuf_size + sizeof(int));
+
+		/* limited by ring_buffer_cpu_meta::nr_subbufs */
+		if (nr_pages > U32_MAX - 1)
+			goto fail_free_buffers;
 		/* Need at least two pages plus the reader page */
 		if (nr_pages < 3)
 			goto fail_free_buffers;
@@ -2869,6 +2873,10 @@ 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;
+
+		/* limited by ring_buffer_desc::nr_page_va */
+		if (nr_pages > U32_MAX - 1)
+			goto fail_free_buffers;
 		if (nr_pages < 2)
 			goto fail_free_buffers;
 	} else {
@@ -7421,8 +7429,8 @@ 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;
@@ -7604,7 +7612,7 @@ 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;
@@ -7834,6 +7842,10 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 	/* prevent another thread from changing buffer/sub-buffer sizes */
 	guard(mutex)(&buffer->mutex);
 
+	/* limited by trace_buffer_meta::nr_subbufs */
+	if (cpu_buffer->nr_pages > U32_MAX - 1)
+		return -E2BIG;
+
 	err = rb_alloc_meta_page(cpu_buffer);
 	if (err)
 		return err;
-- 
2.55.0.691.gc56d675ccc-goog

Re: [PATCH v6 2/2] ring-buffer: Improve nr_pages type

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:

New issues:
- [High] When `nr_pages` is exactly `U32_MAX - 1`, the expression `meta->nr_subbufs + 1` overflows to 0, silently skipping persistent ring buffer validation and wiping data.

Pre-existing issues:
- [Critical] Loop counters iterating over `meta->nr_subbufs` are typed as `signed int`, which wrap to negative values for extremely large buffers, leading to out-of-bounds memory accesses.
- [Critical] The `meta->nr_subbufs` field is read from potentially corrupted persistent memory and used without validation against `nr_pages`, causing a heap out-of-bounds write in `bitmap_clear()`.
- [High] Multiplication of `subbuf_size` and `idx` overflows 32 bits for ring buffers > 4GB, causing severe memory aliasing.
--

--- Patch [2]: [PATCH v6 2/2] ring-buffer: Improve nr_pages type ---
commit 030ec6a7ceec6e71123077bd1fc64e0cf5c27cc8
Author: Vincent Donnefort <vdonnefort@google.com>

ring-buffer: Improve nr_pages type

If 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.

[ ... ]

> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index ec13779922ffb..ec127e2ad0527 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c

[ ... ]

> @@ -1746,8 +1745,8 @@ static void *rb_subbufs_from_meta(struct ring_buffer_cpu_meta *meta)
>  static void *rb_range_buffer(struct ring_buffer_per_cpu *cpu_buffer, 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)

[Severity: High]
This is a pre-existing issue, but does the multiplication of subbuf_size and
idx overflow 32 bits for ring buffers larger than 4GB?

In rb_range_buffer(), we have:

    subbuf_size = meta->subbuf_size;
    idx = meta->buffers[idx];
    ptr = (unsigned long)rb_subbufs_from_meta(meta);
    ptr += subbuf_size * idx;

Since subbuf_size is an unsigned int and idx is an int, the multiplication
evaluates as a 32-bit unsigned integer. For ring buffers over 4GB, this can
overflow and wrap around before being zero-extended and added to the 64-bit
ptr, causing memory aliasing by overwriting earlier subbuffers.

[ ... ]

> @@ -1840,10 +1839,10 @@ 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 int subbuf_size = PAGE_SIZE;
>  	unsigned long buffers_start;
>  	unsigned long buffers_end;
>  	int i;

[Severity: Critical]
This is a pre-existing issue, but will loop counters iterating over
meta->nr_subbufs wrap to negative values for extremely large buffers?

In rb_cpu_meta_valid():

    for (i = 0; i < meta->nr_subbufs; i++) {
        if (meta->buffers[i] < 0 || ...

And in rb_range_meta_init():

    for (i = 0; i < meta->nr_subbufs; i++) {
        meta->buffers[i] = i;

The loop counter i is a signed int. Since nr_pages can now go up to
U32_MAX - 1, if the buffer size pushes meta->nr_subbufs past INT_MAX, i
will wrap to negative. i < meta->nr_subbufs will promote i to unsigned,
evaluating to true, and the negative index will be sign-extended on 64-bit
architectures, causing out-of-bounds reads and writes.

[Severity: Critical]
This is a pre-existing issue, but could meta->nr_subbufs cause a heap
out-of-bounds write if it doesn't match nr_pages?

In rb_cpu_meta_valid():

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

    for (i = 0; i < meta->nr_subbufs; i++) {
        ...
        set_bit(meta->buffers[i], subbuf_mask);

meta->nr_subbufs is read directly from persistent memory and used without
being validated against the dynamically allocated subbuf_mask size, which is
based on nr_pages + 1. If meta->nr_subbufs is maliciously or accidentally set
larger than nr_pages + 1, bitmap_clear() and set_bit() will write zeroes and
ones past the end of the bitmap allocation.

[ ... ]

> @@ -2837,6 +2837,10 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
>  		 */
>  		nr_pages = (size - sizeof(struct ring_buffer_cpu_meta)) /
>  			(subbuf_size + sizeof(int));
> +
> +		/* limited by ring_buffer_cpu_meta::nr_subbufs */
> +		if (nr_pages > U32_MAX - 1)
> +			goto fail_free_buffers;
>  		/* Need at least two pages plus the reader page */
>  		if (nr_pages < 3)
>  			goto fail_free_buffers;

[Severity: High]
Does meta->nr_subbufs + 1 overflow when nr_pages is exactly U32_MAX - 1?

If nr_pages is exactly U32_MAX - 1, meta->nr_subbufs becomes U32_MAX.
In rb_meta_validate_events(), there are loops with this condition:

    for (i = 0; i < meta->nr_subbufs + 1; i++, rb_dec_page(&head_page)) {

U32_MAX + 1 overflows to 0 in 32-bit unsigned arithmetic. This makes the loop
condition i < 0, causing the loops to terminate immediately without rewinding
and validating the persistent ring buffer from a previous boot.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260814154823.755406-1-vdonnefort@google.com?part=2