Re: [PATCH] ring-buffer: Fix buffer max_data_size with max_event_size

[Steven Rostedt <rostedt@goodmis.org>]

On Sat, 9 Dec 2023 17:01:39 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> The maximum ring buffer data size is the maximum size of data that can be
> recorded on the ring buffer. Events must be smaller than the sub buffer
> data size minus any meta data. This size is checked before trying to
> allocate from the ring buffer because the allocation assumes that the size
> will fit on the sub buffer.
> 
> The maximum size was calculated as the size of a sub buffer page (which is
> currently PAGE_SIZE minus the sub buffer header) minus the size of the
> meta data of an individual event. But it missed the possible adding of a
> time stamp for events that are added long enough apart that the event meta
> data can't hold the time delta.
> 
> When an event is added that is greater than the current BUF_MAX_DATA_SIZE
> minus the size of a time stamp, but still less than or equal to
> BUF_MAX_DATA_SIZE, the ring buffer would go into an infinite loop, looking
> for a page that can hold the event. Luckily, there's a check for this loop
> and after 1000 iterations and a warning is emitted and the ring buffer is
> disabled. But this should never happen.
> 
> This can happen when a large event is added first, or after a long period
> where an absolute timestamp is prefixed to the event, increasing its size
> by 8 bytes. This passes the check and then goes into the algorithm that
> causes the infinite loop.
> 
> Fix this by creating a BUF_MAX_EVENT_SIZE to be used to determine if the
> passed in event is too big for the buffer.
> 

Forgot to add:

Cc: stable@vger.kernel.org
Fixes: a4543a2fa9ef3 ("ring-buffer: Get timestamp after event is allocated")

-- Steve


> Reported-by: Kent Overstreet <kent.overstreet@linux.dev> # (on IRC)
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/ring_buffer.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 8d2a4f00eca9..a38e5a3c6803 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -378,6 +378,9 @@ static inline bool test_time_stamp(u64 delta)
>  /* Max payload is BUF_PAGE_SIZE - header (8bytes) */
>  #define BUF_MAX_DATA_SIZE (BUF_PAGE_SIZE - (sizeof(u32) * 2))
>  
> +/* Events may have a time stamp attached to them */
> +#define BUF_MAX_EVENT_SIZE (BUF_MAX_DATA_SIZE - RB_LEN_TIME_EXTEND)
> +
>  int ring_buffer_print_page_header(struct trace_seq *s)
>  {
>  	struct buffer_data_page field;
> @@ -3810,7 +3813,7 @@ ring_buffer_lock_reserve(struct trace_buffer *buffer, unsigned long length)
>  	if (unlikely(atomic_read(&cpu_buffer->record_disabled)))
>  		goto out;
>  
> -	if (unlikely(length > BUF_MAX_DATA_SIZE))
> +	if (unlikely(length > BUF_MAX_EVENT_SIZE))
>  		goto out;
>  
>  	if (unlikely(trace_recursive_lock(cpu_buffer)))
> @@ -3960,7 +3963,7 @@ int ring_buffer_write(struct trace_buffer *buffer,
>  	if (atomic_read(&cpu_buffer->record_disabled))
>  		goto out;
>  
> -	if (length > BUF_MAX_DATA_SIZE)
> +	if (length > BUF_MAX_EVENT_SIZE)
>  		goto out;
>  
>  	if (unlikely(trace_recursive_lock(cpu_buffer)))