linux-kernel - Re: [PATCH] ring-buffer: Fix buffer max_data_size with max_event

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Date:   Mon, 11 Dec 2023 20:40:33 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Kent Overstreet <kent.overstreet@...ux.dev>
Subject: Re: [PATCH] ring-buffer: Fix buffer max_data_size with
 max_event_size

On Sat, 9 Dec 2023 17:09:25 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sat, 9 Dec 2023 17:01:39 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > From: "Steven Rostedt (Google)" <rostedt@...dmis.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@...r.kernel.org
> Fixes: a4543a2fa9ef3 ("ring-buffer: Get timestamp after event is allocated")

Looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thanks,
> 
> -- Steve
> 
> 
> > Reported-by: Kent Overstreet <kent.overstreet@...ux.dev> # (on IRC)
> > Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.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)))
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>