On Sat, 9 Dec 2023 17:09:25 -0500 Steven Rostedt <[email protected]> wrote:
> On Sat, 9 Dec 2023 17:01:39 -0500 > Steven Rostedt <[email protected]> wrote: > > > From: "Steven Rostedt (Google)" <[email protected]> > > > > 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: [email protected] > Fixes: a4543a2fa9ef3 ("ring-buffer: Get timestamp after event is allocated") Looks good to me. Reviewed-by: Masami Hiramatsu (Google) <[email protected]> Thanks, > > -- Steve > > > > Reported-by: Kent Overstreet <[email protected]> # (on IRC) > > Signed-off-by: Steven Rostedt (Google) <[email protected]> > > --- > > 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) <[email protected]>
