On Tue, Jan 23, 2024 at 12:33:37PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
>
> The trace_mmap() checks if the mmapped data has any previously read data
> (the reader.read value of the meta page is non-zero), then it will pre-read
> the kbuf to move its internal reader pointer to the same value.
>
> But when tracefs_cpu_read_buf() calls trace_mmap_load_subbuf(), its
> kbuf->subbuffer will not be the same as the mapped data and
> kbuffer_load_subbuffer() is called on it and it is returned. But that means
> its read pointer has not been updated, and the read data will restart again.
>
> When the kbuf is updated in trace_mmap_load_subbuf() check the tmap->kbuf to
> see if it already read any of the data, and move the kbuffer forward just
> like the trace_mmap() does.
>
> Link: https://lore.kernel.org/linux-trace-devel/Za-Md51snPcIoYFn@xxxxxxxxxx/
>
> Fixes: 2ed14b59 ("libtracefs: Add ring buffer memory mapping APIs")
> Reported-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
That fixes fast-forward, event already read are now skipped.
> ---
> src/tracefs-mmap.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/src/tracefs-mmap.c b/src/tracefs-mmap.c
> index d3af453..a288677 100644
> --- a/src/tracefs-mmap.c
> +++ b/src/tracefs-mmap.c
> @@ -165,6 +165,12 @@ __hidden int trace_mmap_load_subbuf(void *mapping, struct kbuffer *kbuf)
> */
> if (data != kbuffer_subbuffer(kbuf)) {
> kbuffer_load_subbuffer(kbuf, data);
> + /* Move the read pointer forward if need be */
> + if (kbuffer_curr_index(tmap->kbuf)) {
> + int size = kbuffer_curr_offset(tmap->kbuf);
> + char tmpbuf[size];
> + kbuffer_read_buffer(kbuf, tmpbuf, size);
> + }
> return 1;
> }
>
> --
> 2.43.0
>
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