Re: [libgpiod][PATCH v2 2/2] core: fix reading subset of available events

[Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>]

On Sat, Sep 12, 2020 at 10:11 AM Kent Gibson <warthog618@xxxxxxxxx> wrote:
>
> Only read the requested number of events from the kernel rather than
> reading up to 16 and quietly discarding any surplus.
>
> The previous behavour is particularly bad for reading single events as
> userspace must read the events as quickly as they arrive, effectively
> negating the presence of the kernel event kfifo.
>
> Fixes: 44921ecc9a00 (core: provide functions for reading multiple
> line events at once)
>
> Signed-off-by: Kent Gibson <warthog618@xxxxxxxxx>
> ---
>  lib/core.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/lib/core.c b/lib/core.c
> index ad76051..b964272 100644
> --- a/lib/core.c
> +++ b/lib/core.c
> @@ -1090,7 +1090,10 @@ int gpiod_line_event_read_fd_multiple(int fd, struct gpiod_line_event *events,
>
>         memset(evdata, 0, sizeof(evdata));
>
> -       rd = read(fd, evdata, sizeof(evdata));
> +       if (num_events > 16)
> +               num_events = 16;
> +
> +       rd = read(fd, evdata, num_events * sizeof(*evdata));
>         if (rd < 0) {
>                 return -1;
>         } else if ((unsigned int)rd < sizeof(*evdata)) {
> --
> 2.28.0
>

Wow this is a bad one, thanks for catching this!

Will apply shortly and backport it to stable branches too.

Thanks!
Bartosz