Re: [RFC PATCH v1 1/3] bpf: Make ring buffer overwritable.

[Francis Laniel <flaniel@linux.microsoft.com>]

Hi.


Le lundi 15 août 2022, 23:52:22 CEST Andrii Nakryiko a écrit :
> On Wed, Aug 10, 2022 at 10:18 AM Francis Laniel
> 
> <flaniel@linux.microsoft.com> wrote:
> > By default, BPF ring buffer are size bounded, when producers already
> > filled the buffer, they need to wait for the consumer to get those data
> > before adding new ones.
> > In terms of API, bpf_ringbuf_reserve() returns NULL if the buffer is full.
> > 
> > This patch permits making BPF ring buffer overwritable.
> > When producers already wrote as many data as the buffer size, they will
> > begin to over write existing data, so the oldest will be replaced.
> > As a result, bpf_ringbuf_reserve() never returns NULL.
> 
> Part of BPF ringbuf record (first 8 bytes) stores information like
> record size and offset in pages to the beginning of ringbuf map
> metadata. This is used by consumer to know how much data belongs to
> data record, but also for making sure that
> bpf_ringbuf_reserve()/bpf_ringbuf_submit() work correctly and don't
> corrupt kernel memory.
> 
> If we simply allow overwriting this information (and no, spinlock
> doesn't protect from that, you can have multiple producers writing to
> different parts of ringbuf data area in parallel after "reserving"
> their respective records), it completely breaks any sort of
> correctness, both for user-space consumer and kernel-side producers.

Thank you for your answer.
My current implementation is indeed wrong as I based it on the wrong 
assumption than BPF ring buffer could only store data of the same size...
With data of different size, we can have the troubles you described.

I will rework my patches and send a new version once polished but I 
cannot give an ETA.

> > Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>
> > ---
> > 
> >  include/uapi/linux/bpf.h |  3 +++
> >  kernel/bpf/ringbuf.c     | 51 +++++++++++++++++++++++++++++++---------
> >  2 files changed, 43 insertions(+), 11 deletions(-)
> 
> [...]


Best regards.