Subject: Re: [RFC PATCH bpf-next 4/4] libbpf: don't remove eBPF resources when other xsks are present

From: Maciej Fijalkowski <[email-masked]>
Date: Tue, 4 Jun 2019 17:07:22 +0200

On Tue, 4 Jun 2019 10:08:03 +0200
Björn Töpel <[email-masked]> wrote:

> On 2019-06-03 15:19, Maciej Fijalkowski wrote:
> > In case where multiple xsk sockets are attached to a single interface
> > and one of them gets detached, the eBPF maps and program are removed.
> > This should not happen as the rest of xsksocks are still using these
> > resources.
> > 
> > In order to fix that, let's have an additional eBPF map with a single
> > entry that will be used as a xsks count. During the xsk_socket__delete,
> > remove the resources only when this count is equal to 0.  This map is
> > not being accessed from eBPF program, so the verifier is not associating
> > it with the prog, which in turn makes bpf_obj_get_info_by_fd not
> > reporting this map in nr_map_ids field of struct bpf_prog_info. The
> > described behaviour brings the need to have this map pinned, so in
> > case when socket is being created and the libbpf detects the presence of
> > bpf resources, it will be able to access that map.
> >
> 
> This commit is only needed after #3 is applied, right? So, this is a way 
> of refcounting XDP socks?

Yes, but as you pointed out it needs synchronization.

> 
> 
> > Signed-off-by: Maciej Fijalkowski <[email-masked]>
> > ---
> >   tools/lib/bpf/xsk.c | 59 +++++++++++++++++++++++++++++++++++++++++++++--------
> >   1 file changed, 51 insertions(+), 8 deletions(-)
> > 
> > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> > index e28bedb0b078..88d2c931ad14 100644
> > --- a/tools/lib/bpf/xsk.c
> > +++ b/tools/lib/bpf/xsk.c
> > @@ -44,6 +44,8 @@
> >    #define PF_XDP AF_XDP
> >   #endif
> >   
> > +#define XSKS_CNT_MAP_PATH "/sys/fs/bpf/xsks_cnt_map"
> > +
> >   struct xsk_umem {
> >   	struct xsk_ring_prod *fill;
> >   	struct xsk_ring_cons *comp;
> > @@ -65,6 +67,7 @@ struct xsk_socket {
> >   	int prog_fd;
> >   	int qidconf_map_fd;
> >   	int xsks_map_fd;
> > +	int xsks_cnt_map_fd;
> >   	__u32 queue_id;
> >   	char ifname[IFNAMSIZ];
> >   };
> > @@ -372,7 +375,7 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
> >   static int xsk_create_bpf_maps(struct xsk_socket *xsk)
> >   {
> >   	int max_queues;
> > -	int fd;
> > +	int fd, ret;
> >   
> >   	max_queues = xsk_get_max_queues(xsk);
> >   	if (max_queues < 0)
> > @@ -392,6 +395,24 @@ static int xsk_create_bpf_maps(struct xsk_socket *xsk)
> >   	}
> >   	xsk->xsks_map_fd = fd;
> >   
> > +	fd = bpf_create_map_name(BPF_MAP_TYPE_ARRAY, "xsks_cnt_map",
> > +				 sizeof(int), sizeof(int), 1, 0);
> > +	if (fd < 0) {
> > +		close(xsk->qidconf_map_fd);
> > +		close(xsk->xsks_map_fd);
> > +		return fd;
> > +	}
> > +
> > +	ret = bpf_obj_pin(fd, XSKS_CNT_MAP_PATH);
> > +	if (ret < 0) {
> > +		pr_warning("pinning map failed; is bpffs mounted?\n");
> > +		close(xsk->qidconf_map_fd);
> > +		close(xsk->xsks_map_fd);
> > +		close(fd);
> > +		return ret;
> > +	}
> > +	xsk->xsks_cnt_map_fd = fd;
> > +
> >   	return 0;
> >   }
> >   
> > @@ -456,8 +477,10 @@ static int xsk_lookup_bpf_maps(struct xsk_socket *xsk)
> >   		close(fd);
> >   	}
> >   
> > +	xsk->xsks_cnt_map_fd = bpf_obj_get(XSKS_CNT_MAP_PATH);
> >   	err = 0;
> > -	if (xsk->qidconf_map_fd < 0 || xsk->xsks_map_fd < 0) {
> > +	if (xsk->qidconf_map_fd < 0 || xsk->xsks_map_fd < 0 ||
> > +	    xsk->xsks_cnt_map_fd < 0) {
> >   		err = -ENOENT;
> >   		xsk_delete_bpf_maps(xsk);
> >   	}
> > @@ -467,17 +490,25 @@ static int xsk_lookup_bpf_maps(struct xsk_socket *xsk)
> >   	return err;
> >   }
> >   
> > -static void xsk_clear_bpf_maps(struct xsk_socket *xsk)
> > +static void xsk_clear_bpf_maps(struct xsk_socket *xsk, long *xsks_cnt_ptr)
> >   {
> > +	long xsks_cnt, key = 0;
> >   	int qid = false;
> >   
> >   	bpf_map_update_elem(xsk->qidconf_map_fd, &xsk->queue_id, &qid, 0);
> >   	bpf_map_delete_elem(xsk->xsks_map_fd, &xsk->queue_id);
> > +	bpf_map_lookup_elem(xsk->xsks_cnt_map_fd, &key, &xsks_cnt);
> > +	if (xsks_cnt)
> > +		xsks_cnt--;
> > +	bpf_map_update_elem(xsk->xsks_cnt_map_fd, &key, &xsks_cnt, 0);
> > +	if (xsks_cnt_ptr)
> > +		*xsks_cnt_ptr = xsks_cnt;
> 
> This refcount scheme will not work; There's no synchronization between 
> the updates (cross process)!

Ugh, shame. Let me fix this in v2 :(

> 
> >   }
> >   
> >   static int xsk_set_bpf_maps(struct xsk_socket *xsk)
> >   {
> >   	int qid = true, fd = xsk->fd, err;
> > +	long xsks_cnt, key = 0;
> >   
> >   	err = bpf_map_update_elem(xsk->qidconf_map_fd, &xsk->queue_id, &qid, 0);
> >   	if (err)
> > @@ -487,9 +518,18 @@ static int xsk_set_bpf_maps(struct xsk_socket *xsk)
> >   	if (err)
> >   		goto out;
> >   
> > +	err = bpf_map_lookup_elem(xsk->xsks_cnt_map_fd, &key, &xsks_cnt);
> > +	if (err)
> > +		goto out;
> > +
> > +	xsks_cnt++;
> > +	err = bpf_map_update_elem(xsk->xsks_cnt_map_fd, &key, &xsks_cnt, 0);
> > +	if (err)
> > +		goto out;
> > +
> 
> Dito.
> 
> >   	return 0;
> >   out:
> > -	xsk_clear_bpf_maps(xsk);
> > +	xsk_clear_bpf_maps(xsk, NULL);
> >   	return err;
> >   }
> >   
> > @@ -752,13 +792,18 @@ void xsk_socket__delete(struct xsk_socket *xsk)
> >   	size_t desc_sz = sizeof(struct xdp_desc);
> >   	struct xdp_mmap_offsets off;
> >   	socklen_t optlen;
> > +	long xsks_cnt;
> >   	int err;
> >   
> >   	if (!xsk)
> >   		return;
> >   
> > -	xsk_clear_bpf_maps(xsk);
> > -	xsk_delete_bpf_maps(xsk);
> > +	xsk_clear_bpf_maps(xsk, &xsks_cnt);
> > +	unlink(XSKS_CNT_MAP_PATH);
> > +	if (!xsks_cnt) {
> > +		xsk_delete_bpf_maps(xsk);
> > +		xsk_remove_xdp_prog(xsk);
> > +	}
> >   
> >   	optlen = sizeof(off);
> >   	err = getsockopt(xsk->fd, SOL_XDP, XDP_MMAP_OFFSETS, &off, &optlen);
> > @@ -774,8 +819,6 @@ void xsk_socket__delete(struct xsk_socket *xsk)
> >   
> >   	}
> >   
> > -	xsk_remove_xdp_prog(xsk);
> > -
> >   	xsk->umem->refcount--;
> >   	/* Do not close an fd that also has an associated umem connected
> >   	 * to it.
> >