Subject: Re: [PATCH v4 bpf-next 06/17] libbpf: xsk: use bpf_link

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 29 Mar 2021 23:46:41 +0200

Maciej Fijalkowski <[email-masked]> writes:

> On Mon, Mar 29, 2021 at 04:09:33PM +0200, Toke Høiland-Jørgensen wrote:
>> Maciej Fijalkowski <[email-masked]> writes:
>> 
>> > On Mon, Mar 29, 2021 at 01:05:44PM +0200, Toke Høiland-Jørgensen wrote:
>> >> Maciej Fijalkowski <[email-masked]> writes:
>> >> 
>> >> > Currently, if there are multiple xdpsock instances running on a single
>> >> > interface and in case one of the instances is terminated, the rest of
>> >> > them are left in an inoperable state due to the fact of unloaded XDP
>> >> > prog from interface.
>> >> >
>> >> > Consider the scenario below:
>> >> >
>> >> > // load xdp prog and xskmap and add entry to xskmap at idx 10
>> >> > $ sudo ./xdpsock -i ens801f0 -t -q 10
>> >> >
>> >> > // add entry to xskmap at idx 11
>> >> > $ sudo ./xdpsock -i ens801f0 -t -q 11
>> >> >
>> >> > terminate one of the processes and another one is unable to work due to
>> >> > the fact that the XDP prog was unloaded from interface.
>> >> >
>> >> > To address that, step away from setting bpf prog in favour of bpf_link.
>> >> > This means that refcounting of BPF resources will be done automatically
>> >> > by bpf_link itself.
>> >> >
>> >> > Provide backward compatibility by checking if underlying system is
>> >> > bpf_link capable. Do this by looking up/creating bpf_link on loopback
>> >> > device. If it failed in any way, stick with netlink-based XDP prog.
>> >> > therwise, use bpf_link-based logic.
>> >> >
>> >> > When setting up BPF resources during xsk socket creation, check whether
>> >> > bpf_link for a given ifindex already exists via set of calls to
>> >> > bpf_link_get_next_id -> bpf_link_get_fd_by_id -> bpf_obj_get_info_by_fd
>> >> > and comparing the ifindexes from bpf_link and xsk socket.
>> >> >
>> >> > For case where resources exist but they are not AF_XDP related, bail out
>> >> > and ask user to remove existing prog and then retry.
>> >> >
>> >> > Lastly, do a bit of refactoring within __xsk_setup_xdp_prog and pull out
>> >> > existing code branches based on prog_id value onto separate functions
>> >> > that are responsible for resource initialization if prog_id was 0 and
>> >> > for lookup existing resources for non-zero prog_id as that implies that
>> >> > XDP program is present on the underlying net device. This in turn makes
>> >> > it easier to follow, especially the teardown part of both branches.
>> >> >
>> >> > Signed-off-by: Maciej Fijalkowski <[email-masked]>
>> >> 
>> >> The logic is much improved in this version! A few smallish issues below:
>> >
>> > Glad to hear that!
>> >
>> >> 
>> >> > ---
>> >> >  tools/lib/bpf/xsk.c | 259 ++++++++++++++++++++++++++++++++++++--------
>> >> >  1 file changed, 214 insertions(+), 45 deletions(-)
>> >> >
>> >> > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
>> >> > index 526fc35c0b23..c75067f0035f 100644
>> >> > --- a/tools/lib/bpf/xsk.c
>> >> > +++ b/tools/lib/bpf/xsk.c
>> >> > @@ -28,6 +28,7 @@
>> >> >  #include <sys/mman.h>
>> >> >  #include <sys/socket.h>
>> >> >  #include <sys/types.h>
>> >> > +#include <linux/if_link.h>
>> >> >  
>> >> >  #include "bpf.h"
>> >> >  #include "libbpf.h"
>> >> > @@ -70,8 +71,10 @@ struct xsk_ctx {
>> >> >  	int ifindex;
>> >> >  	struct list_head list;
>> >> >  	int prog_fd;
>> >> > +	int link_fd;
>> >> >  	int xsks_map_fd;
>> >> >  	char ifname[IFNAMSIZ];
>> >> > +	bool has_bpf_link;
>> >> >  };
>> >> >  
>> >> >  struct xsk_socket {
>> >> > @@ -409,7 +412,7 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk)
>> >> >  	static const int log_buf_size = 16 * 1024;
>> >> >  	struct xsk_ctx *ctx = xsk->ctx;
>> >> >  	char log_buf[log_buf_size];
>> >> > -	int err, prog_fd;
>> >> > +	int prog_fd;
>> >> >  
>> >> >  	/* This is the fallback C-program:
>> >> >  	 * SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx)
>> >> > @@ -499,14 +502,43 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk)
>> >> >  		return prog_fd;
>> >> >  	}
>> >> >  
>> >> > -	err = bpf_set_link_xdp_fd(xsk->ctx->ifindex, prog_fd,
>> >> > -				  xsk->config.xdp_flags);
>> >> > +	ctx->prog_fd = prog_fd;
>> >> > +	return 0;
>> >> > +}
>> >> > +
>> >> > +static int xsk_create_bpf_link(struct xsk_socket *xsk)
>> >> > +{
>> >> > +	/* bpf_link only accepts XDP_FLAGS_MODES, but xsk->config.xdp_flags
>> >> > +	 * might have set XDP_FLAGS_UPDATE_IF_NOEXIST
>> >> > +	 */
>> >> > +	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts,
>> >> > +			    .flags = (xsk->config.xdp_flags & XDP_FLAGS_MODES));
>> >> 
>> >> This will silently suppress any new flags as well; that's not a good
>> >> idea. Rather mask out the particular flag (UPDATE_IF_NOEXIST) and pass
>> >> everything else through so the kernel can reject invalid ones.
>> >
>> > I'd say it's fine as it matches the check:
>> >
>> > 	/* link supports only XDP mode flags */
>> > 	if (link && (flags & ~XDP_FLAGS_MODES)) {
>> > 		NL_SET_ERR_MSG(extack, "Invalid XDP flags for BPF link attachment");
>> > 		return -EINVAL;
>> > 	}
>> >
>> > from dev_xdp_attach() in net/core/dev.c ?
>> 
>> Yeah, it does today. But what happens when the kernel learns to accept a
>> new flag?
>> 
>> Also, you're masking the error on an invalid flag. If, in the future,
>> the kernel learns to handle a new flag, that check in the kernel will
>> change to accept that new flag. But if userspace tries to pass that to
>> and old kernel, they'll get back an EINVAL. This can be used to detect
>> whether the kernel doesn't support the flag, and can if not, userspace
>> can fall back and do something different.
>> 
>> Whereas with your code, you're just silently zeroing out the invalid
>> flag, so the caller will have no way to detect whether the flag works
>> or not...
>
> I'd rather worry about such feature detection once a new flag is in place
> and this code would actually care about :) but that's me.
>
> I feel like stick has two ends in this case - if we introduce a new flag
> that would be out of the bpf_link's interest (the kernel part), then we
> will have to come back here and explicitly mask it out, just like you
> propose to do so with UPDATE_IF_NOEXIST right now.

Well I'd rather err on the side of throwing errors for new flags than on
silently suppressing them :)

(The partial sharing of flags between netlink and bpf_link is a bit odd
in any case, but that's another discussion)

> What I'm saying is that we need to mask out the FLAGS_REPLACE as well.
> Current code took care of that. So, to move this forward, I can do:
>
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index c75067f0035f..95da0e19f4a5 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -508,11 +508,7 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk)
>  
>  static int xsk_create_bpf_link(struct xsk_socket *xsk)
>  {
> -	/* bpf_link only accepts XDP_FLAGS_MODES, but xsk->config.xdp_flags
> -	 * might have set XDP_FLAGS_UPDATE_IF_NOEXIST
> -	 */
> -	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts,
> -			    .flags = (xsk->config.xdp_flags & XDP_FLAGS_MODES));
> +	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts);
>  	struct xsk_ctx *ctx = xsk->ctx;
>  	__u32 prog_id = 0;
>  	int link_fd;
> @@ -532,6 +528,8 @@ static int xsk_create_bpf_link(struct xsk_socket *xsk)
>  		return -EINVAL;
>  	}
>  
> +	opts.flags = xsk->config.xdp_flags & ~(XDP_FLAGS_UPDATE_IF_NOEXIST | XDP_FLAGS_REPLACE);
> +
>  	link_fd = bpf_link_create(ctx->prog_fd, ctx->ifindex, BPF_XDP, &opts);
>  	if (link_fd < 0) {
>  		pr_warn("bpf_link_create failed: %s\n", strerror(errno));

Yup, this works for me! :)

-Toke