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

From: Maciej Fijalkowski <[email-masked]>
Date: Mon, 29 Mar 2021 15:14:01 +0200

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 ?

> 
> > +	struct xsk_ctx *ctx = xsk->ctx;
> > +	__u32 prog_id = 0;
> > +	int link_fd;
> > +	int err;
> > +
> > +	err = bpf_get_link_xdp_id(ctx->ifindex, &prog_id, xsk->config.xdp_flags);
> >  	if (err) {
> > -		close(prog_fd);
> > +		pr_warn("getting XDP prog id failed\n");
> >  		return err;
> >  	}
> >  
> > -	ctx->prog_fd = prog_fd;
> > +	/* if there's a netlink-based XDP prog loaded on interface, bail out
> > +	 * and ask user to do the removal by himself
> > +	 */
> > +	if (prog_id) {
> > +		pr_warn("Netlink-based XDP prog detected, please unload it in order to launch AF_XDP prog\n");
> > +		return -EINVAL;
> > +	}
> > +
> > +	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));
> > +		return link_fd;
> > +	}
> > +
> > +	ctx->link_fd = link_fd;
> >  	return 0;
> >  }
> >  
> > @@ -625,7 +657,6 @@ static int xsk_lookup_bpf_maps(struct xsk_socket *xsk)
> >  		close(fd);
> >  	}
> >  
> > -	err = 0;
> >  	if (ctx->xsks_map_fd == -1)
> >  		err = -ENOENT;
> >  
> > @@ -642,6 +673,97 @@ static int xsk_set_bpf_maps(struct xsk_socket *xsk)
> >  				   &xsk->fd, 0);
> >  }
> >  
> > +static int xsk_link_lookup(int ifindex, __u32 *prog_id, int *link_fd)
> > +{
> > +	struct bpf_link_info link_info;
> > +	__u32 link_len;
> > +	__u32 id = 0;
> > +	int err;
> > +	int fd;
> > +
> > +	while (true) {
> > +		err = bpf_link_get_next_id(id, &id);
> > +		if (err) {
> > +			if (errno == ENOENT) {
> > +				err = 0;
> > +				break;
> > +			}
> > +			pr_warn("can't get next link: %s\n", strerror(errno));
> > +			break;
> > +		}
> > +
> > +		fd = bpf_link_get_fd_by_id(id);
> > +		if (fd < 0) {
> > +			if (errno == ENOENT)
> > +				continue;
> > +			pr_warn("can't get link by id (%u): %s\n", id, strerror(errno));
> > +			err = -errno;
> > +			break;
> > +		}
> > +
> > +		link_len = sizeof(struct bpf_link_info);
> > +		memset(&link_info, 0, link_len);
> > +		err = bpf_obj_get_info_by_fd(fd, &link_info, &link_len);
> > +		if (err) {
> > +			pr_warn("can't get link info: %s\n", strerror(errno));
> > +			close(fd);
> > +			break;
> > +		}
> > +		if (link_info.type == BPF_LINK_TYPE_XDP) {
> > +			if (link_info.xdp.ifindex == ifindex) {
> > +				*link_fd = fd;
> > +				if (prog_id)
> > +					*prog_id = link_info.prog_id;
> > +				break;
> > +			}
> > +		}
> > +		close(fd);
> > +	}
> > +
> > +	return err;
> > +}
> > +
> > +static bool xsk_probe_bpf_link(void)
> > +{
> > +	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts,
> > +			    .flags = XDP_FLAGS_SKB_MODE);
> > +	struct bpf_load_program_attr prog_attr;
> > +	struct bpf_insn insns[2] = {
> > +		BPF_MOV64_IMM(BPF_REG_0, XDP_PASS),
> > +		BPF_EXIT_INSN()
> > +	};
> > +	int prog_fd, link_fd = -1;
> > +	int ifindex_lo = 1;
> > +	bool ret = false;
> > +	int err;
> > +
> > +	err = xsk_link_lookup(ifindex_lo, NULL, &link_fd);
> > +	if (err)
> > +		return ret;
> > +
> > +	if (link_fd >= 0)
> > +		return true;
> > +
> > +	memset(&prog_attr, 0, sizeof(prog_attr));
> > +	prog_attr.prog_type = BPF_PROG_TYPE_XDP;
> > +	prog_attr.insns = insns;
> > +	prog_attr.insns_cnt = ARRAY_SIZE(insns);
> > +	prog_attr.license = "GPL";
> > +
> > +	prog_fd = bpf_load_program_xattr(&prog_attr, NULL, 0);
> > +	if (prog_fd < 0)
> > +		return ret;
> > +
> > +	link_fd = bpf_link_create(prog_fd, ifindex_lo, BPF_XDP, &opts);
> > +	if (link_fd >= 0)
> > +		ret = true;
> > +
> > +	close(prog_fd);
> > +	close(link_fd);
> 
> This potentially calls close() on an invalid link_fd...

Will fix.

> 
> > +
> > +	return ret;
> > +}
> > +
> >  static int xsk_create_xsk_struct(int ifindex, struct xsk_socket *xsk)
> >  {
> >  	char ifname[IFNAMSIZ];
> > @@ -663,64 +785,108 @@ static int xsk_create_xsk_struct(int ifindex, struct xsk_socket *xsk)
> >  	ctx->ifname[IFNAMSIZ - 1] = 0;
> >  
> >  	xsk->ctx = ctx;
> > +	xsk->ctx->has_bpf_link = xsk_probe_bpf_link();
> >  
> >  	return 0;
> >  }
> >  
> > -static int __xsk_setup_xdp_prog(struct xsk_socket *_xdp,
> > -				int *xsks_map_fd)
> > +static int xsk_init_xdp_res(struct xsk_socket *xsk,
> > +			    int *xsks_map_fd)
> >  {
> > -	struct xsk_socket *xsk = _xdp;
> >  	struct xsk_ctx *ctx = xsk->ctx;
> > -	__u32 prog_id = 0;
> >  	int err;
> >  
> > -	err = bpf_get_link_xdp_id(ctx->ifindex, &prog_id,
> > -				  xsk->config.xdp_flags);
> > +	err = xsk_create_bpf_maps(xsk);
> >  	if (err)
> >  		return err;
> >  
> > -	if (!prog_id) {
> > -		err = xsk_create_bpf_maps(xsk);
> > -		if (err)
> > -			return err;
> > +	err = xsk_load_xdp_prog(xsk);
> > +	if (err)
> > +		goto err_load_xdp_prog;
> >  
> > -		err = xsk_load_xdp_prog(xsk);
> > -		if (err) {
> > -			goto err_load_xdp_prog;
> > -		}
> > -	} else {
> > -		ctx->prog_fd = bpf_prog_get_fd_by_id(prog_id);
> > -		if (ctx->prog_fd < 0)
> > -			return -errno;
> > -		err = xsk_lookup_bpf_maps(xsk);
> > -		if (err) {
> > -			close(ctx->prog_fd);
> > -			return err;
> > -		}
> > -	}
> > +	if (ctx->has_bpf_link)
> > +		err = xsk_create_bpf_link(xsk);
> > +	else
> > +		err = bpf_set_link_xdp_fd(xsk->ctx->ifindex, ctx->prog_fd,
> > +					  xsk->config.xdp_flags);
> >  
> > -	if (xsk->rx) {
> > -		err = xsk_set_bpf_maps(xsk);
> > -		if (err) {
> > -			if (!prog_id) {
> > -				goto err_set_bpf_maps;
> > -			} else {
> > -				close(ctx->prog_fd);
> > -				return err;
> > -			}
> > -		}
> > -	}
> > -	if (xsks_map_fd)
> > -		*xsks_map_fd = ctx->xsks_map_fd;
> > +	if (err)
> > +		goto err_atach_xdp_prog;
> 
> s/atach/attach/

Whoops :)

> -Toke