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Message-ID: <20210329193234.GA9506@ranger.igk.intel.com>
Date: Mon, 29 Mar 2021 21:32:34 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, daniel@...earbox.net,
ast@...nel.org, andrii@...nel.org, bjorn.topel@...el.com,
magnus.karlsson@...el.com, ciara.loftus@...el.com,
john.fastabend@...il.com
Subject: Re: [PATCH v4 bpf-next 06/17] libbpf: xsk: use bpf_link
On Mon, Mar 29, 2021 at 04:09:33PM +0200, Toke Høiland-Jørgensen wrote:
> Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:
>
> > On Mon, Mar 29, 2021 at 01:05:44PM +0200, Toke Høiland-Jørgensen wrote:
> >> Maciej Fijalkowski <maciej.fijalkowski@...el.com> 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 <maciej.fijalkowski@...el.com>
> >>
> >> 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.
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));
>
> -Toke
>
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