Re: [PATCH bpf-next v6 2/3] libbpf: add low level TC-BPF API

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Thu, May 06, 2021 at 03:12:01AM IST, Daniel Borkmann wrote:
> On 5/4/21 2:50 AM, Kumar Kartikeya Dwivedi wrote:
> > This adds functions that wrap the netlink API used for adding,
> > manipulating, and removing traffic control filters.
> >
> > An API summary:
>
> Looks better, few minor comments below:
>
> > A bpf_tc_hook represents a location where a TC-BPF filter can be
> > attached. This means that creating a hook leads to creation of the
> > backing qdisc, while destruction either removes all filters attached to
> > a hook, or destroys qdisc if requested explicitly (as discussed below).
> >
> > The TC-BPF API functions operate on this bpf_tc_hook to attach, replace,
> > query, and detach tc filters.
> >
> > All functions return 0 on success, and a negative error code on failure.
> >
> > bpf_tc_hook_create - Create a hook
> > Parameters:
> > 	@hook - Cannot be NULL, ifindex > 0, attach_point must be set to
> > 		proper enum constant. Note that parent must be unset when
> > 		attach_point is one of BPF_TC_INGRESS or BPF_TC_EGRESS. Note
> > 		that as an exception BPF_TC_INGRESS|BPF_TC_EGRESS is also a
> > 		valid value for attach_point.
> >
> > 		Returns -EOPNOTSUPP when hook has attach_point as BPF_TC_CUSTOM.
> >
> > 		hook's flags member can be BPF_TC_F_REPLACE, which
> > 		creates qdisc in non-exclusive mode (i.e. an existing
> > 		qdisc will be replaced instead of this function failing
> > 		with -EEXIST).
>
> Why supporting BPF_TC_F_REPLACE here? It's not changing any qdisc parameters
> given clsact doesn't have any, no? Iow, what effect are you expecting on this
> with BPF_TC_F_REPLACE & why supporting it? I'd probably just require flags to
> be 0 here, and if hook exists return sth like -EEXIST.
>

Ok, will change.

> > bpf_tc_hook_destroy - Destroy the hook
> > Parameters:
> >          @hook - Cannot be NULL. The behaviour depends on value of
> > 		attach_point.
> >
> > 		If BPF_TC_INGRESS, all filters attached to the ingress
> > 		hook will be detached.
> > 		If BPF_TC_EGRESS, all filters attached to the egress hook
> > 		will be detached.
> > 		If BPF_TC_INGRESS|BPF_TC_EGRESS, the clsact qdisc will be
> > 		deleted, also detaching all filters.
> >
> > 		As before, parent must be unset for these attach_points,
> > 		and set for BPF_TC_CUSTOM. flags must also be unset.
> >
> > 		It is advised that if the qdisc is operated on by many programs,
> > 		then the program at least check that there are no other existing
> > 		filters before deleting the clsact qdisc. An example is shown
> > 		below:
> >
> > 		DECLARE_LIBBPF_OPTS(bpf_tc_hook, .ifindex = if_nametoindex("lo"),
> > 				    .attach_point = BPF_TC_INGRESS);
> > 		/* set opts as NULL, as we're not really interested in
> > 		 * getting any info for a particular filter, but just
> > 	 	 * detecting its presence.
> > 		 */
> > 		r = bpf_tc_query(&hook, NULL);
> > 		if (r == -ENOENT) {
> > 			/* no filters */
> > 			hook.attach_point = BPF_TC_INGRESS|BPF_TC_EGREESS;
> > 			return bpf_tc_hook_destroy(&hook);
> > 		} else {
> > 			/* failed or r == 0, the latter means filters do exist */
> > 			return r;
> > 		}
> >
> > 		Note that there is a small race between checking for no
> > 		filters and deleting the qdisc. This is currently unavoidable.
> >
> > 		Returns -EOPNOTSUPP when hook has attach_point as BPF_TC_CUSTOM.
> >
> > bpf_tc_attach - Attach a filter to a hook
> > Parameters:
> > 	@hook - Cannot be NULL. Represents the hook the filter will be
> > 		attached to. Requirements for ifindex and attach_point are
> > 		same as described in bpf_tc_hook_create, but BPF_TC_CUSTOM
> > 		is also supported.  In that case, parent must be set to the
> > 		handle where the filter will be attached (using TC_H_MAKE).
> > 		flags member must be unset.
> >
> > 		E.g. To set parent to 1:16 like in tc command line,
> > 		     the equivalent would be TC_H_MAKE(1 << 16, 16)
>
> Small nit: I wonder whether from libbpf side we should just support a more
> user friendly TC_H_MAKE, so you'd have: BPF_TC_CUSTOM + BPF_TC_PARENT(1, 16).
>

Something like this was there in v1. I'll add this macro again (I guess the most surprising part of
TC_H_MAKE is that it won't shift the major number).

> > 	@opts - Cannot be NULL.
> >
> > 		The following opts are optional:
> > 			handle - The handle of the filter
> > 			priority - The priority of the filter
> > 				   Must be >= 0 and <= UINT16_MAX
>
> It should probably be mentioned that if they are not specified, then they
> are auto-allocated from kernel.

Right, I'll add a small note.

>
> > 		The following opts must be set:
> > 			prog_fd - The fd of the loaded SCHED_CLS prog
> > 		The following opts must be unset:
> > 			prog_id - The ID of the BPF prog
> > 		The following opts are optional:
> > 			flags - Currently only BPF_TC_F_REPLACE is
> > 				allowed. It allows replacing an existing
> > 				filter instead of failing with -EEXIST.
> >
> > 		The following opts will be filled by bpf_tc_attach on a
> > 		successful attach operation if they are unset:
> > 			handle - The handle of the attached filter
> > 			priority - The priority of the attached filter
> > 			prog_id - The ID of the attached SCHED_CLS prog
> >
> > 		This way, the user can know what the auto allocated
> > 		values for optional opts like handle and priority are
> > 		for the newly attached filter, if they were unset.
> >
> > 		Note that some other attributes are set to some default
> > 		values listed below (this holds for all bpf_tc_* APIs):
> > 			protocol - ETH_P_ALL
> > 			mode - direct action
> > 			chain index - 0
> > 			class ID - 0 (this can be set by writing to the
> > 			skb->tc_classid field from the BPF program)
> >
> > bpf_tc_detach
> > Parameters:
> > 	@hook: Cannot be NULL. Represents the hook the filter will be
> > 		detached from. Requirements are same as described above
> > 		in bpf_tc_attach.
> >
> > 	@opts:	Cannot be NULL.
> >
> > 		The following opts must be set:
> > 			handle
> > 			priority
> > 		The following opts must be unset:
> > 			prog_fd
> > 			prog_id
> > 			flags
> >
> > bpf_tc_query
> > Parameters:
> > 	@hook: Cannot be NULL. Represents the hook where the filter
> > 	       lookup will be performed. Requires are same as described
> > 	       above in bpf_tc_attach.
> >
> > 	@opts: Can be NULL.
>
> Shouldn't it be: Cannot be NULL?
>

This allows you to check the existence of a filter. If set to NULL we skip writing anything to opts,
but we still return -ENOENT or 0 depending on whether atleast one filter exists (based on the
default attributes that we choose). This is used in multiple places in the test, to determine
whether no filters exists.

> > 	       The following opts are optional:
> > 			handle
> > 			priority
> > 			prog_fd
> > 			prog_id
>
> What is the use case to set prog_fd here?
>

It allows you to search with the prog_id of the program represented by fd. It's just a convenience
thing, we end up doing a call to get the prog_id for you, and since the parameter is already there,
it seemed ok to support this.

> > 	       The following opts must be unset:
> > 			flags
> >
> > 	       However, only one of prog_fd and prog_id must be
> > 	       set. Setting both leads to an error. Setting none is
> > 	       allowed.
> >
> > 	       The following fields will be filled by bpf_tc_query on a
> > 	       successful lookup if they are unset:
> > 			handle
> > 			priority
> > 			prog_id
> >
> > 	       Based on the specified optional parameters, the matching
> > 	       data for the first matching filter is filled in and 0 is
> > 	       returned. When setting prog_fd, the prog_id will be
> > 	       matched against prog_id of the loaded SCHED_CLS prog
> > 	       represented by prog_fd.
> >
> > 	       To uniquely identify a filter, e.g. to detect its presence,
> > 	       it is recommended to set both handle and priority fields.
>
> What if prog_id is not unique, but part of multiple instances? Do we need
> to support this case?

We return the first filter that matches on the prog_id. I think it is worthwhile to support this, as
long as the kernel's sequence of returning filters is stable (which it is), we keep returning the
same filter's handle/priority, so you can essentially pop filters attached to a hook one by one by
passing in unset opts and getting its details (or setting one of the parameters and making the
lookup domain smaller).

In simple words, setting one of the parameters that will be filled leads to only returning an entry
that matches them. This is similar to what tc filter show's dump allows you to do.

>
> Why not just bpf_tc_query() with non-NULL hook and non-NULL opts where
> handle and priority is required to be set, and rest must be 0?
>

There is also a usecase for us where we need to query the existing filter on a hook without knowing
its handle/priority. Shaun also mentioned something similar, where they then go on to check the tag
they get from the returned prog_id to determine what to do next.

> > Some usage examples (using bpf skeleton infrastructure):
> >
> > BPF program (test_tc_bpf.c):
> >
> > 	#include <linux/bpf.h>
> > 	#include <bpf/bpf_helpers.h>
> >
> > 	SEC("classifier")
> > 	int cls(struct __sk_buff *skb)
> > 	{
> > 		return 0;
> > 	}
> >
> > Userspace loader:
> >
> > 	DECLARE_LIBBPF_OPTS(bpf_tc_opts, opts, 0);
> > 	struct test_tc_bpf *skel = NULL;
> > 	int fd, r;
> >
> > 	skel = test_tc_bpf__open_and_load();
> > 	if (!skel)
> > 		return -ENOMEM;
> >
> > 	fd = bpf_program__fd(skel->progs.cls);
> >
> > 	DECLARE_LIBBPF_OPTS(bpf_tc_hook, hook, .ifindex =
> > 			    if_nametoindex("lo"), .attach_point =
> > 			    BPF_TC_INGRESS);
> > 	/* Create clsact qdisc */
> > 	r = bpf_tc_hook_create(&hook);
> > 	if (r < 0)
> > 		goto end;
> >
> > 	DECLARE_LIBBPF_OPTS(bpf_tc_opts, opts, .prog_fd = fd);
>
> Given we had DECLARE_LIBBPF_OPTS earlier, can't we just set:
> opts.prog_fd = fd here?

Right, will fix.

>
> > 	r = bpf_tc_attach(&hook, &opts);
> > 	if (r < 0)
> > 		goto end;
> > 	/* Print the auto allocated handle and priority */
> > 	printf("Handle=%u", opts.handle);
> > 	printf("Priority=%u", opts.priority);
> >
> > 	opts.prog_fd = opts.prog_id = 0;
> > 	bpf_tc_detach(&hook, &opts);
>
> Here we detach ...
>
> > end:
> > 	test_tc_bpf__destroy(skel);
> >
> > This is equivalent to doing the following using tc command line:
> >    # tc qdisc add dev lo clsact
> >    # tc filter add dev lo ingress bpf obj foo.o sec classifier da
>
> ... so this is not equivalent to your tc cmdline description.
>

I'll add a tc filter del.

> > Another example replacing a filter (extending prior example):
> >
> > 	/* We can also choose both (or one), let's try replacing an
> > 	 * existing filter.
> > 	 */
> > 	DECLARE_LIBBPF_OPTS(bpf_tc_opts, replace_opts, .handle =
> > 			    opts.handle, .priority = opts.priority,
> > 			    .prog_fd = fd);
> > 	r = bpf_tc_attach(&hook, &replace_opts);
> > 	if (r == -EEXIST) {
> > 		/* Expected, now use BPF_TC_F_REPLACE to replace it */
> > 		replace_opts.flags = BPF_TC_F_REPLACE;
> > 		return bpf_tc_attach(&hook, &replace_opts);
> > 	} else if (r < 0) {
> > 		return r;
> > 	}
> > 	/* There must be no existing filter with these
> > 	 * attributes, so cleanup and return an error.
> > 	 */
> > 	replace_opts.flags = replace_opts.prog_fd = replace_opts.prog_id = 0;
> > 	bpf_tc_detach(&hook, &replace_opts);
> > 	return -1;
> >
> > To obtain info of a particular filter:
> >
> > 	/* Find info for filter with handle 1 and priority 50 */
> > 	DECLARE_LIBBPF_OPTS(bpf_tc_opts, info_opts, .handle = 1,
> > 			    .priority = 50);
> > 	r = bpf_tc_query(&hook, &info_opts);
> > 	if (r == -ENOENT)
> > 		printf("Filter not found");
> > 	else if (r < 0)
> > 		return r;
> > 	printf("Prog ID: %u", info_opts.prog_id);
> > 	return 0;
> >
> > We can also match using prog_id to find the same filter:
> >
> > 	DECLARE_LIBBPF_OPTS(bpf_tc_opts, info_opts2, .prog_id =
> > 			    info_opts.prog_id);
> > 	r = bpf_tc_query(&hook, &info_opts2);
> > 	if (r == -ENOENT)
> > 		printf("Filter not found");
> > 	else if (r < 0)
> > 		return r;
> > 	/* If we know there's only one filter for this loaded prog,
> > 	 * it is safe to assert that the handle and priority are
> > 	 * as expected.
> > 	 */
> > 	assert(info_opts2.handle == 1);
> > 	assert(info_opts2.priority == 50);
>
> What if a given prog_id is attached to multiple instances?
>

The first match is returned.

> > 	return 0;
> >
> > Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> > ---
> >   tools/lib/bpf/libbpf.h   |  42 ++++
> >   tools/lib/bpf/libbpf.map |   5 +
> >   tools/lib/bpf/netlink.c  | 473 ++++++++++++++++++++++++++++++++++++++-
> >   3 files changed, 519 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index bec4e6a6e31d..09d1a4fb10f9 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -775,6 +775,48 @@ LIBBPF_API int bpf_linker__add_file(struct bpf_linker *linker, const char *filen
> >   LIBBPF_API int bpf_linker__finalize(struct bpf_linker *linker);
> >   LIBBPF_API void bpf_linker__free(struct bpf_linker *linker);
> > +enum bpf_tc_attach_point {
> > +	BPF_TC_INGRESS = 1 << 0,
> > +	BPF_TC_EGRESS  = 1 << 1,
> > +	BPF_TC_CUSTOM  = 1 << 2,
> > +};
> > +
> > +enum bpf_tc_flags {
> > +	BPF_TC_F_REPLACE = 1 << 0,
> > +};
> > +
> > +struct bpf_tc_hook {
> > +	size_t sz;
> > +	int ifindex;
> > +	int flags;
>
> nit: __u32 flags; (or rather dropping as discussed)
>

I'll drop it for now.

> > +	enum bpf_tc_attach_point attach_point;
> > +	__u32 parent;
> > +	size_t :0;
> > +};
> > +
> > +#define bpf_tc_hook__last_field parent
> > +
> > +struct bpf_tc_opts {
> > +	size_t sz;
> > +	int prog_fd;
> > +	int flags;
>
> nit: __u32 flags;
>

Ok.

> > +	__u32 prog_id;
> > +	__u32 handle;
> > +	__u32 priority;
> > +	size_t :0;
> > +};
> > +
> > +#define bpf_tc_opts__last_field priority
> > +
> > +LIBBPF_API int bpf_tc_hook_create(struct bpf_tc_hook *hook);
> > +LIBBPF_API int bpf_tc_hook_destroy(struct bpf_tc_hook *hook);
> > +LIBBPF_API int bpf_tc_attach(const struct bpf_tc_hook *hook,
> > +			     struct bpf_tc_opts *opts);
> > +LIBBPF_API int bpf_tc_detach(const struct bpf_tc_hook *hook,
> > +			     const struct bpf_tc_opts *opts);
> > +LIBBPF_API int bpf_tc_query(const struct bpf_tc_hook *hook,
> > +			    struct bpf_tc_opts *opts);
> > +
> >   #ifdef __cplusplus
> >   } /* extern "C" */
> >   #endif
> > diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> > index b9b29baf1df8..6c96729050dc 100644
> > --- a/tools/lib/bpf/libbpf.map
> > +++ b/tools/lib/bpf/libbpf.map
> > @@ -361,4 +361,9 @@ LIBBPF_0.4.0 {
> >   		bpf_linker__new;
> >   		bpf_map__inner_map;
> >   		bpf_object__set_kversion;
> > +		bpf_tc_attach;
> > +		bpf_tc_detach;
> > +		bpf_tc_hook_create;
> > +		bpf_tc_hook_destroy;
> > +		bpf_tc_query;
> >   } LIBBPF_0.3.0;
> > diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
> > index 8a01d9eed5f9..95c87f87a178 100644
> > --- a/tools/lib/bpf/netlink.c
> > +++ b/tools/lib/bpf/netlink.c
> > @@ -4,7 +4,10 @@
> >   #include <stdlib.h>
> >   #include <memory.h>
> >   #include <unistd.h>
> > +#include <arpa/inet.h>
> >   #include <linux/bpf.h>
> > +#include <linux/if_ether.h>
> > +#include <linux/pkt_cls.h>
> >   #include <linux/rtnetlink.h>
> >   #include <sys/socket.h>
> >   #include <errno.h>
> > @@ -73,6 +76,12 @@ static int libbpf_netlink_open(__u32 *nl_pid)
> >   	return ret;
> >   }
> > +enum {
> > +	BPF_NL_CONT,
> > +	BPF_NL_NEXT,
> > +	BPF_NL_DONE,
>
> nit: I don't think we need BPF_ prefix here given it's not specific to BPF.
>

Ok, will rename.

> > +};
> > +
> >   static int bpf_netlink_recv(int sock, __u32 nl_pid, int seq,
> >   			    __dump_nlmsg_t _fn, libbpf_dump_nlmsg_t fn,
> >   			    void *cookie)
> > @@ -84,6 +93,7 @@ static int bpf_netlink_recv(int sock, __u32 nl_pid, int seq,
> >   	int len, ret;
> >   	while (multipart) {
> > +start:
> >   		multipart = false;
> >   		len = recv(sock, buf, sizeof(buf), 0);
> >   		if (len < 0) {
> [...]

--
Kartikeya