Re: [PATCH 13/19] bpf: Add support to link multi func tracing program

Jiri Olsa
2021-06-07T18:26:04Z

On Sun, Jun 06, 2021 at 10:36:57PM -0700, Yonghong Song wrote:
>
>
> On 6/5/21 4:10 AM, Jiri Olsa wrote:
> > Adding support to attach multiple functions to tracing program
> > by using the link_create/link_update interface.
> >
> > Adding multi_btf_ids/multi_btf_ids_cnt pair to link_create struct
> > API, that define array of functions btf ids that will be attached
> > to prog_fd.
> >
> > The prog_fd needs to be multi prog tracing program (BPF_F_MULTI_FUNC).
> >
> > The new link_create interface creates new BPF_LINK_TYPE_TRACING_MULTI
> > link type, which creates separate bpf_trampoline and registers it
> > as direct function for all specified btf ids.
> >
> > The new bpf_trampoline is out of scope (bpf_trampoline_lookup) of
> > standard trampolines, so all registered functions need to be free
> > of direct functions, otherwise the link fails.
>
> I am not sure how severe such a limitation could be in practice.
> It is possible in production some non-multi fentry/fexit program
> may run continuously. Does kprobe program impact this as well?

I did not find a way how to combine current trampolines with the
new ones for multiple programs.. what you described is a limitation
of the current approach

I'm not sure about kprobes and trampolines, but the limitation
should be same as we do have for current trampolines.. I'll check

>
> >
> > The new bpf_trampoline will store and pass to bpf program the highest
> > number of arguments from all given functions.
> >
> > New programs (fentry or fexit) can be added to the existing trampoline
> > through the link_update interface via new_prog_fd descriptor.
>
> Looks we do not support replacing old programs. Do we support
> removing old programs?

we don't.. it's not what bpftrace would do, it just adds programs
to trace and close all when it's done.. I think interface for removal
could be added if you think it's needed

>
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > include/linux/bpf.h | 3 +
> > include/uapi/linux/bpf.h | 5 +
> > kernel/bpf/syscall.c | 185 ++++++++++++++++++++++++++++++++-
> > kernel/bpf/trampoline.c | 53 +++++++---
> > tools/include/uapi/linux/bpf.h | 5 +
> > 5 files changed, 237 insertions(+), 14 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 23221e0e8d3c..99a81c6c22e6 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -661,6 +661,7 @@ struct bpf_trampoline {
> > struct bpf_tramp_image *cur_image;
> > u64 selector;
> > struct module *mod;
> > + bool multi;
> > };
> > struct bpf_attach_target_info {
> > @@ -746,6 +747,8 @@ void bpf_ksym_add(struct bpf_ksym *ksym);
> > void bpf_ksym_del(struct bpf_ksym *ksym);
> > int bpf_jit_charge_modmem(u32 pages);
> > void bpf_jit_uncharge_modmem(u32 pages);
> > +struct bpf_trampoline *bpf_trampoline_multi_alloc(void);
> > +void bpf_trampoline_multi_free(struct bpf_trampoline *tr);
> > #else
> > static inline int bpf_trampoline_link_prog(struct bpf_prog *prog,
> > struct bpf_trampoline *tr)
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index ad9340fb14d4..5fd6ff64e8dc 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -1007,6 +1007,7 @@ enum bpf_link_type {
> > BPF_LINK_TYPE_ITER = 4,
> > BPF_LINK_TYPE_NETNS = 5,
> > BPF_LINK_TYPE_XDP = 6,
> > + BPF_LINK_TYPE_TRACING_MULTI = 7,
> > MAX_BPF_LINK_TYPE,
> > };
> > @@ -1454,6 +1455,10 @@ union bpf_attr {
> > __aligned_u64 iter_info; /* extra bpf_iter_link_info */
> > __u32 iter_info_len; /* iter_info length */
> > };
> > + struct {
> > + __aligned_u64 multi_btf_ids; /* addresses to attach */
> > + __u32 multi_btf_ids_cnt; /* addresses count */
> > + };
> > };
> > } link_create;
> [...]
> > +static int bpf_tracing_multi_link_fill_link_info(const struct bpf_link *link,
> > + struct bpf_link_info *info)
> > +{
> > + struct bpf_tracing_multi_link *tr_link =
> > + container_of(link, struct bpf_tracing_multi_link, link);
> > +
> > + info->tracing.attach_type = tr_link->attach_type;
> > + return 0;
> > +}
> > +
> > +static int check_multi_prog_type(struct bpf_prog *prog)
> > +{
> > + if (!prog->aux->multi_func &&
> > + prog->type != BPF_PROG_TYPE_TRACING)
>
> I think prog->type != BPF_PROG_TYPE_TRACING is not needed, it should have
> been checked during program load time?
>
> > + return -EINVAL;
> > + if (prog->expected_attach_type != BPF_TRACE_FENTRY &&
> > + prog->expected_attach_type != BPF_TRACE_FEXIT)
> > + return -EINVAL;
> > + return 0;
> > +}
> > +
> > +static int bpf_tracing_multi_link_update(struct bpf_link *link,
> > + struct bpf_prog *new_prog,
> > + struct bpf_prog *old_prog __maybe_unused)
> > +{
> > + struct bpf_tracing_multi_link *tr_link =
> > + container_of(link, struct bpf_tracing_multi_link, link);
> > + int err;
> > +
> > + if (check_multi_prog_type(new_prog))
> > + return -EINVAL;
> > +
> > + err = bpf_trampoline_link_prog(new_prog, tr_link->tr);
> > + if (err)
> > + return err;
> > +
> > + err = modify_ftrace_direct_multi(&tr_link->ops,
> > + (unsigned long) tr_link->tr->cur_image->image);
> > + return WARN_ON(err);
>
> Why WARN_ON here? Some comments will be good.
>
> > +}
> > +
> > +static const struct bpf_link_ops bpf_tracing_multi_link_lops = {
> > + .release = bpf_tracing_multi_link_release,
> > + .dealloc = bpf_tracing_multi_link_dealloc,
> > + .show_fdinfo = bpf_tracing_multi_link_show_fdinfo,
> > + .fill_link_info = bpf_tracing_multi_link_fill_link_info,
> > + .update_prog = bpf_tracing_multi_link_update,
> > +};
> > +
> [...]
> > +
> > struct bpf_raw_tp_link {
> > struct bpf_link link;
> > struct bpf_raw_event_map *btp;
> > @@ -3043,6 +3222,8 @@ attach_type_to_prog_type(enum bpf_attach_type attach_type)
> > case BPF_CGROUP_SETSOCKOPT:
> > return BPF_PROG_TYPE_CGROUP_SOCKOPT;
> > case BPF_TRACE_ITER:
> > + case BPF_TRACE_FENTRY:
> > + case BPF_TRACE_FEXIT:
> > return BPF_PROG_TYPE_TRACING;
> > case BPF_SK_LOOKUP:
> > return BPF_PROG_TYPE_SK_LOOKUP;
> > @@ -4099,6 +4280,8 @@ static int tracing_bpf_link_attach(const union bpf_attr *attr, bpfptr_t uattr,
> > if (prog->expected_attach_type == BPF_TRACE_ITER)
> > return bpf_iter_link_attach(attr, uattr, prog);
> > + else if (prog->aux->multi_func)
> > + return bpf_tracing_multi_attach(prog, attr);
> > else if (prog->type == BPF_PROG_TYPE_EXT)
> > return bpf_tracing_prog_attach(prog,
> > attr->link_create.target_fd,
> > @@ -4106,7 +4289,7 @@ static int tracing_bpf_link_attach(const union bpf_attr *attr, bpfptr_t uattr,
> > return -EINVAL;
> > }
> > -#define BPF_LINK_CREATE_LAST_FIELD link_create.iter_info_len
> > +#define BPF_LINK_CREATE_LAST_FIELD link_create.multi_btf_ids_cnt
>
> It is okay that we don't change this. link_create.iter_info_len
> has the same effect since it is a union.
>
> > static int link_create(union bpf_attr *attr, bpfptr_t uattr)
> > {
> > enum bpf_prog_type ptype;
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > index 2755fdcf9fbf..660b8197c27f 100644
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -58,7 +58,7 @@ void bpf_image_ksym_del(struct bpf_ksym *ksym)
> > PAGE_SIZE, true, ksym->name);
> > }
> > -static struct bpf_trampoline *bpf_trampoline_alloc(void)
> > +static struct bpf_trampoline *bpf_trampoline_alloc(bool multi)
> > {
> > struct bpf_trampoline *tr;
> > int i;
> > @@ -72,6 +72,7 @@ static struct bpf_trampoline *bpf_trampoline_alloc(void)
> > mutex_init(&tr->mutex);
> > for (i = 0; i < BPF_TRAMP_MAX; i++)
> > INIT_HLIST_HEAD(&tr->progs_hlist[i]);
> > + tr->multi = multi;
> > return tr;
> > }
> > @@ -88,7 +89,7 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
> > goto out;
> > }
> > }
> > - tr = bpf_trampoline_alloc();
> > + tr = bpf_trampoline_alloc(false);
> > if (tr) {
> > tr->key = key;
> > hlist_add_head(&tr->hlist, head);
> > @@ -343,14 +344,16 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
> > struct bpf_tramp_image *im;
> > struct bpf_tramp_progs *tprogs;
> > u32 flags = BPF_TRAMP_F_RESTORE_REGS;
> > - int err, total;
> > + bool update = !tr->multi;
> > + int err = 0, total;
> > tprogs = bpf_trampoline_get_progs(tr, &total);
> > if (IS_ERR(tprogs))
> > return PTR_ERR(tprogs);
> > if (total == 0) {
> > - err = unregister_fentry(tr, tr->cur_image->image);
> > + if (update)
> > + err = unregister_fentry(tr, tr->cur_image->image);
> > bpf_tramp_image_put(tr->cur_image);
> > tr->cur_image = NULL;
> > tr->selector = 0;
> > @@ -363,9 +366,15 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
> > goto out;
> > }
> > + if (tr->multi)
> > + flags |= BPF_TRAMP_F_IP_ARG;
> > +
> > if (tprogs[BPF_TRAMP_FEXIT].nr_progs ||
> > - tprogs[BPF_TRAMP_MODIFY_RETURN].nr_progs)
> > + tprogs[BPF_TRAMP_MODIFY_RETURN].nr_progs) {
> > flags = BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME;
> > + if (tr->multi)
> > + flags |= BPF_TRAMP_F_ORIG_STACK | BPF_TRAMP_F_IP_ARG;
>
> BPF_TRAMP_F_IP_ARG is not needed. It has been added before.

it's erased in 2 lines above.. which reminds me that I forgot to check
if that's a bug or intended ;-)

jirka