On Wed, Mar 27, 2024 at 10:24:26PM -0700, Andrii Nakryiko wrote:
> BPF link for some program types is passed as a "context" which can be
> used by those BPF programs to look up additional information. E.g., for
> multi-kprobes and multi-uprobes, link is used to fetch BPF cookie values.
>
> Because of this runtime dependency, when bpf_link refcnt drops to zero
> there could still be active BPF programs running accessing link data.
>
> This patch adds generic support to defer bpf_link dealloc callback to
> after RCU GP, if requested. This is done by exposing two different
> deallocation callbacks, one synchronous and one deferred. If deferred
> one is provided, bpf_link_free() will schedule dealloc_deferred()
> callback to happen after RCU GP.
>
> BPF is using two flavors of RCU: "classic" non-sleepable one and RCU
> tasks trace one. The latter is used when sleepable BPF programs are
> used. bpf_link_free() accommodates that by checking underlying BPF
> program's sleepable flag, and goes either through normal RCU GP only for
> non-sleepable, or through RCU tasks trace GP *and* then normal RCU GP
> (taking into account rcu_trace_implies_rcu_gp() optimization), if BPF
> program is sleepable.
>
> We use this for multi-kprobe and multi-uprobe links, which dereference
> link during program run. We also preventively switch raw_tp link to use
> deferred dealloc callback, as upcoming changes in bpf-next tree expose
> raw_tp link data (specifically, cookie value) to BPF program at runtime
> as well.
nice catch.. I thought there'd be more link types accesing link
data in runtime.. but looks like it's just [ku]probe_multi
Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
jirka
>
> Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
> Fixes: 89ae89f53d20 ("bpf: Add multi uprobe link")
> Reported-by: syzbot+981935d9485a560bfbcb@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: syzbot+2cb5a6c573e98db598cc@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: syzbot+62d8b26793e8a2bd0516@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> ---
> include/linux/bpf.h | 16 +++++++++++++++-
> kernel/bpf/syscall.c | 35 ++++++++++++++++++++++++++++++++---
> kernel/trace/bpf_trace.c | 4 ++--
> 3 files changed, 49 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 4f20f62f9d63..890e152d553e 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1574,12 +1574,26 @@ struct bpf_link {
> enum bpf_link_type type;
> const struct bpf_link_ops *ops;
> struct bpf_prog *prog;
> - struct work_struct work;
> + /* rcu is used before freeing, work can be used to schedule that
> + * RCU-based freeing before that, so they never overlap
> + */
> + union {
> + struct rcu_head rcu;
> + struct work_struct work;
> + };
> };
>
> struct bpf_link_ops {
> void (*release)(struct bpf_link *link);
> + /* deallocate link resources callback, called without RCU grace period
> + * waiting
> + */
> void (*dealloc)(struct bpf_link *link);
> + /* deallocate link resources callback, called after RCU grace period;
> + * if underlying BPF program is sleepable we go through tasks trace
> + * RCU GP and then "classic" RCU GP
> + */
> + void (*dealloc_deferred)(struct bpf_link *link);
> int (*detach)(struct bpf_link *link);
> int (*update_prog)(struct bpf_link *link, struct bpf_prog *new_prog,
> struct bpf_prog *old_prog);
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index ae2ff73bde7e..c287925471f6 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3024,17 +3024,46 @@ void bpf_link_inc(struct bpf_link *link)
> atomic64_inc(&link->refcnt);
> }
>
> +static void bpf_link_defer_dealloc_rcu_gp(struct rcu_head *rcu)
> +{
> + struct bpf_link *link = container_of(rcu, struct bpf_link, rcu);
> +
> + /* free bpf_link and its containing memory */
> + link->ops->dealloc_deferred(link);
> +}
> +
> +static void bpf_link_defer_dealloc_mult_rcu_gp(struct rcu_head *rcu)
> +{
> + if (rcu_trace_implies_rcu_gp())
> + bpf_link_defer_dealloc_rcu_gp(rcu);
> + else
> + call_rcu(rcu, bpf_link_defer_dealloc_rcu_gp);
> +}
> +
> /* bpf_link_free is guaranteed to be called from process context */
> static void bpf_link_free(struct bpf_link *link)
> {
> + bool sleepable = false;
> +
> bpf_link_free_id(link->id);
> if (link->prog) {
> + sleepable = link->prog->sleepable;
> /* detach BPF program, clean up used resources */
> link->ops->release(link);
> bpf_prog_put(link->prog);
> }
> - /* free bpf_link and its containing memory */
> - link->ops->dealloc(link);
> + if (link->ops->dealloc_deferred) {
> + /* schedule BPF link deallocation; if underlying BPF program
> + * is sleepable, we need to first wait for RCU tasks trace
> + * sync, then go through "classic" RCU grace period
> + */
> + if (sleepable)
> + call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
> + else
> + call_rcu(&link->rcu, bpf_link_defer_dealloc_rcu_gp);
> + }
> + if (link->ops->dealloc)
> + link->ops->dealloc(link);
> }
>
> static void bpf_link_put_deferred(struct work_struct *work)
> @@ -3544,7 +3573,7 @@ static int bpf_raw_tp_link_fill_link_info(const struct bpf_link *link,
>
> static const struct bpf_link_ops bpf_raw_tp_link_lops = {
> .release = bpf_raw_tp_link_release,
> - .dealloc = bpf_raw_tp_link_dealloc,
> + .dealloc_deferred = bpf_raw_tp_link_dealloc,
> .show_fdinfo = bpf_raw_tp_link_show_fdinfo,
> .fill_link_info = bpf_raw_tp_link_fill_link_info,
> };
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 0b73fe5f7206..9dc605f08a23 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2728,7 +2728,7 @@ static int bpf_kprobe_multi_link_fill_link_info(const struct bpf_link *link,
>
> static const struct bpf_link_ops bpf_kprobe_multi_link_lops = {
> .release = bpf_kprobe_multi_link_release,
> - .dealloc = bpf_kprobe_multi_link_dealloc,
> + .dealloc_deferred = bpf_kprobe_multi_link_dealloc,
> .fill_link_info = bpf_kprobe_multi_link_fill_link_info,
> };
>
> @@ -3242,7 +3242,7 @@ static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
>
> static const struct bpf_link_ops bpf_uprobe_multi_link_lops = {
> .release = bpf_uprobe_multi_link_release,
> - .dealloc = bpf_uprobe_multi_link_dealloc,
> + .dealloc_deferred = bpf_uprobe_multi_link_dealloc,
> .fill_link_info = bpf_uprobe_multi_link_fill_link_info,
> };
>
> --
> 2.43.0
>
