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bpf: support deferring bpf_link dealloc to after RCU grace period

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commit 1a80dbcb2d upstream.

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.

Fixes: 0dcac27254 ("bpf: Add multi kprobe link")
Fixes: 89ae89f53d ("bpf: Add multi uprobe link")
Reported-by: syzbot+981935d9485a560bfbcb@syzkaller.appspotmail.com
Reported-by: syzbot+2cb5a6c573e98db598cc@syzkaller.appspotmail.com
Reported-by: syzbot+62d8b26793e8a2bd0516@syzkaller.appspotmail.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240328052426.3042617-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Andrii Nakryiko 2024-03-27 22:24:26 -07:00 committed by Greg Kroah-Hartman
parent d8b4ea738c
commit 5d8d447777
3 changed files with 49 additions and 6 deletions
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16
include/linux/bpf.h
View File

@ -1568,12 +1568,26 @@ 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);

35
kernel/bpf/syscall.c
View File

@ -2895,17 +2895,46 @@ 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->aux->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)
@ -3415,7 +3444,7 @@ 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,
};

4
kernel/trace/bpf_trace.c
View File

@ -2713,7 +2713,7 @@ 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,
};
@ -3227,7 +3227,7 @@ 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,
};