FAILED: patch "[PATCH] bpf: support deferring bpf_link dealloc to after RCU grace" failed to apply to 6.6-stable tree

gregkh
2024-04-05T09:50:51Z

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 1a80dbcb2dbaf6e4c216e62e30fa7d3daa8001ce
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2024040548-lid-mahogany-fd86@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 1a80dbcb2dbaf6e4c216e62e30fa7d3daa8001ce Mon Sep 17 00:00:00 2001
From: Andrii Nakryiko <andrii@kernel.org>
Date: Wed, 27 Mar 2024 22:24:26 -0700
Subject: [PATCH] bpf: support deferring bpf_link dealloc to after RCU grace
period

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: 0dcac2725406 ("bpf: Add multi kprobe link")
Fixes: 89ae89f53d20 ("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>

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,
};