On 12/14/23 6:24 AM, Florian Westphal wrote:
D. Wythe <alibuda@xxxxxxxxxxxxxxxxx> wrote:
From: "D. Wythe" <alibuda@xxxxxxxxxxxxxxxxx>
To support the prog update, we need to ensure that the prog seen
within the hook is always valid. Considering that hooks are always
protected by rcu_read_lock(), which provide us the ability to use a
new RCU-protected context to access the prog.
Signed-off-by: D. Wythe <alibuda@xxxxxxxxxxxxxxxxx>
---
net/netfilter/nf_bpf_link.c | 124 +++++++++++++++++++++++++++++++++++++++-----
1 file changed, 111 insertions(+), 13 deletions(-)
diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
index e502ec0..918c470 100644
--- a/net/netfilter/nf_bpf_link.c
+++ b/net/netfilter/nf_bpf_link.c
@@ -8,17 +8,11 @@
#include <net/netfilter/nf_bpf_link.h>
#include <uapi/linux/netfilter_ipv4.h>
-static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb,
- const struct nf_hook_state *s)
+struct bpf_nf_hook_ctx
{
- const struct bpf_prog *prog = bpf_prog;
- struct bpf_nf_ctx ctx = {
- .state = s,
- .skb = skb,
- };
-
- return bpf_prog_run(prog, &ctx);
-}
+ struct bpf_prog *prog;
+ struct rcu_head rcu;
+};
I don't understand the need for this structure. AFAICS bpf_prog_put()
will always release the program via call_rcu()?
If it doesn't, we are probably already in trouble as-is without this
patch, I don't think anything that prevents us from ending up calling already
released bpf prog, or releasing it while another cpu is still running it
if bpf_prog_put releases the actual underlying prog instantly.
A BPF expert could confirm bpf-prog-put-is-call-rcu.
Hi Florian,
I must admit that I did not realize that bpf_prog is released
under RCU ...
struct bpf_nf_link {
struct bpf_link link;
@@ -26,8 +20,59 @@ struct bpf_nf_link {
struct net *net;
u32 dead;
const struct nf_defrag_hook *defrag_hook;
+ /* protect link update in parallel */
+ struct mutex update_lock;
+ struct bpf_nf_hook_ctx __rcu *hook_ctx;
What kind of replacements-per-second rate are you aiming for?
I think
static DEFINE_MUTEX(bpf_nf_mutex);
is enough.
I'm okay with that.
Then bpf_nf_link gains
struct bpf_prog __rcu *prog
and possibly a trailing struct rcu_head, see below.
Yes, that's what we need.
+static void bpf_nf_hook_ctx_free_rcu(struct bpf_nf_hook_ctx *hook_ctx)
+{
+ call_rcu(&hook_ctx->rcu, __bpf_nf_hook_ctx_free_rcu);
+}
Don't understand the need for call_rcu either, see below.
+static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb,
+ const struct nf_hook_state *s)
+{
+ const struct bpf_nf_link *link = bpf_link;
+ struct bpf_nf_hook_ctx *hook_ctx;
+ struct bpf_nf_ctx ctx = {
+ .state = s,
+ .skb = skb,
+ };
+
+ hook_ctx = rcu_dereference(link->hook_ctx);
This could then just rcu_deref link->prog.
+ return bpf_prog_run(hook_ctx->prog, &ctx);
+}
+
#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)
static const struct nf_defrag_hook *
get_proto_defrag_hook(struct bpf_nf_link *link,
@@ -120,6 +165,10 @@ static void bpf_nf_link_release(struct bpf_link *link)
if (!cmpxchg(&nf_link->dead, 0, 1)) {
nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops);
bpf_nf_disable_defrag(nf_link);
+ /* Wait for outstanding hook to complete before the
+ * link gets released.
+ */
+ synchronize_rcu();
}
Could you convert bpf_nf_link_dealloc to release via kfree_rcu instead?
Got it.
@@ -162,7 +212,42 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link,
static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
struct bpf_prog *old_prog)
{
- return -EOPNOTSUPP;
+ struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link);
+ struct bpf_nf_hook_ctx *hook_ctx;
+ int err = 0;
+
+ mutex_lock(&nf_link->update_lock);
+
I think you need to check link->dead here too.
Got that.
+ /* bpf_nf_link_release() ensures that after its execution, there will be
+ * no ongoing or upcoming execution of nf_hook_run_bpf() within any context.
+ * Therefore, within nf_hook_run_bpf(), the link remains valid at all times."
+ */
+ link->hook_ops.priv = link;
ATM we only need to make sure the bpf prog itself stays alive until after
all concurrent rcu critical sections have completed.
After this change, struct bpf_link gets passed instead, so we need to
keep that alive too.
Which works with synchronize_rcu, sure, but that seems a bit overkill here.
Got it! Thank you very much for your suggestion.
I will address those issues you mentioned in the next version.
Best wishes,
D. Wythe
