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Message-ID: <8787f5c0-fed0-b8fa-997b-4d17d9966f13@iogearbox.net>
Date: Thu, 21 Dec 2023 14:21:44 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: 'Alexei Starovoitov ' <ast@...nel.org>,
'Andrii Nakryiko ' <andrii@...nel.org>, netdev@...r.kernel.org,
kernel-team@...a.com, Aditi Ghag <aditi.ghag@...valent.com>,
bpf@...r.kernel.org
Subject: Re: [PATCH bpf 1/2] bpf: Avoid iter->offset making backward progress
in bpf_iter_udp
On 12/21/23 5:45 AM, Martin KaFai Lau wrote:
> On 12/20/23 11:10 AM, Martin KaFai Lau wrote:
>> Good catch. It will unnecessary skip in the following batch/bucket if there is changes in the current batch/bucket.
>>
>> From looking at the loop again, I think it is better not to change the iter->offset during the for loop. Only update iter->offset after the for loop has concluded.
>>
>> The non-zero iter->offset is only useful for the first bucket, so does a test on the first bucket (state->bucket == bucket) before skipping sockets. Something like this:
>>
>> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
>> index 89e5a806b82e..a993f364d6ae 100644
>> --- a/net/ipv4/udp.c
>> +++ b/net/ipv4/udp.c
>> @@ -3139,6 +3139,7 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
>> struct net *net = seq_file_net(seq);
>> struct udp_table *udptable;
>> unsigned int batch_sks = 0;
>> + int bucket, bucket_offset;
>> bool resized = false;
>> struct sock *sk;
>>
>> @@ -3162,14 +3163,14 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
>> iter->end_sk = 0;
>> iter->st_bucket_done = false;
>> batch_sks = 0;
>> + bucket = state->bucket;
>> + bucket_offset = 0;
>>
>> for (; state->bucket <= udptable->mask; state->bucket++) {
>> struct udp_hslot *hslot2 = &udptable->hash2[state->bucket];
>>
>> - if (hlist_empty(&hslot2->head)) {
>> - iter->offset = 0;
>> + if (hlist_empty(&hslot2->head))
>> continue;
>> - }
>>
>> spin_lock_bh(&hslot2->lock);
>> udp_portaddr_for_each_entry(sk, &hslot2->head) {
>> @@ -3177,8 +3178,9 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
>> /* Resume from the last iterated socket at the
>> * offset in the bucket before iterator was stopped.
>> */
>> - if (iter->offset) {
>> - --iter->offset;
>> + if (state->bucket == bucket &&
>> + bucket_offset < iter->offset) {
>> + ++bucket_offset;
>> continue;
>> }
>> if (iter->end_sk < iter->max_sk) {
>> @@ -3192,10 +3194,10 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
>>
>> if (iter->end_sk)
>> break;
>> + }
>>
>> - /* Reset the current bucket's offset before moving to the next bucket. */
>> + if (state->bucket != bucket)
>> iter->offset = 0;
>> - }
>>
>> /* All done: no batch made. */
>> if (!iter->end_sk)
>
> I think I found another bug in the current bpf_iter_udp_batch(). The "state->bucket--;" at the end of the batch() function is wrong also. It does not need to go back to the previous bucket. After realloc with a larger batch array, it should retry on the "state->bucket" as is. I tried to force the bind() to use bucket 0 and bind a larger so_reuseport set (24 sockets). WARN_ON(state->bucket < 0) triggered.
>
> Going back to this bug (backward progress on --iter->offset), I think it is a bit cleaner to always reset iter->offset to 0 and advance iter->offset to the resume_offset only when needed. Something like this:
Hm, my assumption was.. why not do something like the below, and fully start over?
I'm mostly puzzled about the side-effects here, in particular, if for the rerun the sockets
in the bucket could already have changed.. maybe I'm still missing something - what do
we need to deal with exactly worst case when we need to go and retry everything, and what
guarantees do we have?
(only compile tested)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 89e5a806b82e..ca62a4bb7bec 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -3138,7 +3138,8 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
struct udp_iter_state *state = &iter->state;
struct net *net = seq_file_net(seq);
struct udp_table *udptable;
- unsigned int batch_sks = 0;
+ int orig_bucket, orig_offset;
+ unsigned int i, batch_sks = 0;
bool resized = false;
struct sock *sk;
@@ -3149,7 +3150,8 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
}
udptable = udp_get_table_seq(seq, net);
-
+ orig_bucket = state->bucket;
+ orig_offset = iter->offset;
again:
/* New batch for the next bucket.
* Iterate over the hash table to find a bucket with sockets matching
@@ -3211,9 +3213,15 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
if (!resized && !bpf_iter_udp_realloc_batch(iter, batch_sks * 3 / 2)) {
resized = true;
/* After allocating a larger batch, retry one more time to grab
- * the whole bucket.
+ * the whole bucket. Drop the current refs since for the next
+ * attempt the composition could have changed, thus start over.
*/
- state->bucket--;
+ for (i = 0; i < iter->end_sk; i++) {
+ sock_put(iter->batch[i]);
+ iter->batch[i] = NULL;
+ }
+ state->bucket = orig_bucket;
+ iter->offset = orig_offset;
goto again;
}
done:
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