[PATCH net-next] ipv4: fib: fix route re-dump in inet_dump_fib() on multi-batch dump

[Pengfei Zhang <zhangfeionline@gmail.com>]

inet_dump_fib() saves its progress in cb->args[1] as a positional
index within the current hash chain.  Between batches, a concurrent
fib_new_table() can insert a new table at the chain head, shifting
all existing entries.  On resume the saved index lands on a different
table, causing already-dumped tables to be re-dumped and the
originally suspended table to restart from the beginning.

Fix by storing tb->tb_id in cb->args[1] instead of a positional
index, mirroring the fix applied to inet6_dump_fib().

Fixes: 1b43af5480c3 ("[IPV6]: Increase number of possible routing tables to 2^32")
Signed-off-by: Pengfei Zhang <zhangfeionline@gmail.com>
---
Consider a hash slot containing two tables [A(pos=0), B(pos=1)] where
B is large enough to require multiple batches.  On the first batch, B
suspends mid-walk and the loop saves:

  cb->args[1] = e;   /* e=1, position of B in the chain */

The lock is then released.  At this point a concurrent fib_new_table()
inserts table C at the chain head via hlist_add_head_rcu(), making the
chain [C(pos=0), A(pos=1), B(pos=2)].

On the next batch, inet_dump_fib() resumes with s_e=1 and iterates:

  s_e = cb->args[1];   /* s_e = 1 */
  hlist_for_each_entry_rcu(tb, head, tb_hlist) {
      if (e < s_e)     /* skip C at pos=0 */
          goto next;
      /* e=1: tb now points to A, not B */
      if (dumped)
          memset(...);  /* resets B's suspended progress */
      fib_table_dump(tb, ...);   /* re-dumps A from scratch */
      dumped = 1;
      /* e=2: tb now points to B */
      fib_table_dump(tb, ...);   /* re-dumps B from beginning */
  }

Routes from A are dumped twice, and the portion of B that was already
dumped in the first batch is dumped again.

 net/ipv4/fib_frontend.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 42212970d..65fa245af 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1019,10 +1019,11 @@ static int inet_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 		.dump_routes = true,
 		.dump_exceptions = true,
 	};
-	unsigned int e = 0, s_e, h, s_h;
 	struct hlist_head *head;
 	int dumped = 0, err = 0;
+	unsigned int h, s_h;
 	struct fib_table *tb;
+	u32 s_id;
 
 	rcu_read_lock();
 	if (cb->strict_check) {
@@ -1054,29 +1055,28 @@ static int inet_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 	}
 
 	s_h = cb->args[0];
-	s_e = cb->args[1];
+	s_id = cb->args[1];
 
 	err = 0;
-	for (h = s_h; h < FIB_TABLE_HASHSZ; h++, s_e = 0) {
-		e = 0;
+	for (h = s_h; h < FIB_TABLE_HASHSZ; h++, s_id = 0) {
 		head = &net->ipv4.fib_table_hash[h];
 		hlist_for_each_entry_rcu(tb, head, tb_hlist) {
-			if (e < s_e)
-				goto next;
+			if (s_id && tb->tb_id != s_id)
+				continue;
+
+			s_id = 0;
 			if (dumped)
 				memset(&cb->args[2], 0, sizeof(cb->args) -
 						 2 * sizeof(cb->args[0]));
+			cb->args[1] = tb->tb_id;
 			err = fib_table_dump(tb, skb, cb, &filter);
 			if (err < 0)
 				goto out;
 			dumped = 1;
-next:
-			e++;
 		}
 	}
 out:
 
-	cb->args[1] = e;
 	cb->args[0] = h;
 
 unlock:
-- 
2.34.1