tcp_bpf: Fix the sk_mem_uncharge logic in tcp_bpf_sendmsg

The current sk memory accounting logic in __SK_REDIRECT is pre-uncharging
tosend bytes, which is either msg->sg.size or a smaller value apply_bytes.
Potential problems with this strategy are as follows:
- If the actual sent bytes are smaller than tosend, we need to charge some
bytes back, as in line 487, which is okay but seems not clean.
- When tosend is set to apply_bytes, as in line 417, and (ret < 0), we may
miss uncharging (msg->sg.size - apply_bytes) bytes.

415 tosend = msg->sg.size;
416 if (psock->apply_bytes && psock->apply_bytes < tosend)
417   tosend = psock->apply_bytes;
...
443 sk_msg_return(sk, msg, tosend);
444 release_sock(sk);
446 origsize = msg->sg.size;
447 ret = tcp_bpf_sendmsg_redir(sk_redir, redir_ingress,
448                             msg, tosend, flags);
449 sent = origsize - msg->sg.size;
...
454 lock_sock(sk);
455 if (unlikely(ret < 0)) {
456   int free = sk_msg_free_nocharge(sk, msg);
458   if (!cork)
459     *copied -= free;
460 }
...
487 if (eval == __SK_REDIRECT)
488   sk_mem_charge(sk, tosend - sent);

When running the selftest test_txmsg_redir_wait_sndmem with txmsg_apply,
the following warning will be reported,
------------[ cut here ]------------
WARNING: CPU: 6 PID: 57 at net/ipv4/af_inet.c:156 inet_sock_destruct+0x190/0x1a0
Modules linked in:
CPU: 6 UID: 0 PID: 57 Comm: kworker/6:0 Not tainted 6.12.0-rc1.bm.1-amd64+ #43
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
Workqueue: events sk_psock_destroy
RIP: 0010:inet_sock_destruct+0x190/0x1a0
RSP: 0018:ffffad0a8021fe08 EFLAGS: 00010206
RAX: 0000000000000011 RBX: ffff9aab4475b900 RCX: ffff9aab481a0800
RDX: 0000000000000303 RSI: 0000000000000011 RDI: ffff9aab4475b900
RBP: ffff9aab4475b990 R08: 0000000000000000 R09: ffff9aab40050ec0
R10: 0000000000000000 R11: ffff9aae6fdb1d01 R12: ffff9aab49c60400
R13: ffff9aab49c60598 R14: ffff9aab49c60598 R15: dead000000000100
FS:  0000000000000000(0000) GS:ffff9aae6fd80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffec7e47bd8 CR3: 00000001a1a1c004 CR4: 0000000000770ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<TASK>
? __warn+0x89/0x130
? inet_sock_destruct+0x190/0x1a0
? report_bug+0xfc/0x1e0
? handle_bug+0x5c/0xa0
? exc_invalid_op+0x17/0x70
? asm_exc_invalid_op+0x1a/0x20
? inet_sock_destruct+0x190/0x1a0
__sk_destruct+0x25/0x220
sk_psock_destroy+0x2b2/0x310
process_scheduled_works+0xa3/0x3e0
worker_thread+0x117/0x240
? __pfx_worker_thread+0x10/0x10
kthread+0xcf/0x100
? __pfx_kthread+0x10/0x10
ret_from_fork+0x31/0x40
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
</TASK>
---[ end trace 0000000000000000 ]---

In __SK_REDIRECT, a more concise way is delaying the uncharging after sent
bytes are finalized, and uncharge this value. When (ret < 0), we shall
invoke sk_msg_free.

Same thing happens in case __SK_DROP, when tosend is set to apply_bytes,
we may miss uncharging (msg->sg.size - apply_bytes) bytes. The same
warning will be reported in selftest.

468 case __SK_DROP:
469 default:
470 sk_msg_free_partial(sk, msg, tosend);
471 sk_msg_apply_bytes(psock, tosend);
472 *copied -= (tosend + delta);
473 return -EACCES;

So instead of sk_msg_free_partial we can do sk_msg_free here.

Fixes: 604326b ("bpf, sockmap: convert to generic sk_msg interface")
Fixes: 8ec95b9 ("bpf, sockmap: Fix the sk->sk_forward_alloc warning of sk_stream_kill_queues")
Signed-off-by: Zijian Zhang <[email protected]>

net/ipv4/tcp_bpf.c

@@ -441,7 +441,6 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
 			cork = true;
 			psock->cork = NULL;
 		}
-		sk_msg_return(sk, msg, tosend);
 		release_sock(sk);
 
 		origsize = msg->sg.size;
@@ -453,8 +452,9 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
 			sock_put(sk_redir);
 
 		lock_sock(sk);
+		sk_mem_uncharge(sk, sent);
 		if (unlikely(ret < 0)) {
-			int free = sk_msg_free_nocharge(sk, msg);
+			int free = sk_msg_free(sk, msg);
 
 			if (!cork)
 				*copied -= free;
@@ -468,7 +468,7 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
 		break;
 	case __SK_DROP:
 	default:
-		sk_msg_free_partial(sk, msg, tosend);
+		sk_msg_free(sk, msg);
 		sk_msg_apply_bytes(psock, tosend);
 		*copied -= (tosend + delta);
 		return -EACCES;
@@ -484,11 +484,8 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
 		}
 		if (msg &&
 		    msg->sg.data[msg->sg.start].page_link &&
-		    msg->sg.data[msg->sg.start].length) {
-			if (eval == __SK_REDIRECT)
-				sk_mem_charge(sk, tosend - sent);
+		    msg->sg.data[msg->sg.start].length)
 			goto more_data;
-		}
 	}
 	return ret;
 }