Subject: Fw: [Bug 214339] New: sendmsg return value may be positive while send errors

From: Stephen Hemminger <[email-masked]>
Date: Tue, 7 Sep 2021 12:10:52 -0700

Begin forwarded message:

Date: Tue, 07 Sep 2021 09:23:54 +0000
From: [email-masked]
To: [email-masked]
Subject: [Bug 214339] New: sendmsg return value may be positive while send errors

https://bugzilla.kernel.org/show_bug.cgi?id=214339

            Bug ID: 214339
           Summary: sendmsg return value may be positive while send errors
           Product: Networking
           Version: 2.5
    Kernel Version: 4.9.99
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: IPV4
          Assignee: [email-masked]
          Reporter: [email-masked]
        Regression: No

in file udp.c, a function named udp_sendmsg has a code like this:

        /* Lockless fast path for the non-corking case. */
        if (!corkreq) {
                skb = ip_make_skb(sk, fl4, getfrag, msg, ulen,
                                  sizeof(struct udphdr), &ipc, &rt,
                                  msg->msg_flags);
                err = PTR_ERR(skb);
                if (!IS_ERR_OR_NULL(skb))
                        err = udp_send_skb(skb, fl4);
                goto out;
        }

but function ip_make_skb may return a null, then err will be set to 0;and out
like this:

out:
        ip_rt_put(rt);
        if (free)
                kfree(ipc.opt);
        if (!err)
                return len;  // return a positive value

actually, because lock of kernel memory or socket_buffer,the ip_make_skb failed
means the send operation failed. but a positive value is returnd here.
finnally, users regard the operation was success, but actually it failed in
kernel.

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