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SubjectRe: [PATCH 2/2] af_unix: If we don't care about credentials coallesce all messages
dingtianhong <dingtianhong@huawei.com> writes:

> On 2013/4/4 10:14, Eric W. Biederman wrote:
>>
>> It was reported that the following LSB test case failed
>> https://lsbbugs.linuxfoundation.org/attachment.cgi?id=2144 because we
>> were not coallescing unix stream messages when the application was
>> expecting us to.
>>
>> The problem was that the first send was before the socket was accepted
>> and thus sock->sk_socket was NULL in maybe_add_creds, and the second
>> send after the socket was accepted had a non-NULL value for sk->socket
>> and thus we could tell the credentials were not needed so we did not
>> bother.
>>
>> The unnecessary credentials on the first message cause
>> unix_stream_recvmsg to start verifying that all messages had the same
>> credentials before coallescing and then the coallescing failed because
>> the second message had no credentials.
>>
>> Ignoring credentials when we don't care in unix_stream_recvmsg fixes a
>> long standing pessimization which would fail to coallesce messages when
>> reading from a unix stream socket if the senders were different even if
>> we did not care about their credentials.
>>
>> I have tested this and verified that the in the LSB test case mentioned
>> above that the messages do coallesce now, while the were failing to
>> coallesce without this change.
>>
>> Reported-by: Karel Srot <ksrot@redhat.com>
>> Reported-by: Ding Tianhong <dingtianhong@huawei.com>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>> net/unix/af_unix.c | 2 +-
>> 1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>> index f153a8d..2db702d 100644
>> --- a/net/unix/af_unix.c
>> +++ b/net/unix/af_unix.c
>> @@ -1993,7 +1993,7 @@ again:
>> if ((UNIXCB(skb).pid != siocb->scm->pid) ||
>> (UNIXCB(skb).cred != siocb->scm->cred))
>> break;
>> - } else {
>> + } else if (test_bit(SOCK_PASSCRED, &sock->flags)) {
>> /* Copy credentials */
>> scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
>> check_creds = 1;
>>
>
> As your opinion, I think the way is better:
>
> if (test_bit(SOCK_PASSCRED, &sock->flags)) {
> if (check_creds) {
> /* Never glue messages from different writers */
> if ((UNIXCB(skb).pid != siocb->scm->pid) ||
> (UNIXCB(skb).cred != siocb->scm->cred))
> break;
> } else {
> /* Copy credentials */
> scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
> check_creds = 1;
> }
> }

It is a smidge clearer in intent, but there is no functional
difference. The lines get really long.

Shrug.

Patches are always welcome.

Beyond getting something correct for the right reasons I don't care.

Eric



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