Re: [V4.4-rc6 Regression] af_unix: Revert 'lock_interruptible' in stream receive code

[Rainer Weikusat <rweikusat@mobileactivedefense.com>]

Joseph Salisbury <joseph.salisbury@canonical.com> writes:
> Hi Rainer,
>
> A kernel bug report was opened against Ubuntu [0].  After a kernel
> bisect, it was found that reverting the following commit resolved this bug:
>
> commit 3822b5c2fc62e3de8a0f33806ff279fb7df92432
> Author: Rainer Weikusat <rweikusat@mobileactivedefense.com>
> Date:   Wed Dec 16 20:09:25 2015 +0000
>
>     af_unix: Revert 'lock_interruptible' in stream receive code
>
>       
> The regression was introduced as of v4.4-rc6.
>
> I was hoping to get your feedback, since you are the patch author.  Do
> you think gathering any additional data will help diagnose this issue,
> or would it be best to submit a revert request?

Funny little problem :-). The code using the interruptible lock cleared
err as side effect hence the

out:
	return copied ? : err;

at the end of unix_stream_read_generic didn't return the -ENOTSUP put
into err at the start of the function if copied was zero after the loop
because the size of the passed data buffer was zero.

The following patch should fix this:

---------
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 49d5093..c3e1a08 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2300,6 +2300,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state)
        else
                skip = 0;
 
+       err = 0;
        do {
                int chunk;
                bool drop_skb;
----------

I was just about to go the the supermarket to buy an apple when I
received the mail. I didn't even compile the change above yet, however,
I'll do so once I'm back and then submit something formal.

Here's a test program which can be compiled with a C compiler:
------------
#define _GNU_SOURCE
    
#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <assert.h>
#include <errno.h>
#include <string.h>

int main(void)
{
    enum { server, client, size };
    int socket_fd[size];
    int const opt = 1;

    assert(socketpair(AF_LOCAL, SOCK_STREAM, 0, socket_fd) == 0);

    char const msg[] = "A random message";
    send(socket_fd[client], msg, sizeof msg, MSG_DONTWAIT | MSG_NOSIGNAL);

    assert(setsockopt(socket_fd[server], SOL_SOCKET, SO_PASSCRED, &opt, sizeof(opt)) != -1);

    union {
        struct cmsghdr cmh;
        char control[CMSG_SPACE(sizeof(struct ucred))];
    } control_un;

    control_un.cmh.cmsg_len = CMSG_LEN(sizeof(struct ucred));
    control_un.cmh.cmsg_level = SOL_SOCKET;
    control_un.cmh.cmsg_type = SCM_CREDENTIALS;

    struct msghdr msgh;
    msgh.msg_name = NULL;
    msgh.msg_namelen = 0;
    msgh.msg_iov = NULL;
    msgh.msg_iovlen = 0;
    msgh.msg_control = control_un.control;
    msgh.msg_controllen = sizeof(control_un.control);

    errno = 0;

    if (recvmsg(socket_fd[server], &msgh, MSG_PEEK) == -1)
    {
        printf("Error: %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }
    else
    {
        printf("Success!\n");
        exit(EXIT_SUCCESS);
    }
}