Allow Unix sockets to be treated like normal files.

某天忽然想要用echo/cat的方式直接對unix socket做存取，結果得到error，只好有請google大神，覓得此良方Allow Unix sockets to be treated like normal files，try了一下沒問題。

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdlib.h>

#define handle_error(msg) \
        do { perror(msg); exit(EXIT_FAILURE); } while (0)

int main(int argc, char *argv[])
{
    int srv_fd, cli_fd;
    socklen_t cli_len;
    struct sockaddr_un srv_addr, cli_addr;
    char buf[128] = "Brook: ";
    ssize_t len;

    unlink("server_socket");
    if ((srv_fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
        handle_error("socket");
    }

    srv_addr.sun_family = AF_UNIX;
    strcpy(srv_addr.sun_path, "/tmp/unix_sock");
    if (bind(srv_fd, (struct sockaddr *)&srv_addr, sizeof(srv_addr)) < 0) {
        handle_error("bind");
    }

    if (listen(srv_fd, 1) < 0) {
         handle_error("listen");
    }

    while (1) {
        cli_fd = accept(srv_fd, (struct sockaddr *)&cli_addr, &cli_len);
        len = read(cli_fd, buf + 6, sizeof(buf) - 6);
        buf[6 + len] = 0;
        write(cli_fd, buf, strlen(buf));
        close(cli_fd);
    }
    return 0;
}

參考資料：
• http://lwn.net/Articles/415651/ , net/unix: Allow Unix sockets to be treated like normal files.

github：
https://github.com/brook-kuo/Linux_Module/tree/master/socket/unix_as_normal_file

關於bridge中hard_header_len/needed_headroom的問題

我的interface在收送上本來沒有問題，但是當它和其他interface作bridge之後就發生crash了，後來才發現是head不夠大，於是參考這篇文章"关于bridge中hard_header_len值的问题"，做了些修改，原因是因為我的device在預留head room時，是hard_header_len+needed_headroom。而且有些local generate的packet會使用LL_RESERVED_SPACE()這macro來保留head room，而LL_RESERVED_SPACE()也會使用到needed_headroom，所以最保險的作法就是同時設定bridge的hard_header_len和needed_headroom。

--- a/net/bridge/br_if.c
+++ b/net/bridge/br_if.c
@@ -369,6 +369,10 @@ void br_features_recompute(struct net_bridge *br)
 {
        struct net_bridge_port *p;
        unsigned long features, mask;
+#if 1 // Brook.
+    unsigned short max_hard_header_len = ETH_HLEN;
+    unsigned short max_needed_headroom = 0;
+#endif

        features = mask = br->feature_mask;
        if (list_empty(&br->port_list))
@@ -379,8 +383,22 @@ void br_features_recompute(struct net_bridge *br)
        list_for_each_entry(p, &br->port_list, list) {
                features = netdev_increment_features(features,
                                                     p->dev->features, mask);
+#if 1 // Brook.
+        if (p->dev->hard_header_len > max_hard_header_len) {
+            max_hard_header_len = p->dev->hard_header_len;
+        }
+
+        if (p->dev->needed_headroom > max_needed_headroom) {
+            max_needed_headroom = p->dev->needed_headroom;
+        }
+#endif
        }

+#if 1 // Brook.
+    br->dev->hard_header_len = max_hard_header_len;
+    br->dev->needed_headroom = max_needed_headroom;
+#endif
+
 done:
        br->dev->features = netdev_fix_features(features, NULL);
 }

參考資料：
1. 关于bridge中hard_header_len值的问题
2. struct net_device结构中添加了needed_headroom/needed_tailroom成员