commit d8730f8cec648952ba22935d73c2484fc88c9424
author Peter Maydell <peter.maydell@linaro.org> Tue Nov 06 15:13:23 2018 +0000
committer Samuel Thibault <samuel.thibault@ens-lyon.org> Sat Nov 10 15:07:53 2018 +0100
tree e45fa909fdd363ffef59b24009ffa80d1832985d
parent ab8ddaed8dc2eb92ef017b695fcaa37004b0973b

slirp: fork_exec(): create and connect child socket before fork()

Currently fork_exec() fork()s, and then creates and connects the
child socket which it uses for communication with the parent in
the child process. This is awkward because the child has no
mechanism to report failure back to the parent, which might end
up blocked forever in accept(). The child code also has an issue
pointed out by Coverity (CID 1005727), where if the qemu_socket()
call fails it will pass -1 as a file descriptor to connect().

Fix these issues by moving the creation of the child's end of
the socket to before the fork(), where we are in a position to
handle a possible failure.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

misc.c

diff --git a/misc.c b/misc.c
index 0edb993..92a7bb4 100644
--- a/misc.c
+++ b/misc.c
@@ -80,9 +80,10 @@
  */
 int fork_exec(struct socket *so, const char *ex, int do_pty)
 {
-    int s;
-    struct sockaddr_in addr;
+    int s, cs;
+    struct sockaddr_in addr, csaddr;
     socklen_t addrlen = sizeof(addr);
+    socklen_t csaddrlen = sizeof(csaddr);
     int opt;
     const char *argv[256];
     /* don't want to clobber the original */
@@ -115,10 +116,35 @@
         }
     }
 
+    if (getsockname(s, (struct sockaddr *)&csaddr, &csaddrlen) < 0) {
+        closesocket(s);
+        return 0;
+    }
+    cs = qemu_socket(AF_INET, SOCK_STREAM, 0);
+    if (cs < 0) {
+        closesocket(s);
+        return 0;
+    }
+    csaddr.sin_addr = loopback_addr;
+    /*
+     * This connect won't block because we've already listen()ed on
+     * the server end (even though we won't accept() the connection
+     * until later on).
+     */
+    do {
+        ret = connect(cs, (struct sockaddr *)&csaddr, csaddrlen);
+    } while (ret < 0 && errno == EINTR);
+    if (ret < 0) {
+        closesocket(s);
+        closesocket(cs);
+        return 0;
+    }
+
     pid = fork();
     switch (pid) {
     case -1:
         error_report("Error: fork failed: %s", strerror(errno));
+        closesocket(cs);
         close(s);
         return 0;
 
@@ -126,21 +152,10 @@
         setsid();
 
         /* Set the DISPLAY */
-        getsockname(s, (struct sockaddr *)&addr, &addrlen);
         close(s);
-        /*
-         * Connect to the socket
-         * XXX If any of these fail, we're in trouble!
-         */
-        s = qemu_socket(AF_INET, SOCK_STREAM, 0);
-        addr.sin_addr = loopback_addr;
-        do {
-            ret = connect(s, (struct sockaddr *)&addr, addrlen);
-        } while (ret < 0 && errno == EINTR);
-
-        dup2(s, 0);
-        dup2(s, 1);
-        dup2(s, 2);
+        dup2(cs, 0);
+        dup2(cs, 1);
+        dup2(cs, 2);
         for (s = getdtablesize() - 1; s >= 3; s--)
             close(s);
 
@@ -175,12 +190,10 @@
 
     default:
         qemu_add_child_watch(pid);
+        closesocket(cs);
         /*
-         * XXX this could block us...
-         * XXX Should set a timer here, and if accept() doesn't
-         * return after X seconds, declare it a failure
-         * The only reason this will block forever is if socket()
-         * of connect() fail in the child process
+         * This should never block, because we already connect()ed
+         * on the child end before we forked.
          */
         do {
             so->s = accept(s, (struct sockaddr *)&addr, &addrlen);