[PATCH] uml: fix hang on run_helper() failure on uml_net

Fix an hang on a pipe when run_helper() fails when called by change_tramp()
(i.e.  when calling uml_net) - reproduced the bug and verified this fixes it.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 8 insertions, 6 deletions

diff --git a/arch/um/drivers/net_user.c b/arch/um/drivers/net_user.c
index 0e2f06187ea7..0a7786e00cfb 100644
--- a/arch/um/drivers/net_user.c
+++ b/arch/um/drivers/net_user.c
@@ -182,7 +182,9 @@ static int change_tramp(char **argv, char *output, int output_len)
         pe_data.stdout = fds[1];
         pid = run_helper(change_pre_exec, &pe_data, argv, NULL);
 
-        read_output(fds[0], output, output_len);
+        if (pid > 0)    /* Avoid hang as we won't get data in failure case. */
+                read_output(fds[0], output, output_len);
+
         os_close_file(fds[0]);
         os_close_file(fds[1]);
 
diff --git a/arch/um/os-Linux/helper.c b/arch/um/os-Linux/helper.c
index 6490a4ff40ac..6987d1d247a2 100644
--- a/arch/um/os-Linux/helper.c
+++ b/arch/um/os-Linux/helper.c
@@ -43,7 +43,7 @@ static int helper_child(void *arg)
                 (*data->pre_exec)(data->pre_data);
         execvp(argv[0], argv);
         errval = errno;
-        printk("execvp of '%s' failed - errno = %d\n", argv[0], errno);
+        printk("helper_child - execve of '%s' failed - errno = %d\n", argv[0], errno);
         os_write_file(data->fd, &errval, sizeof(errval));
         kill(os_getpid(), SIGKILL);
         return(0);
@@ -92,15 +92,15 @@ int run_helper(void (*pre_exec)(void *), void *pre_data, char **argv,
         close(fds[1]);
         fds[1] = -1;
 
-        /*Read the errno value from the child.*/
+        /* Read the errno value from the child, if the exec failed, or get 0 if
+         * the exec succeeded because the pipe fd was set as close-on-exec. */
         n = os_read_file(fds[0], &ret, sizeof(ret));
-        if(n < 0){
+        if (n < 0) {
                 printk("run_helper : read on pipe failed, ret = %d\n", -n);
                 ret = n;
                 kill(pid, SIGKILL);
                 CATCH_EINTR(waitpid(pid, NULL, 0));
-        }
-        else if(n != 0){
+        } else if(n != 0){
                 CATCH_EINTR(n = waitpid(pid, NULL, 0));
                 ret = -errno;
         } else {