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Subject[PATCH 12/17] uml: fix hang on run_helper() failure on uml_net
Date
From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

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>
---

arch/um/drivers/net_user.c | 4 +++-
arch/um/os-Linux/helper.c | 10 +++++-----
2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/um/drivers/net_user.c b/arch/um/drivers/net_user.c
index 0e2f061..0a7786e 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, cha
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 6490a4f..6987d1d 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 *),
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 {
-
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