[PATCH 6/6] kernel: add a kernel_wait helper

Christoph Hellwig
2020-06-15T13:00:32Z

Add a helper that waits for a pid and stores the status in the passed
in kernel pointer. Use it to fix the usage of kernel_wait4 in
call_usermodehelper_exec_sync that only happens to work due to the
implicit set_fs(KERNEL_DS) for kernel threads.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
include/linux/sched/task.h | 1 +
kernel/exit.c | 16 ++++++++++++++++
kernel/umh.c | 29 ++++-------------------------
3 files changed, 21 insertions(+), 25 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 38359071236ad7..a80007df396e95 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -102,6 +102,7 @@ struct task_struct *fork_idle(int);
struct mm_struct *copy_init_mm(void);
extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
extern long kernel_wait4(pid_t, int __user *, int, struct rusage *);
+int kernel_wait(pid_t pid, int *stat);

extern void free_task(struct task_struct *tsk);

diff --git a/kernel/exit.c b/kernel/exit.c
index 727150f2810338..fd598846df0b17 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1626,6 +1626,22 @@ long kernel_wait4(pid_t upid, int __user *stat_addr, int options,
return ret;
}

+int kernel_wait(pid_t pid, int *stat)
+{
+ struct wait_opts wo = {
+ .wo_type = PIDTYPE_PID,
+ .wo_pid = find_get_pid(pid),
+ .wo_flags = WEXITED,
+ };
+ int ret;
+
+ ret = do_wait(&wo);
+ if (ret > 0 && wo.wo_stat)
+ *stat = wo.wo_stat;
+ put_pid(wo.wo_pid);
+ return ret;
+}
+
SYSCALL_DEFINE4(wait4, pid_t, upid, int __user *, stat_addr,
int, options, struct rusage __user *, ru)
{
diff --git a/kernel/umh.c b/kernel/umh.c
index 1284823dbad338..6fd948e478bec4 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -126,37 +126,16 @@ static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info)
{
pid_t pid;

- /* If SIGCLD is ignored kernel_wait4 won't populate the status. */
+ /* If SIGCLD is ignored do_wait won't populate the status. */
kernel_sigaction(SIGCHLD, SIG_DFL);
pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);
- if (pid < 0) {
+ if (pid < 0)
sub_info->retval = pid;
- } else {
- int ret = -ECHILD;
- /*
- * Normally it is bogus to call wait4() from in-kernel because
- * wait4() wants to write the exit code to a userspace address.
- * But call_usermodehelper_exec_sync() always runs as kernel
- * thread (workqueue) and put_user() to a kernel address works
- * OK for kernel threads, due to their having an mm_segment_t
- * which spans the entire address space.
- *
- * Thus the __user pointer cast is valid here.
- */
- kernel_wait4(pid, (int __user *)&ret, 0, NULL);
-
- /*
- * If ret is 0, either call_usermodehelper_exec_async failed and
- * the real error code is already in sub_info->retval or
- * sub_info->retval is 0 anyway, so don't mess with it then.
- */
- if (ret)
- sub_info->retval = ret;
- }
+ else
+ kernel_wait(pid, &sub_info->retval);

/* Restore default kernel sig handler */
kernel_sigaction(SIGCHLD, SIG_IGN);
-
umh_complete(sub_info);
}

--
2.26.2