kernel: add a kernel_wait helper

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Link: http://lkml.kernel.org/r/20200721130449.5008-1-hch@lst.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index ae3060f..a989650 100644 (file)
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -88,6 +88,7 @@ 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 c2d2961..733e80f 100644 (file)
--- 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 a25433f..fcf3ee8 100644 (file)
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -119,37 +119,16 @@ 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);
 }