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SubjectRe: [PATCH] kernel: add a kernel_wait helper
Christoph Hellwig <hch@lst.de> writes:

> 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.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

> 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 79f139a7ca03c6..733430921f47d7 100644
> --- a/kernel/umh.c
> +++ b/kernel/umh.c
> @@ -130,37 +130,16 @@ 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);
> }

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