Commit 8043fc14 authored by Christoph Hellwig's avatar Christoph Hellwig Committed by Linus Torvalds
Browse files

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: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Acked-by: default avatar"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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f48ff83e

include/linux/sched/task.h

+1 −0
Original line number Diff line number Diff line
@@ -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);

kernel/exit.c

+16 −0
Original line number Diff line number Diff line
@@ -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)

{

kernel/umh.c

+4 −25
Original line number Diff line number Diff line
@@ -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);

}