+ kernel-add-a-kernel_wait-helper.patch added to -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: kernel: add a kernel_wait helper
has been added to the -mm tree.  Its filename is
     kernel-add-a-kernel_wait-helper.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/kernel-add-a-kernel_wait-helper.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/kernel-add-a-kernel_wait-helper.patch

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------------------------------------------------------
From: Christoph Hellwig <hch@lst.de>
Subject: 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.

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

 include/linux/sched/task.h |    1 +
 kernel/exit.c              |   16 ++++++++++++++++
 kernel/umh.c               |   29 ++++-------------------------
 3 files changed, 21 insertions(+), 25 deletions(-)

--- a/include/linux/sched/task.h~kernel-add-a-kernel_wait-helper
+++ a/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);
 
--- a/kernel/exit.c~kernel-add-a-kernel_wait-helper
+++ a/kernel/exit.c
@@ -1626,6 +1626,22 @@ long kernel_wait4(pid_t upid, int __user
 	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)
 {
--- a/kernel/umh.c~kernel-add-a-kernel_wait-helper
+++ a/kernel/umh.c
@@ -130,37 +130,16 @@ static void call_usermodehelper_exec_syn
 {
 	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);
 }
 
_

Patches currently in -mm which might be from hch@lst.de are

syscalls-use-uaccess_kernel-in-addr_limit_user_check.patch
nds32-use-uaccess_kernel-in-show_regs.patch
riscv-include-asm-pgtableh-in-asm-uaccessh.patch
uaccess-remove-segment_eq.patch
uaccess-add-force_uaccess_beginend-helpers.patch
uaccess-add-force_uaccess_beginend-helpers-v2.patch
exec-use-force_uaccess_begin-during-exec-and-exit.patch
kernel-add-a-kernel_wait-helper.patch