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.

Christoph Hellwig committed 6 years ago

Commit 8043fc11 parent f48ff833 files · +21 −25Browse files

Unified · Split

3 files

include/linux/sched/task.h +1 −0
kernel/exit.c +16 −0
kernel/umh.c +4 −25

include/linux/sched/task.h+1−0

@@ -88,6 +88,7 @@ struct task_struct *fork_idle(int); · 87 unmodified lines
88	88	struct mm_struct *copy_init_mm(void);
89	89	extern pid_t kernel_thread(int (fn)(void ), void *arg, unsigned long flags);
90	90	extern long kernel_wait4(pid_t, int __user , int, struct rusage );
	91	+int kernel_wait(pid_t pid, int *stat);
91	92
92	93	extern void free_task(struct task_struct *tsk);
93	94

kernel/exit.c+16−0

@@ -1626,6 +1626,22 @@ long kernel_wait4(pid_t upid, int __user *stat_addr, int options, · 1625 unmodified lines
1626	1626	return ret;
1627	1627	}
1628	1628
	1629	+int kernel_wait(pid_t pid, int *stat)
	1630	+{
	1631	+ struct wait_opts wo = {
	1632	+ .wo_type = PIDTYPE_PID,
	1633	+ .wo_pid = find_get_pid(pid),
	1634	+ .wo_flags = WEXITED,
	1635	+ };
	1636	+ int ret;
	1637	+
	1638	+ ret = do_wait(&wo);
	1639	+ if (ret > 0 && wo.wo_stat)
	1640	+ *stat = wo.wo_stat;
	1641	+ put_pid(wo.wo_pid);
	1642	+ return ret;
	1643	+}
	1644	+
1629	1645	SYSCALL_DEFINE4(wait4, pid_t, upid, int __user *, stat_addr,
1630	1646	int, options, struct rusage __user *, ru)
1631	1647	{

kernel/umh.c+4−25

@@ -119,37 +119,16 @@ static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info) · 118 unmodified lines
119	119	{
120	120	pid_t pid;
121	121
122		− /* If SIGCLD is ignored kernel_wait4 won't populate the status. */
	122	+ /* If SIGCLD is ignored do_wait won't populate the status. */
123	123	kernel_sigaction(SIGCHLD, SIG_DFL);
124	124	pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);
125		− if (pid < 0) {
	125	+ if (pid < 0)
126	126	sub_info->retval = pid;
127		− } else {
128		− int ret = -ECHILD;
129		− /*
130		− * Normally it is bogus to call wait4() from in-kernel because
131		− * wait4() wants to write the exit code to a userspace address.
132		− * But call_usermodehelper_exec_sync() always runs as kernel
133		− * thread (workqueue) and put_user() to a kernel address works
134		− * OK for kernel threads, due to their having an mm_segment_t
135		− * which spans the entire address space.
136		− *
137		− * Thus the __user pointer cast is valid here.
138		− */
139		− kernel_wait4(pid, (int __user *)&ret, 0, NULL);
140		−
141		− /*
142		− * If ret is 0, either call_usermodehelper_exec_async failed and
143		− * the real error code is already in sub_info->retval or
144		− * sub_info->retval is 0 anyway, so don't mess with it then.
145		− */
146		− if (ret)
147		− sub_info->retval = ret;
148		− }
	127	+ else
	128	+ kernel_wait(pid, &sub_info->retval);
149	129
150	130	/* Restore default kernel sig handler */
151	131	kernel_sigaction(SIGCHLD, SIG_IGN);
152		−
153	132	umh_complete(sub_info);
154	133	}
155	134