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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>
This commit is contained in:
Christoph Hellwig 2020-08-11 18:34:10 -07:00 committed by Linus Torvalds
parent f48ff83e9c
commit 8043fc147a
3 changed files with 21 additions and 25 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/sched/task.h
View File

@ -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);

16
kernel/exit.c
View File

@ -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)
{

29
kernel/umh.c
View File

@ -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);
}