Re: [PATCH 2/3] kernel/fork, cred.c: allow copy_process to take user

[Mike Christie]

On 6/29/21 8:04 AM, Christian Brauner wrote:
> On Wed, Jun 23, 2021 at 10:08:03PM -0500, Mike Christie wrote:
>> This allows kthread to pass copy_process the user we want to check for the
>> RLIMIT_NPROC limit for and also charge for the new process. It will be used
>> by vhost where userspace has that driver create threads but the kthreadd
>> thread is checked/charged.
>>
>> Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
>> ---
>>  include/linux/cred.h |  3 ++-
>>  kernel/cred.c        |  7 ++++---
>>  kernel/fork.c        | 12 +++++++-----
>>  3 files changed, 13 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/cred.h b/include/linux/cred.h
>> index 14971322e1a0..9a2c1398cdd4 100644
>> --- a/include/linux/cred.h
>> +++ b/include/linux/cred.h
>> @@ -153,7 +153,8 @@ struct cred {
>>  
>>  extern void __put_cred(struct cred *);
>>  extern void exit_creds(struct task_struct *);
>> -extern int copy_creds(struct task_struct *, unsigned long);
>> +extern int copy_creds(struct task_struct *, unsigned long,
>> +		      struct user_struct *);
>>  extern const struct cred *get_task_cred(struct task_struct *);
>>  extern struct cred *cred_alloc_blank(void);
>>  extern struct cred *prepare_creds(void);
>> diff --git a/kernel/cred.c b/kernel/cred.c
>> index e1d274cd741b..e006aafa8f05 100644
>> --- a/kernel/cred.c
>> +++ b/kernel/cred.c
>> @@ -330,7 +330,8 @@ struct cred *prepare_exec_creds(void)
>>   * The new process gets the current process's subjective credentials as its
>>   * objective and subjective credentials
>>   */
>> -int copy_creds(struct task_struct *p, unsigned long clone_flags)
>> +int copy_creds(struct task_struct *p, unsigned long clone_flags,
>> +	       struct user_struct *user)
>>  {
>>  	struct cred *new;
>>  	int ret;
>> @@ -351,7 +352,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
>>  		kdebug("share_creds(%p{%d,%d})",
>>  		       p->cred, atomic_read(&p->cred->usage),
>>  		       read_cred_subscribers(p->cred));
>> -		atomic_inc(&p->cred->user->processes);
>> +		atomic_inc(&user->processes);
> 
> Hey Mike,
> 
> This won't work anymore since this has moved into ucounts. So in v5.14
> atomic_inc(&p->cred->user->processes);
> will have been replaced by
> inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
> 
Will do.

> From what I can see from your code vhost will always create this kthread
> for current. So you could e.g. add an internal flag/bitfield entry to
> struct kernel_clone_args that you can use to tell copy_creds() that you
> want to charge this thread against current's process limit.

If I understood you, I don't think a flag/bit will work. When vhost does
a kthread call we do kthread_create -> __kthread_create_on_node. This creates
a tmp kthread_create_info struct and adds it to the kthread_create_list list.
It then wakes up the kthreadd thread. kthreadd will then loop over the list,
and do the:

kernel_thread -> kernel_clone -> copy_process ->  copy_creds

So copy_creds sees current == kthreadd.

I think I would have to add a task_struct pointer to kernel_clone_args
and kthread_create_info. If copy_creds sees kernel_clone_args->user_task
then it would use that.