linux-kernel - Re: [PATCH 2/3] kernel/fork, cred.c: allow copy

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Date:   Thu, 1 Jul 2021 18:59:13 -0500
From:   michael.christie@...cle.com
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, jasowang@...hat.com, stefanha@...hat.com,
        christian@...uner.io, akpm@...ux-foundation.org,
        peterz@...radead.org
Subject: Re: [PATCH 2/3] kernel/fork, cred.c: allow copy_process to take user

On 6/29/21 11:53 AM, Mike Christie wrote:
> 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@...cle.com>
>>> ---
>>>  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.

One question/clarification. For 5.14, I could pass in the struct task_struct
or struct ucounts (in a previous mail I wrote user_struct).

I could also just have vhost.c do inc_rlimit_ucounts and is_ucounts_overlimit
directly.