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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Stephen Brennan <stephen.s.brennan@oracle.com>
Cc:     Alexey Dobriyan <adobriyan@gmail.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        linux-security-module@vger.kernel.org,
        Paul Moore <paul@paul-moore.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        Eric Paris <eparis@parisplace.org>, selinux@vger.kernel.org,
        Casey Schaufler <casey@schaufler-ca.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Matthew Wilcox <willy@infradead.org>
References: <20201130200619.84819-1-stephen.s.brennan@oracle.com>
Date:   Mon, 30 Nov 2020 14:46:17 -0600
In-Reply-To: <20201130200619.84819-1-stephen.s.brennan@oracle.com> (Stephen
        Brennan's message of "Mon, 30 Nov 2020 12:06:19 -0800")
Message-ID: <87zh2yh8ti.fsf@x220.int.ebiederm.org>
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Subject: Re: [PATCH] proc: Allow pid_revalidate() during LOOKUP_RCU
Precedence: bulk

Stephen Brennan <stephen.s.brennan@oracle.com> writes:

> The pid_revalidate() function requires dropping from RCU into REF lookup
> mode. When many threads are resolving paths within /proc in parallel,
> this can result in heavy spinlock contention as each thread tries to
> grab a reference to the /proc dentry (and drop it shortly thereafter).
>
> Allow the pid_revalidate() function to execute under LOOKUP_RCU. When
> updates must be made to the inode due to the owning task performing
> setuid(), drop out of RCU and into REF mode.

So rather than get_task_rcu_user.  I think what we want is a function
that verifies task->rcu_users > 0.

Which frankly is just "pid_task(proc_pid(inode), PIDTYPE_PID)".

Which is something that we can do unconditionally in pid_revalidate.

Skipping the update of the inode is probably the only thing that needs
to be skipped.

It looks like the code can safely rely on the the security_task_to_inode
in proc_pid_make_inode and remove the security_task_to_inode in
pid_update_inode.


> Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
> ---
>
> I'd like to use this patch as an RFC on this approach for reducing spinlock
> contention during many parallel path lookups in the /proc filesystem. The
> contention can be triggered by, for example, running ~100 parallel instances of
> "TZ=/etc/localtime ps -fe >/dev/null" on a 100CPU machine. The %sys utilization
> in such a case reaches around 90%, and profiles show two code paths with high
> utilization:

Do you have a real world work-load that is behaves something like this
micro benchmark?  I am just curious how severe the problem you are
trying to solve is.

>
>     walk_component
>       lookup_fast
>         unlazy_child
>           legitimize_root
>             __legitimize_path
>               lockref_get_not_dead
>
>     terminate_walk
>       dput
>         dput
>
> By applying this patch, %sys utilization falls to around 60% under the same
> workload.
>
> One item I'd like to highlight about this patch is that the
> security_task_to_inode() hook is called less frequently as a result. I don't
> know whether this is a major concern, which is why I've included security
> reviewers as well.
>
>  fs/proc/base.c      | 50 ++++++++++++++++++++++++++++++++-------------
>  fs/proc/internal.h  |  5 +++++
>  include/linux/pid.h |  2 ++
>  kernel/pid.c        | 12 +++++++++++
>  4 files changed, 55 insertions(+), 14 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ebea9501afb8..038056f94ed0 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1813,12 +1813,29 @@ int pid_getattr(const struct path *path, struct kstat *stat,
>  /*
>   * Set <pid>/... inode ownership (can change due to setuid(), etc.)
>   */
> -void pid_update_inode(struct task_struct *task, struct inode *inode)
> +static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
> +							   unsigned int flags)
>  {
> -	task_dump_owner(task, inode->i_mode, &inode->i_uid, &inode->i_gid);
> +	kuid_t uid;
> +	kgid_t gid;
> +
> +	task_dump_owner(task, inode->i_mode, &uid, &gid);
> +	if (uid_eq(uid, inode->i_uid) && gid_eq(gid, inode->i_gid) &&
> +			!(inode->i_mode & (S_ISUID | S_ISGID)))
> +		return 1;
> +	if (flags & LOOKUP_RCU)
> +		return -ECHILD;
>  
> +	inode->i_uid = uid;
> +	inode->i_gid = gid;
>  	inode->i_mode &= ~(S_ISUID | S_ISGID);
>  	security_task_to_inode(task, inode);
> +	return 1;
> +}
> +
> +void pid_update_inode(struct task_struct *task, struct inode *inode)
> +{
> +	do_pid_update_inode(task, inode, 0);
>  }
>  
>  /*
> @@ -1830,19 +1847,24 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
>  {
>  	struct inode *inode;
>  	struct task_struct *task;
> -
> -	if (flags & LOOKUP_RCU)
> -		return -ECHILD;
> -
> -	inode = d_inode(dentry);
> -	task = get_proc_task(inode);
> -
> -	if (task) {
> -		pid_update_inode(task, inode);
> -		put_task_struct(task);
> -		return 1;
> +	int rv = 0;
> +
> +	if (flags & LOOKUP_RCU) {
> +		inode = d_inode_rcu(dentry);
> +		task = get_proc_task_rcu(inode);
> +		if (task) {
> +			rv = do_pid_update_inode(task, inode, flags);
> +			put_task_struct_rcu_user(task);
> +		}
> +	} else {
> +		inode = d_inode(dentry);
> +		task = get_proc_task(inode);
> +		if (task) {
> +			rv = do_pid_update_inode(task, inode, flags);
> +			put_task_struct(task);
> +		}
>  	}
> -	return 0;
> +	return rv;
>  }
>  
>  static inline bool proc_inode_is_dead(struct inode *inode)
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index cd0c8d5ce9a1..aa6df65ad3eb 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -121,6 +121,11 @@ static inline struct task_struct *get_proc_task(const struct inode *inode)
>  	return get_pid_task(proc_pid(inode), PIDTYPE_PID);
>  }
>  
> +static inline struct task_struct *get_proc_task_rcu(const struct inode *inode)
> +{
> +	return get_pid_task_rcu_user(proc_pid(inode), PIDTYPE_PID);
> +}
> +
>  void task_dump_owner(struct task_struct *task, umode_t mode,
>  		     kuid_t *ruid, kgid_t *rgid);
>  
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 9645b1194c98..0b2c54f85e6d 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -86,6 +86,8 @@ static inline struct pid *get_pid(struct pid *pid)
>  extern void put_pid(struct pid *pid);
>  extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
>  extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
> +extern struct task_struct *get_pid_task_rcu_user(struct pid *pid,
> +						 enum pid_type type);
>  
>  extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
>  
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 0a9f2e437217..05acbd15cfa6 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -390,6 +390,18 @@ struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
>  }
>  EXPORT_SYMBOL_GPL(get_pid_task);
>  
> +struct task_struct *get_pid_task_rcu_user(struct pid *pid, enum pid_type type)
> +{
> +	struct task_struct *task;
> +
> +	task = pid_task(pid, type);
> +	if (task && refcount_inc_not_zero(&task->rcu_users))
> +		return task;
> +
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(get_pid_task_rcu_user);
> +
>  struct pid *find_get_pid(pid_t nr)
>  {
>  	struct pid *pid;