Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

[Alexey Gladkov <gladkov.alexey@gmail.com>]

On Mon, Feb 10, 2020 at 07:36:08PM -0600, Eric W. Biederman wrote:
> Alexey Gladkov <gladkov.alexey@gmail.com> writes:
> 
> > This allows to flush dcache entries of a task on multiple procfs mounts
> > per pid namespace.
> >
> > The RCU lock is used because the number of reads at the task exit time
> > is much larger than the number of procfs mounts.
> 
> A couple of quick comments.
> 
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Signed-off-by: Djalal Harouni <tixxdz@gmail.com>
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> > ---
> >  fs/proc/base.c                | 20 +++++++++++++++-----
> >  fs/proc/root.c                | 27 ++++++++++++++++++++++++++-
> >  include/linux/pid_namespace.h |  2 ++
> >  include/linux/proc_fs.h       |  2 ++
> >  4 files changed, 45 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 4ccb280a3e79..24b7c620ded3 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -3133,7 +3133,7 @@ static const struct inode_operations proc_tgid_base_inode_operations = {
> >  	.permission	= proc_pid_permission,
> >  };
> >  
> > -static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
> > +static void proc_flush_task_mnt_root(struct dentry *mnt_root, pid_t pid, pid_t tgid)
> Perhaps just rename things like:
> > +static void proc_flush_task_root(struct dentry *root, pid_t pid, pid_t tgid)
> >  {
> 
> I don't think the mnt_ prefix conveys any information, and it certainly
> makes everything longer and more cumbersome.
> 
> >  	struct dentry *dentry, *leader, *dir;
> >  	char buf[10 + 1];
> > @@ -3142,7 +3142,7 @@ static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
> >  	name.name = buf;
> >  	name.len = snprintf(buf, sizeof(buf), "%u", pid);
> >  	/* no ->d_hash() rejects on procfs */
> > -	dentry = d_hash_and_lookup(mnt->mnt_root, &name);
> > +	dentry = d_hash_and_lookup(mnt_root, &name);
> >  	if (dentry) {
> >  		d_invalidate(dentry);
> >  		dput(dentry);
> > @@ -3153,7 +3153,7 @@ static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
> >  
> >  	name.name = buf;
> >  	name.len = snprintf(buf, sizeof(buf), "%u", tgid);
> > -	leader = d_hash_and_lookup(mnt->mnt_root, &name);
> > +	leader = d_hash_and_lookup(mnt_root, &name);
> >  	if (!leader)
> >  		goto out;
> >  
> > @@ -3208,14 +3208,24 @@ void proc_flush_task(struct task_struct *task)
> >  	int i;
> >  	struct pid *pid, *tgid;
> >  	struct upid *upid;
> > +	struct dentry *mnt_root;
> > +	struct proc_fs_info *fs_info;
> >  
> >  	pid = task_pid(task);
> >  	tgid = task_tgid(task);
> >  
> >  	for (i = 0; i <= pid->level; i++) {
> >  		upid = &pid->numbers[i];
> > -		proc_flush_task_mnt(upid->ns->proc_mnt, upid->nr,
> > -					tgid->numbers[i].nr);
> > +
> > +		rcu_read_lock();
> > +		list_for_each_entry_rcu(fs_info, &upid->ns->proc_mounts, pidns_entry) {
> > +			mnt_root = fs_info->m_super->s_root;
> > +			proc_flush_task_mnt_root(mnt_root, upid->nr, tgid->numbers[i].nr);
> > +		}
> > +		rcu_read_unlock();
> > +
> > +		mnt_root = upid->ns->proc_mnt->mnt_root;
> > +		proc_flush_task_mnt_root(mnt_root, upid->nr, tgid->numbers[i].nr);
> 
> I don't think this following of proc_mnt is needed.  It certainly
> shouldn't be.  The loop through all of the super blocks should be
> enough.

Yes, thanks!

> Once this change goes through.  UML can be given it's own dedicated
> proc_mnt for the initial pid namespace, and proc_mnt can be removed
> entirely.

After you deleted the old sysctl syscall we could probably do it.

> Unless something has changed recently UML is the only other user of
> pid_ns->proc_mnt.  That proc_mnt really only exists to make the loop in
> proc_flush_task easy to write.

Now I think, is there any way to get rid of proc_mounts or even
proc_flush_task somehow.

> It also probably makes sense to take the rcu_read_lock() over
> that entire for loop.

Al Viro pointed out to me that I cannot use rcu locks here :(

-- 
Rgrds, legion