linux-kernel - Re: [PATCH] tracefs/eventfs: Use root and instance inodes as default ownership

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Date: Wed, 3 Jan 2024 21:17:29 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, Christian Brauner
 <brauner@...nel.org>, linux-fsdevel@...r.kernel.org, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] tracefs/eventfs: Use root and instance inodes as
 default ownership

On Thu, 4 Jan 2024 01:59:10 +0000
Al Viro <viro@...iv.linux.org.uk> wrote:

> On Wed, Jan 03, 2024 at 08:32:46PM -0500, Steven Rostedt wrote:
> 
> > +static struct inode *instance_inode(struct dentry *parent, struct inode *inode)
> > +{
> > +	struct tracefs_inode *ti;
> > +	struct inode *root_inode;
> > +
> > +	root_inode = d_inode(inode->i_sb->s_root);
> > +
> > +	/* If parent is NULL then use root inode */
> > +	if (!parent)
> > +		return root_inode;
> > +
> > +	/* Find the inode that is flagged as an instance or the root inode */
> > +	do {
> > +		inode = d_inode(parent);
> > +		if (inode == root_inode)
> > +			return root_inode;
> > +
> > +		ti = get_tracefs(inode);
> > +
> > +		if (ti->flags & TRACEFS_INSTANCE_INODE)
> > +			return inode;
> > +	} while ((parent = parent->d_parent));  
> 
> *blink*
> 
> This is equivalent to
> 		...
> 		parent = parent->d_parent;
> 	} while (true);

Yeah, that loop went through a few iterations as I first thought that root
was a tracefs_inode and the get_tracefs() would work on it. No, it was not,
and it caused a cash. But I didn't rewrite the loop well after fixing that.

I was also not sure if parent could be NULL, and wanted to protect against it.

> 
> ->d_parent is *never* NULL.  And what the hell is that loop supposed to do,  
> anyway?  Find the nearest ancestor tagged with TRACEFS_INSTANCE_INODE?
> 
> If root is not marked that way, I would suggest
> 	if (!parent)
> 		parent = inode->i_sb->s_root;
> 	while (!IS_ROOT(parent)) {
> 		struct tracefs_inode *ti = get_tracefs(parent->d_inode);
> 		if (ti->flags & TRACEFS_INSTANCE_INODE)
> 			break;
> 		parent = parent->d_parent;
> 	}
> 	return parent->d_inode;

Sure, I could rewrite it that way too.

Thanks,

-- Steve