linux-kernel - Re: [patch 1/9] procfs: Move /proc/pid/fd[info] handling code to fd.[ch]

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Date:	Sat, 25 Aug 2012 23:43:25 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>,
	aneesh.kumar@...ux.vnet.ibm.com, bfields@...ldses.org
Subject: Re: [patch 1/9] procfs: Move /proc/pid/fd[info] handling code to
 fd.[ch]

On Sat, Aug 25, 2012 at 08:12:18PM +0100, Al Viro wrote:
> On Sat, Aug 25, 2012 at 10:58:29PM +0400, Cyrill Gorcunov wrote:
> > On Sat, Aug 25, 2012 at 06:55:04PM +0100, Al Viro wrote:
> > > > Well, this could be simplified indeed, if I understand you correctly
> > > > you propose just save f_mode in flexible array and use it instead
> > > > of struct file, right? (which will require to rewrite code a bit)
> > > 
> > > Yes.  FWIW, proc_fill_cache() is really atrocious ;-/  Not to mention
> > 
> > OK, thanks. I'm putting this cleanup task in my big todo list. Hope I'll
> > manage on the next week with it.
> > 
> > > anything else, if we ever get a negative dentry there, we have a dentry
> > > leak.  I don't think it's possible in practice, but...  Furthermore,
> > 
> > could you please elaborate, you mean this string?
> 
> I mean that if we get to that if (... || !child->d_inode) and end up
> evaluating the last part at all, we have acquired a reference to that
> struct dentry.  And if that last part ends up being true (i.e. if it's
> a negative dentry), we'll return from function without having dropped
> the reference we'd acquired.

Would the patch below improve the code? Look, I've not dropped
find_inode_number call since it's a bit unclear for me what
would happen if !child case hit

	child = d_lookup(dir, &qname);
	if (!child) {
		struct dentry *new = d_alloc(dir, &qname);
		if (new) {
			child = instantiate(dir->d_inode, new, task, ptr);
			if (child)
				dput(new);
			else
				child = new;
		}
	}

can we be sure that i_ino won't be zero here?
---
 fs/proc/base.c |   27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

Index: linux-2.6.git/fs/proc/base.c
===================================================================
--- linux-2.6.git.orig/fs/proc/base.c
+++ linux-2.6.git/fs/proc/base.c
@@ -1650,7 +1650,6 @@ int proc_fill_cache(struct file *filp, v
 	instantiate_t instantiate, struct task_struct *task, const void *ptr)
 {
 	struct dentry *child, *dir = filp->f_path.dentry;
-	struct inode *inode;
 	struct qstr qname;
 	ino_t ino = 0;
 	unsigned type = DT_UNKNOWN;
@@ -1661,8 +1660,7 @@ int proc_fill_cache(struct file *filp, v
 
 	child = d_lookup(dir, &qname);
 	if (!child) {
-		struct dentry *new;
-		new = d_alloc(dir, &qname);
+		struct dentry *new = d_alloc(dir, &qname);
 		if (new) {
 			child = instantiate(dir->d_inode, new, task, ptr);
 			if (child)
@@ -1671,19 +1669,20 @@ int proc_fill_cache(struct file *filp, v
 				child = new;
 		}
 	}
-	if (!child || IS_ERR(child) || !child->d_inode)
-		goto end_instantiate;
-	inode = child->d_inode;
-	if (inode) {
-		ino = inode->i_ino;
-		type = inode->i_mode >> 12;
-	}
+	if (IS_ERR_OR_NULL(child))
+		goto err;
+	if (!child->d_inode)
+		goto err_put;
+	ino = child->d_inode->i_ino;
+	type = child->d_inode->i_mode >> 12;
+err_put:
 	dput(child);
-end_instantiate:
-	if (!ino)
+err:
+	if (!ino) {
 		ino = find_inode_number(dir, &qname);
-	if (!ino)
-		ino = 1;
+		if (!ino)
+			ino = 1;
+	}
 	return filldir(dirent, name, len, filp->f_pos, ino, type);
 }
 
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