linux-kernel - Re: [PATCH] tmpfs: implement generic xattr support

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Date:	Thu, 12 May 2011 16:00:40 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Michal Suchanek <hramrach@...trum.cz>,
	Andreas Dilger <adilger@...ger.ca>,
	Jiri Kosina <jkosina@...e.cz>,
	Ric Wheeler <ricwheeler@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Howells <dhowells@...hat.com>,
	Ian Kent <ikent@...hat.com>, Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Eric Paris <eparis@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Morris <jmorris@...ei.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>
Subject: Re: [PATCH] tmpfs: implement generic xattr support

Miklos Szeredi <miklos@...redi.hu> writes:

>>> +	info = SHMEM_I(dentry->d_inode);
>>> +
>>> +	spin_lock(&dentry->d_inode->i_lock);
>>
>> Not important, but I suggest you use info->lock throughout for this,
>> instead of dentry->d_inode->i_lock: in each case you need "info" for
>> info->xattr_list (and don't need "inode" at all I think), so info->lock
>> seems appropriate, and may be in the same cacheline once I make
>> shmem_inode_info smaller.  But don't worry if you'd prefer to leave
>> it.
>
> Makes sense.  I updated the locking.

This uncovered a nasty bug lurking in there: the "info" area, including
lock and xattr_list, may be overwritten by inline symlinks.  Because
xattr_list is near the end, this wasn't noticed with casual testing, but
info->lock would immediately Oops on getxattr for symlinks.

I propose the following solution.  It results in slightly less space for
inline symlinks, but correct operation for xattrs.  Does the anonymous
union/struct solution look acceptable?

Thanks,
Miklos

Index: linux-2.6/include/linux/shmem_fs.h
===================================================================
--- linux-2.6.orig/include/linux/shmem_fs.h	2011-05-12 15:59:08.000000000 +0200
+++ linux-2.6/include/linux/shmem_fs.h	2011-05-12 15:58:25.000000000 +0200
@@ -11,15 +11,39 @@
 
 struct shmem_inode_info {
 	spinlock_t		lock;
-	unsigned long		flags;
-	unsigned long		alloced;	/* data pages alloced to file */
-	unsigned long		swapped;	/* subtotal assigned to swap */
-	unsigned long		next_index;	/* highest alloced index + 1 */
-	struct shared_policy	policy;		/* NUMA memory alloc policy */
-	struct page		*i_indirect;	/* top indirect blocks page */
-	swp_entry_t		i_direct[SHMEM_NR_DIRECT]; /* first blocks */
-	struct list_head	swaplist;	/* chain of maybes on swap */
-	struct list_head	xattr_list;	/* list of shmem_xattr */
+
+	/* list of shmem_xattr */
+	struct list_head	xattr_list;
+
+	union {
+		char			inline_symlink[0];
+
+		/* Members not used by inline symlinks: */
+		struct {
+			unsigned long		flags;
+
+			/* data pages alloced to file */
+			unsigned long		alloced;
+
+			/* subtotal assigned to swap */
+			unsigned long		swapped;
+
+			/* highest alloced index + 1 */
+			unsigned long		next_index;
+
+			/* NUMA memory alloc policy */
+			struct shared_policy	policy;
+
+			/* top indirect blocks page */
+			struct page		*i_indirect;
+
+			/* first blocks */
+			swp_entry_t		i_direct[SHMEM_NR_DIRECT];
+
+			/* chain of maybes on swap */
+			struct list_head	swaplist;
+		};
+	};
 	struct inode		vfs_inode;
 };
 
Index: linux-2.6/mm/shmem.c
===================================================================
--- linux-2.6.orig/mm/shmem.c	2011-05-12 15:59:08.000000000 +0200
+++ linux-2.6/mm/shmem.c	2011-05-12 15:50:10.000000000 +0200
@@ -2029,9 +2029,9 @@ static int shmem_symlink(struct inode *d
 
 	info = SHMEM_I(inode);
 	inode->i_size = len-1;
-	if (len <= (char *)inode - (char *)info) {
+	if (len <= (char *)inode - info->inline_symlink) {
 		/* do it inline */
-		memcpy(info, symname, len);
+		memcpy(info->inline_symlink, symname, len);
 		inode->i_op = &shmem_symlink_inline_operations;
 	} else {
 		error = shmem_getpage(inode, 0, &page, SGP_WRITE, NULL);
@@ -2057,7 +2057,7 @@ static int shmem_symlink(struct inode *d
 
 static void *shmem_follow_link_inline(struct dentry *dentry, struct nameidata *nd)
 {
-	nd_set_link(nd, (char *)SHMEM_I(dentry->d_inode));
+	nd_set_link(nd, SHMEM_I(dentry->d_inode)->inline_symlink);
 	return NULL;
 }
 



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