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Subject[53/85] UBIFS: prepare to fix a horrid bug
3.2.49-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

commit 33f1a63ae84dfd9ad298cf275b8f1887043ced36 upstream.

Al Viro pointed me to the fact that '->readdir()' and '->llseek()' have no
mutual exclusion, which means the 'ubifs_dir_llseek()' can be run while we are
in the middle of 'ubifs_readdir()'.

First of all, this means that 'file->private_data' can be freed while
'ubifs_readdir()' uses it. But this particular patch does not fix the problem.
This patch is only a preparation, and the fix will follow next.

In this patch we make 'ubifs_readdir()' stop using 'file->f_pos' directly,
because 'file->f_pos' can be changed by '->llseek()' at any point. This may
lead 'ubifs_readdir()' to returning inconsistent data: directory entry names
may correspond to incorrect file positions.

So here we introduce a local variable 'pos', read 'file->f_pose' once at very
the beginning, and then stick to 'pos'. The result of this is that when
'ubifs_dir_llseek()' changes 'file->f_pos' while we are in the middle of
'ubifs_readdir()', the latter "wins".

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/ubifs/dir.c | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)

--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -357,15 +357,16 @@ static unsigned int vfs_dent_type(uint8_
static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
{
int err, over = 0;
+ loff_t pos = file->f_pos;
struct qstr nm;
union ubifs_key key;
struct ubifs_dent_node *dent;
struct inode *dir = file->f_path.dentry->d_inode;
struct ubifs_info *c = dir->i_sb->s_fs_info;

- dbg_gen("dir ino %lu, f_pos %#llx", dir->i_ino, file->f_pos);
+ dbg_gen("dir ino %lu, f_pos %#llx", dir->i_ino, pos);

- if (file->f_pos > UBIFS_S_KEY_HASH_MASK || file->f_pos == 2)
+ if (pos > UBIFS_S_KEY_HASH_MASK || pos == 2)
/*
* The directory was seek'ed to a senseless position or there
* are no more entries.
@@ -373,15 +374,15 @@ static int ubifs_readdir(struct file *fi
return 0;

/* File positions 0 and 1 correspond to "." and ".." */
- if (file->f_pos == 0) {
+ if (pos == 0) {
ubifs_assert(!file->private_data);
over = filldir(dirent, ".", 1, 0, dir->i_ino, DT_DIR);
if (over)
return 0;
- file->f_pos = 1;
+ file->f_pos = pos = 1;
}

- if (file->f_pos == 1) {
+ if (pos == 1) {
ubifs_assert(!file->private_data);
over = filldir(dirent, "..", 2, 1,
parent_ino(file->f_path.dentry), DT_DIR);
@@ -397,7 +398,7 @@ static int ubifs_readdir(struct file *fi
goto out;
}

- file->f_pos = key_hash_flash(c, &dent->key);
+ file->f_pos = pos = key_hash_flash(c, &dent->key);
file->private_data = dent;
}

@@ -405,17 +406,16 @@ static int ubifs_readdir(struct file *fi
if (!dent) {
/*
* The directory was seek'ed to and is now readdir'ed.
- * Find the entry corresponding to @file->f_pos or the
- * closest one.
+ * Find the entry corresponding to @pos or the closest one.
*/
- dent_key_init_hash(c, &key, dir->i_ino, file->f_pos);
+ dent_key_init_hash(c, &key, dir->i_ino, pos);
nm.name = NULL;
dent = ubifs_tnc_next_ent(c, &key, &nm);
if (IS_ERR(dent)) {
err = PTR_ERR(dent);
goto out;
}
- file->f_pos = key_hash_flash(c, &dent->key);
+ file->f_pos = pos = key_hash_flash(c, &dent->key);
file->private_data = dent;
}

@@ -427,7 +427,7 @@ static int ubifs_readdir(struct file *fi
ubifs_inode(dir)->creat_sqnum);

nm.len = le16_to_cpu(dent->nlen);
- over = filldir(dirent, dent->name, nm.len, file->f_pos,
+ over = filldir(dirent, dent->name, nm.len, pos,
le64_to_cpu(dent->inum),
vfs_dent_type(dent->type));
if (over)
@@ -443,7 +443,7 @@ static int ubifs_readdir(struct file *fi
}

kfree(file->private_data);
- file->f_pos = key_hash_flash(c, &dent->key);
+ file->f_pos = pos = key_hash_flash(c, &dent->key);
file->private_data = dent;
cond_resched();
}


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