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Subject[PATCH] configfs: Fix failing symlink() making rmdir() fail
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Joel,

What about this other solution? By the way, it does not even need to
rename CONFIGFS_USET_IN_MKDIR.

Louis

--
Dr Louis Rilling Kerlabs
Skype: louis.rilling Batiment Germanium
Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes
http://www.kerlabs.com/ 35700 RennesOn a similar pattern as mkdir() vs rmdir(), a failing symlink() may make rmdir()
fail for the symlink's parent and the symlink's target as well.

failing symlink() making target's rmdir() fail:

process 1: process 2:
symlink("A/S" -> "B")
allow_link()
create_link()
attach to "B" links list
rmdir("B")
detach_prep("B")
error because of new link
configfs_create_link("A", "S")
error (eg -ENOMEM)

failing symlink() making parent's rmdir() fail:

process 1: process 2:
symlink("A/D/S" -> "B")
allow_link()
create_link()
attach to "B" links list
configfs_create_link("A/D", "S")
make_dirent("A/D", "S")
rmdir("A")
detach_prep("A")
detach_prep("A/D")
error because of "S"
create("S")
error (eg -ENOMEM)

We cannot use the same solution as for mkdir() vs rmdir(), since rmdir() on the
target cannot wait on the i_mutex of the new symlink's parent without risking a
deadlock (with other symlink() or sys_rename()). Instead we define a global
mutex protecting all configfs symlinks attachment, so that rmdir() can avoid the
races above.

Signed-off-by: Louis Rilling <louis.rilling@kerlabs.com>
---
fs/configfs/configfs_internal.h | 1 +
fs/configfs/dir.c | 10 ++++++++++
fs/configfs/symlink.c | 8 +++++++-
3 files changed, 18 insertions(+), 1 deletions(-)
diff --git a/fs/configfs/configfs_internal.h b/fs/configfs/configfs_internal.h
index da015c1..5f61b26 100644
--- a/fs/configfs/configfs_internal.h
+++ b/fs/configfs/configfs_internal.h
@@ -51,6 +51,7 @@ struct configfs_dirent {
#define CONFIGFS_USET_IN_MKDIR 0x0200
#define CONFIGFS_NOT_PINNED (CONFIGFS_ITEM_ATTR)

+extern struct mutex configfs_symlink_mutex;
extern spinlock_t configfs_dirent_lock;

extern struct vfsmount * configfs_mount;
diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index f2a12d0..2c873fd 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -1202,6 +1202,11 @@ static int configfs_rmdir(struct inode *dir, struct dentry *dentry)
return -EINVAL;
}

+ /*
+ * Ensure that no racing symlink() will make detach_prep() fail while
+ * the new link is temporarily attached
+ */
+ mutex_lock(&configfs_symlink_mutex);
spin_lock(&configfs_dirent_lock);
do {
struct mutex *wait_mutex;
@@ -1210,6 +1215,7 @@ static int configfs_rmdir(struct inode *dir, struct dentry *dentry)
if (ret) {
configfs_detach_rollback(dentry);
spin_unlock(&configfs_dirent_lock);
+ mutex_unlock(&configfs_symlink_mutex);
if (ret != -EAGAIN) {
config_item_put(parent_item);
return ret;
@@ -1219,10 +1225,12 @@ static int configfs_rmdir(struct inode *dir, struct dentry *dentry)
mutex_lock(wait_mutex);
mutex_unlock(wait_mutex);

+ mutex_lock(&configfs_symlink_mutex);
spin_lock(&configfs_dirent_lock);
}
} while (ret == -EAGAIN);
spin_unlock(&configfs_dirent_lock);
+ mutex_unlock(&configfs_symlink_mutex);

/* Get a working ref for the duration of this function */
item = configfs_get_config_item(dentry);
@@ -1512,11 +1520,13 @@ void configfs_unregister_subsystem(struct configfs_subsystem *subsys)
mutex_lock_nested(&configfs_sb->s_root->d_inode->i_mutex,
I_MUTEX_PARENT);
mutex_lock_nested(&dentry->d_inode->i_mutex, I_MUTEX_CHILD);
+ mutex_lock(&configfs_symlink_mutex);
spin_lock(&configfs_dirent_lock);
if (configfs_detach_prep(dentry, NULL)) {
printk(KERN_ERR "configfs: Tried to unregister non-empty subsystem!\n");
}
spin_unlock(&configfs_dirent_lock);
+ mutex_unlock(&configfs_symlink_mutex);
configfs_detach_group(&group->cg_item);
dentry->d_inode->i_flags |= S_DEAD;
mutex_unlock(&dentry->d_inode->i_mutex);
diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index b66908d..21bd751 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -31,6 +31,9 @@
#include <linux/configfs.h>
#include "configfs_internal.h"

+/* Protects attachments of new symlinks */
+DEFINE_MUTEX(configfs_symlink_mutex);
+
static int item_depth(struct config_item * item)
{
struct config_item * p = item;
@@ -146,8 +149,11 @@ int configfs_symlink(struct inode *dir, struct dentry *dentry, const char *symna
goto out_put;

ret = type->ct_item_ops->allow_link(parent_item, target_item);
- if (!ret)
+ if (!ret) {
+ mutex_lock(&configfs_symlink_mutex);
ret = create_link(parent_item, target_item, dentry);
+ mutex_unlock(&configfs_symlink_mutex);
+ }

config_item_put(target_item);
path_put(&nd.path);
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