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kernfs: implement kernfs_get_parent(), kernfs_name/path() and friends
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kernfs_node->parent and ->name are currently marked as "published"
indicating that kernfs users may access them directly; however, those
fields may get updated by kernfs_rename[_ns]() and unrestricted access
may lead to erroneous values or oops.

Protect ->parent and ->name updates with a irq-safe spinlock
kernfs_rename_lock and implement the following accessors for these
fields.

* kernfs_name()		- format the node's name into the specified buffer
* kernfs_path()		- format the node's path into the specified buffer
* pr_cont_kernfs_name()	- pr_cont a node's name (doesn't need buffer)
* pr_cont_kernfs_path()	- pr_cont a node's path (doesn't need buffer)
* kernfs_get_parent()	- pin and return a node's parent

All can be called under any context.  The recursive sysfs_pathname()
in fs/sysfs/dir.c is replaced with kernfs_path() and
sysfs_rename_dir_ns() is updated to use kernfs_get_parent() instead of
dereferencing parent directly.

v2: Dummy definition of kernfs_path() for !CONFIG_KERNFS was missing
    static inline making it cause a lot of build warnings.  Add it.

Signed-off-by: Tejun Heo <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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htejun authored and gregkh committed Feb 8, 2014
1 parent 0c23b22 commit 3eef34a
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Showing 3 changed files with 203 additions and 42 deletions.
175 changes: 165 additions & 10 deletions fs/kernfs/dir.c
Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,8 @@
#include "kernfs-internal.h"

DEFINE_MUTEX(kernfs_mutex);
static DEFINE_SPINLOCK(kernfs_rename_lock); /* kn->parent and ->name */
static char kernfs_pr_cont_buf[PATH_MAX]; /* protected by rename_lock */

#define rb_to_kn(X) rb_entry((X), struct kernfs_node, rb)

Expand All @@ -37,6 +39,141 @@ static bool kernfs_lockdep(struct kernfs_node *kn)
#endif
}

static int kernfs_name_locked(struct kernfs_node *kn, char *buf, size_t buflen)
{
return strlcpy(buf, kn->parent ? kn->name : "/", buflen);
}

static char * __must_check kernfs_path_locked(struct kernfs_node *kn, char *buf,
size_t buflen)
{
char *p = buf + buflen;
int len;

*--p = '\0';

do {
len = strlen(kn->name);
if (p - buf < len + 1) {
buf[0] = '\0';
p = NULL;
break;
}
p -= len;
memcpy(p, kn->name, len);
*--p = '/';
kn = kn->parent;
} while (kn && kn->parent);

return p;
}

/**
* kernfs_name - obtain the name of a given node
* @kn: kernfs_node of interest
* @buf: buffer to copy @kn's name into
* @buflen: size of @buf
*
* Copies the name of @kn into @buf of @buflen bytes. The behavior is
* similar to strlcpy(). It returns the length of @kn's name and if @buf
* isn't long enough, it's filled upto @buflen-1 and nul terminated.
*
* This function can be called from any context.
*/
int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
{
unsigned long flags;
int ret;

spin_lock_irqsave(&kernfs_rename_lock, flags);
ret = kernfs_name_locked(kn, buf, buflen);
spin_unlock_irqrestore(&kernfs_rename_lock, flags);
return ret;
}

/**
* kernfs_path - build full path of a given node
* @kn: kernfs_node of interest
* @buf: buffer to copy @kn's name into
* @buflen: size of @buf
*
* Builds and returns the full path of @kn in @buf of @buflen bytes. The
* path is built from the end of @buf so the returned pointer usually
* doesn't match @buf. If @buf isn't long enough, @buf is nul terminated
* and %NULL is returned.
*/
char *kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen)
{
unsigned long flags;
char *p;

spin_lock_irqsave(&kernfs_rename_lock, flags);
p = kernfs_path_locked(kn, buf, buflen);
spin_unlock_irqrestore(&kernfs_rename_lock, flags);
return p;
}

/**
* pr_cont_kernfs_name - pr_cont name of a kernfs_node
* @kn: kernfs_node of interest
*
* This function can be called from any context.
*/
void pr_cont_kernfs_name(struct kernfs_node *kn)
{
unsigned long flags;

spin_lock_irqsave(&kernfs_rename_lock, flags);

kernfs_name_locked(kn, kernfs_pr_cont_buf, sizeof(kernfs_pr_cont_buf));
pr_cont("%s", kernfs_pr_cont_buf);

spin_unlock_irqrestore(&kernfs_rename_lock, flags);
}

/**
* pr_cont_kernfs_path - pr_cont path of a kernfs_node
* @kn: kernfs_node of interest
*
* This function can be called from any context.
*/
void pr_cont_kernfs_path(struct kernfs_node *kn)
{
unsigned long flags;
char *p;

spin_lock_irqsave(&kernfs_rename_lock, flags);

p = kernfs_path_locked(kn, kernfs_pr_cont_buf,
sizeof(kernfs_pr_cont_buf));
if (p)
pr_cont("%s", p);
else
pr_cont("<name too long>");

spin_unlock_irqrestore(&kernfs_rename_lock, flags);
}

/**
* kernfs_get_parent - determine the parent node and pin it
* @kn: kernfs_node of interest
*
* Determines @kn's parent, pins and returns it. This function can be
* called from any context.
*/
struct kernfs_node *kernfs_get_parent(struct kernfs_node *kn)
{
struct kernfs_node *parent;
unsigned long flags;

spin_lock_irqsave(&kernfs_rename_lock, flags);
parent = kn->parent;
kernfs_get(parent);
spin_unlock_irqrestore(&kernfs_rename_lock, flags);

return parent;
}

/**
* kernfs_name_hash
* @name: Null terminated string to hash
Expand Down Expand Up @@ -1103,8 +1240,14 @@ int kernfs_remove_by_name_ns(struct kernfs_node *parent, const char *name,
int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
const char *new_name, const void *new_ns)
{
struct kernfs_node *old_parent;
const char *old_name = NULL;
int error;

/* can't move or rename root */
if (!kn->parent)
return -EINVAL;

mutex_lock(&kernfs_mutex);

error = -ENOENT;
Expand All @@ -1126,26 +1269,38 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
new_name = kstrdup(new_name, GFP_KERNEL);
if (!new_name)
goto out;

if (kn->flags & KERNFS_STATIC_NAME)
kn->flags &= ~KERNFS_STATIC_NAME;
else
kfree(kn->name);

kn->name = new_name;
} else {
new_name = NULL;
}

/*
* Move to the appropriate place in the appropriate directories rbtree.
*/
kernfs_unlink_sibling(kn);
kernfs_get(new_parent);
kernfs_put(kn->parent);
kn->ns = new_ns;
kn->hash = kernfs_name_hash(kn->name, kn->ns);

/* rename_lock protects ->parent and ->name accessors */
spin_lock_irq(&kernfs_rename_lock);

old_parent = kn->parent;
kn->parent = new_parent;

kn->ns = new_ns;
if (new_name) {
if (!(kn->flags & KERNFS_STATIC_NAME))
old_name = kn->name;
kn->flags &= ~KERNFS_STATIC_NAME;
kn->name = new_name;
}

spin_unlock_irq(&kernfs_rename_lock);

kn->hash = kernfs_name_hash(new_name, new_ns);
kernfs_link_sibling(kn);

kernfs_put(old_parent);
kfree(old_name);

error = 0;
out:
mutex_unlock(&kernfs_mutex);
Expand Down
44 changes: 13 additions & 31 deletions fs/sysfs/dir.c
Original file line number Diff line number Diff line change
Expand Up @@ -19,39 +19,18 @@

DEFINE_SPINLOCK(sysfs_symlink_target_lock);

/**
* sysfs_pathname - return full path to sysfs dirent
* @kn: kernfs_node whose path we want
* @path: caller allocated buffer of size PATH_MAX
*
* Gives the name "/" to the sysfs_root entry; any path returned
* is relative to wherever sysfs is mounted.
*/
static char *sysfs_pathname(struct kernfs_node *kn, char *path)
{
if (kn->parent) {
sysfs_pathname(kn->parent, path);
strlcat(path, "/", PATH_MAX);
}
strlcat(path, kn->name, PATH_MAX);
return path;
}

void sysfs_warn_dup(struct kernfs_node *parent, const char *name)
{
char *path;
char *buf, *path = NULL;

path = kzalloc(PATH_MAX, GFP_KERNEL);
if (path) {
sysfs_pathname(parent, path);
strlcat(path, "/", PATH_MAX);
strlcat(path, name, PATH_MAX);
}
buf = kzalloc(PATH_MAX, GFP_KERNEL);
if (buf)
path = kernfs_path(parent, buf, PATH_MAX);

WARN(1, KERN_WARNING "sysfs: cannot create duplicate filename '%s'\n",
path ? path : name);
WARN(1, KERN_WARNING "sysfs: cannot create duplicate filename '%s/%s'\n",
path, name);

kfree(path);
kfree(buf);
}

/**
Expand Down Expand Up @@ -122,9 +101,13 @@ void sysfs_remove_dir(struct kobject *kobj)
int sysfs_rename_dir_ns(struct kobject *kobj, const char *new_name,
const void *new_ns)
{
struct kernfs_node *parent = kobj->sd->parent;
struct kernfs_node *parent;
int ret;

return kernfs_rename_ns(kobj->sd, parent, new_name, new_ns);
parent = kernfs_get_parent(kobj->sd);
ret = kernfs_rename_ns(kobj->sd, parent, new_name, new_ns);
kernfs_put(parent);
return ret;
}

int sysfs_move_dir_ns(struct kobject *kobj, struct kobject *new_parent_kobj,
Expand All @@ -133,7 +116,6 @@ int sysfs_move_dir_ns(struct kobject *kobj, struct kobject *new_parent_kobj,
struct kernfs_node *kn = kobj->sd;
struct kernfs_node *new_parent;

BUG_ON(!kn->parent);
new_parent = new_parent_kobj && new_parent_kobj->sd ?
new_parent_kobj->sd : sysfs_root_kn;

Expand Down
26 changes: 25 additions & 1 deletion include/linux/kernfs.h
Original file line number Diff line number Diff line change
Expand Up @@ -91,7 +91,12 @@ struct kernfs_node {
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
/* the following two fields are published */
/*
* Use kernfs_get_parent() and kernfs_name/path() instead of
* accessing the following two fields directly. If the node is
* never moved to a different parent, it is safe to access the
* parent directly.
*/
struct kernfs_node *parent;
const char *name;

Expand Down Expand Up @@ -229,6 +234,12 @@ static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
return kn->flags & KERNFS_NS;
}

int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen);
char * __must_check kernfs_path(struct kernfs_node *kn, char *buf,
size_t buflen);
void pr_cont_kernfs_name(struct kernfs_node *kn);
void pr_cont_kernfs_path(struct kernfs_node *kn);
struct kernfs_node *kernfs_get_parent(struct kernfs_node *kn);
struct kernfs_node *kernfs_find_and_get_ns(struct kernfs_node *parent,
const char *name, const void *ns);
void kernfs_get(struct kernfs_node *kn);
Expand Down Expand Up @@ -283,6 +294,19 @@ static inline void kernfs_enable_ns(struct kernfs_node *kn) { }
static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
{ return false; }

static inline int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
{ return -ENOSYS; }

static inline char * __must_check kernfs_path(struct kernfs_node *kn, char *buf,
size_t buflen)
{ return NULL; }

static inline void pr_cont_kernfs_name(struct kernfs_node *kn) { }
static inline void pr_cont_kernfs_path(struct kernfs_node *kn) { }

static inline struct kernfs_node *kernfs_get_parent(struct kernfs_node *kn)
{ return NULL; }

static inline struct kernfs_node *
kernfs_find_and_get_ns(struct kernfs_node *parent, const char *name,
const void *ns)
Expand Down

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