[PATCH 3/7] namei: stash the sampled ->d_seq into nameidata

Al Viro
2022-07-04T23:15:14Z

New field: nd->next_seq. Set to 0 outside of RCU mode, holds the sampled
value for the next dentry to be considered. Used instead of an arseload
of local variables, arguments, etc.

step_into() has lost seq argument; nd->next_seq is used, so dentry passed
to it must be the one ->next_seq is about.

There are two requirements for RCU pathwalk:
1) it should not give a hard failure (other than -ECHILD) unless
non-RCU pathwalk might fail that way given suitable timings.
2) it should not succeed unless non-RCU pathwalk might succeed
with the same end location given suitable timings.

The use of seq numbers is the way we achieve that. Invariant we want
to maintain is:
if RCU pathwalk can reach the state with given nd->path, nd->inode
and nd->seq after having traversed some part of pathname, it must be possible
for non-RCU pathwalk to reach the same nd->path and nd->inode after having
traversed the same part of pathname, and observe the nd->path.dentry->d_seq
equal to what RCU pathwalk has in nd->seq

For transition from parent to child, we sample child's ->d_seq
and verify that parent's ->d_seq remains unchanged.
For transitions from child to parent we sample parent's ->d_seq
and verify that child's ->d_seq has not changed.
For transition from mountpoint to root of mounted we sample
the ->d_seq of root and verify that nobody has touched mount_lock since
the beginning of pathwalk. That guarantees that mount we'd found had
been there all along, with these mountpoint and root of mounted.
It would be possible for a non-RCU pathwalk to reach the previous state,
find the same mount and observe its root at the moment we'd sampled
->d_seq of that
For transitions from root of mounted to mountpoint we sample
->d_seq of mountpoint and verify that mount_lock had not been touched
since the beginning of pathwalk. The same reasoning as in the
previous case applies.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/namei.c | 104 ++++++++++++++++++++++++++---------------------------
1 file changed, 52 insertions(+), 52 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index ecdb9ac21ece..c7c9e88add85 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -567,7 +567,7 @@ struct nameidata {
struct path root;
struct inode *inode; /* path.dentry.d_inode */
unsigned int flags, state;
- unsigned seq, m_seq, r_seq;
+ unsigned seq, next_seq, m_seq, r_seq;
int last_type;
unsigned depth;
int total_link_count;
@@ -772,6 +772,7 @@ static bool try_to_unlazy(struct nameidata *nd)
goto out;
if (unlikely(!legitimize_root(nd)))
goto out;
+ nd->seq = nd->next_seq = 0;
rcu_read_unlock();
BUG_ON(nd->inode != parent->d_inode);
return true;
@@ -780,6 +781,7 @@ static bool try_to_unlazy(struct nameidata *nd)
nd->path.mnt = NULL;
nd->path.dentry = NULL;
out:
+ nd->seq = nd->next_seq = 0;
rcu_read_unlock();
return false;
}
@@ -788,7 +790,6 @@ static bool try_to_unlazy(struct nameidata *nd)
* try_to_unlazy_next - try to switch to ref-walk mode.
* @nd: nameidata pathwalk data
* @dentry: next dentry to step into
- * @seq: seq number to check @dentry against
* Returns: true on success, false on failure
*
* Similar to try_to_unlazy(), but here we have the next dentry already
@@ -797,7 +798,7 @@ static bool try_to_unlazy(struct nameidata *nd)
* Nothing should touch nameidata between try_to_unlazy_next() failure and
* terminate_walk().
*/
-static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry, unsigned seq)
+static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry)
{
BUG_ON(!(nd->flags & LOOKUP_RCU));

@@ -818,7 +819,7 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry, unsi
*/
if (unlikely(!lockref_get_not_dead(&dentry->d_lockref)))
goto out;
- if (unlikely(read_seqcount_retry(&dentry->d_seq, seq)))
+ if (unlikely(read_seqcount_retry(&dentry->d_seq, nd->next_seq)))
goto out_dput;
/*
* Sequence counts matched. Now make sure that the root is
@@ -826,6 +827,7 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry, unsi
*/
if (unlikely(!legitimize_root(nd)))
goto out_dput;
+ nd->seq = nd->next_seq = 0;
rcu_read_unlock();
return true;

@@ -834,9 +836,11 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry, unsi
out1:
nd->path.dentry = NULL;
out:
+ nd->seq = nd->next_seq = 0;
rcu_read_unlock();
return false;
out_dput:
+ nd->seq = nd->next_seq = 0;
rcu_read_unlock();
dput(dentry);
return false;
@@ -1467,7 +1471,7 @@ EXPORT_SYMBOL(follow_down);
* we meet a managed dentry that would need blocking.
*/
static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
- struct inode **inode, unsigned *seqp)
+ struct inode **inode)
{
struct dentry *dentry = path->dentry;
unsigned int flags = dentry->d_flags;
@@ -1496,7 +1500,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
path->mnt = &mounted->mnt;
dentry = path->dentry = mounted->mnt.mnt_root;
nd->state |= ND_JUMPED;
- *seqp = read_seqcount_begin(&dentry->d_seq);
+ nd->next_seq = read_seqcount_begin(&dentry->d_seq);
*inode = dentry->d_inode;
/*
* We don't need to re-check ->d_seq after this
@@ -1505,6 +1509,8 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
* becoming unpinned.
*/
flags = dentry->d_flags;
+ // makes sure that non-RCU pathwalk could reach
+ // this state.
if (read_seqretry(&mount_lock, nd->m_seq))
return false;
continue;
@@ -1517,8 +1523,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
}

static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
- struct path *path, struct inode **inode,
- unsigned int *seqp)
+ struct path *path, struct inode **inode)
{
bool jumped;
int ret;
@@ -1526,16 +1531,15 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
path->mnt = nd->path.mnt;
path->dentry = dentry;
if (nd->flags & LOOKUP_RCU) {
- unsigned int seq = *seqp;
- if (unlikely(!*inode))
- return -ENOENT;
- if (likely(__follow_mount_rcu(nd, path, inode, seqp)))
+ unsigned int seq = nd->next_seq;
+ if (likely(__follow_mount_rcu(nd, path, inode)))
return 0;
- if (!try_to_unlazy_next(nd, dentry, seq))
- return -ECHILD;
- // *path might've been clobbered by __follow_mount_rcu()
+ // *path and nd->next_seq might've been clobbered
path->mnt = nd->path.mnt;
path->dentry = dentry;
+ nd->next_seq = seq;
+ if (!try_to_unlazy_next(nd, dentry))
+ return -ECHILD;
}
ret = traverse_mounts(path, &jumped, &nd->total_link_count, nd->flags);
if (jumped) {
@@ -1550,7 +1554,6 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
mntput(path->mnt);
} else {
*inode = d_backing_inode(path->dentry);
- *seqp = 0; /* out of RCU mode, so the value doesn't matter */
}
return ret;
}
@@ -1610,8 +1613,7 @@ static struct dentry *__lookup_hash(const struct qstr *name,
}

static struct dentry *lookup_fast(struct nameidata *nd,
- struct inode **inode,
- unsigned *seqp)
+ struct inode **inode)
{
struct dentry *dentry, *parent = nd->path.dentry;
int status = 1;
@@ -1622,8 +1624,7 @@ static struct dentry *lookup_fast(struct nameidata *nd,
* going to fall back to non-racy lookup.
*/
if (nd->flags & LOOKUP_RCU) {
- unsigned seq;
- dentry = __d_lookup_rcu(parent, &nd->last, &seq);
+ dentry = __d_lookup_rcu(parent, &nd->last, &nd->next_seq);
if (unlikely(!dentry)) {
if (!try_to_unlazy(nd))
return ERR_PTR(-ECHILD);
@@ -1635,7 +1636,7 @@ static struct dentry *lookup_fast(struct nameidata *nd,
* the dentry name information from lookup.
*/
*inode = d_backing_inode(dentry);
- if (unlikely(read_seqcount_retry(&dentry->d_seq, seq)))
+ if (unlikely(read_seqcount_retry(&dentry->d_seq, nd->next_seq)))
return ERR_PTR(-ECHILD);

/*
@@ -1648,11 +1649,10 @@ static struct dentry *lookup_fast(struct nameidata *nd,
if (unlikely(__read_seqcount_retry(&parent->d_seq, nd->seq)))
return ERR_PTR(-ECHILD);

- *seqp = seq;
status = d_revalidate(dentry, nd->flags);
if (likely(status > 0))
return dentry;
- if (!try_to_unlazy_next(nd, dentry, seq))
+ if (!try_to_unlazy_next(nd, dentry))
return ERR_PTR(-ECHILD);
if (status == -ECHILD)
/* we'd been told to redo it in non-rcu mode */
@@ -1733,7 +1733,7 @@ static inline int may_lookup(struct user_namespace *mnt_userns,
return inode_permission(mnt_userns, nd->inode, MAY_EXEC);
}

-static int reserve_stack(struct nameidata *nd, struct path *link, unsigned seq)
+static int reserve_stack(struct nameidata *nd, struct path *link)
{
if (unlikely(nd->total_link_count++ >= MAXSYMLINKS))
return -ELOOP;
@@ -1748,7 +1748,7 @@ static int reserve_stack(struct nameidata *nd, struct path *link, unsigned seq)
if (nd->flags & LOOKUP_RCU) {
// we need to grab link before we do unlazy. And we can't skip
// unlazy even if we fail to grab the link - cleanup needs it
- bool grabbed_link = legitimize_path(nd, link, seq);
+ bool grabbed_link = legitimize_path(nd, link, nd->next_seq);

if (!try_to_unlazy(nd) || !grabbed_link)
return -ECHILD;
@@ -1762,11 +1762,11 @@ static int reserve_stack(struct nameidata *nd, struct path *link, unsigned seq)
enum {WALK_TRAILING = 1, WALK_MORE = 2, WALK_NOFOLLOW = 4};

static const char *pick_link(struct nameidata *nd, struct path *link,
- struct inode *inode, unsigned seq, int flags)
+ struct inode *inode, int flags)
{
struct saved *last;
const char *res;
- int error = reserve_stack(nd, link, seq);
+ int error = reserve_stack(nd, link);

if (unlikely(error)) {
if (!(nd->flags & LOOKUP_RCU))
@@ -1776,7 +1776,7 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
last = nd->stack + nd->depth++;
last->link = *link;
clear_delayed_call(&last->done);
- last->seq = seq;
+ last->seq = nd->next_seq;

if (flags & WALK_TRAILING) {
error = may_follow_link(nd, inode);
@@ -1838,12 +1838,14 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
* to do this check without having to look at inode->i_op,
* so we keep a cache of "no, this doesn't need follow_link"
* for the common case.
+ *
+ * NOTE: dentry must be what nd->next_seq had been sampled from.
*/
static const char *step_into(struct nameidata *nd, int flags,
- struct dentry *dentry, struct inode *inode, unsigned seq)
+ struct dentry *dentry, struct inode *inode)
{
struct path path;
- int err = handle_mounts(nd, dentry, &path, &inode, &seq);
+ int err = handle_mounts(nd, dentry, &path, &inode);

if (err < 0)
return ERR_PTR(err);
@@ -1858,23 +1860,22 @@ static const char *step_into(struct nameidata *nd, int flags,
}
nd->path = path;
nd->inode = inode;
- nd->seq = seq;
+ nd->seq = nd->next_seq;
return NULL;
}
if (nd->flags & LOOKUP_RCU) {
/* make sure that d_is_symlink above matches inode */
- if (read_seqcount_retry(&path.dentry->d_seq, seq))
+ if (read_seqcount_retry(&path.dentry->d_seq, nd->next_seq))
return ERR_PTR(-ECHILD);
} else {
if (path.mnt == nd->path.mnt)
mntget(path.mnt);
}
- return pick_link(nd, &path, inode, seq, flags);
+ return pick_link(nd, &path, inode, flags);
}

static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
- struct inode **inodep,
- unsigned *seqp)
+ struct inode **inodep)
{
struct dentry *parent, *old;

@@ -1891,6 +1892,7 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
nd->path = path;
nd->inode = path.dentry->d_inode;
nd->seq = seq;
+ // makes sure that non-RCU pathwalk could reach this state
if (unlikely(read_seqretry(&mount_lock, nd->m_seq)))
return ERR_PTR(-ECHILD);
/* we know that mountpoint was pinned */
@@ -1898,7 +1900,8 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
old = nd->path.dentry;
parent = old->d_parent;
*inodep = parent->d_inode;
- *seqp = read_seqcount_begin(&parent->d_seq);
+ nd->next_seq = read_seqcount_begin(&parent->d_seq);
+ // makes sure that non-RCU pathwalk could reach this state
if (unlikely(read_seqcount_retry(&old->d_seq, nd->seq)))
return ERR_PTR(-ECHILD);
if (unlikely(!path_connected(nd->path.mnt, parent)))
@@ -1909,14 +1912,13 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
return ERR_PTR(-ECHILD);
if (unlikely(nd->flags & LOOKUP_BENEATH))
return ERR_PTR(-ECHILD);
- *seqp = nd->seq;
+ nd->next_seq = nd->seq;
*inodep = nd->path.dentry->d_inode;
return nd->path.dentry;
}

static struct dentry *follow_dotdot(struct nameidata *nd,
- struct inode **inodep,
- unsigned *seqp)
+ struct inode **inodep)
{
struct dentry *parent;

@@ -1940,14 +1942,12 @@ static struct dentry *follow_dotdot(struct nameidata *nd,
dput(parent);
return ERR_PTR(-ENOENT);
}
- *seqp = 0;
*inodep = parent->d_inode;
return parent;

in_root:
if (unlikely(nd->flags & LOOKUP_BENEATH))
return ERR_PTR(-EXDEV);
- *seqp = 0;
*inodep = nd->path.dentry->d_inode;
return dget(nd->path.dentry);
}
@@ -1958,7 +1958,6 @@ static const char *handle_dots(struct nameidata *nd, int type)
const char *error = NULL;
struct dentry *parent;
struct inode *inode;
- unsigned seq;

if (!nd->root.mnt) {
error = ERR_PTR(set_root(nd));
@@ -1966,12 +1965,12 @@ static const char *handle_dots(struct nameidata *nd, int type)
return error;
}
if (nd->flags & LOOKUP_RCU)
- parent = follow_dotdot_rcu(nd, &inode, &seq);
+ parent = follow_dotdot_rcu(nd, &inode);
else
- parent = follow_dotdot(nd, &inode, &seq);
+ parent = follow_dotdot(nd, &inode);
if (IS_ERR(parent))
return ERR_CAST(parent);
- error = step_into(nd, WALK_NOFOLLOW, parent, inode, seq);
+ error = step_into(nd, WALK_NOFOLLOW, parent, inode);
if (unlikely(error))
return error;

@@ -1996,7 +1995,6 @@ static const char *walk_component(struct nameidata *nd, int flags)
{
struct dentry *dentry;
struct inode *inode;
- unsigned seq;
/*
* "." and ".." are special - ".." especially so because it has
* to be able to know about the current root directory and
@@ -2007,7 +2005,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
put_link(nd);
return handle_dots(nd, nd->last_type);
}
- dentry = lookup_fast(nd, &inode, &seq);
+ dentry = lookup_fast(nd, &inode);
if (IS_ERR(dentry))
return ERR_CAST(dentry);
if (unlikely(!dentry)) {
@@ -2017,7 +2015,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
}
if (!(flags & WALK_MORE) && nd->depth)
put_link(nd);
- return step_into(nd, flags, dentry, inode, seq);
+ return step_into(nd, flags, dentry, inode);
}

/*
@@ -2372,6 +2370,8 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
flags &= ~LOOKUP_RCU;
if (flags & LOOKUP_RCU)
rcu_read_lock();
+ else
+ nd->seq = nd->next_seq = 0;

nd->flags = flags;
nd->state |= ND_JUMPED;
@@ -2473,8 +2473,9 @@ static int handle_lookup_down(struct nameidata *nd)
{
if (!(nd->flags & LOOKUP_RCU))
dget(nd->path.dentry);
+ nd->next_seq = nd->seq;
return PTR_ERR(step_into(nd, WALK_NOFOLLOW,
- nd->path.dentry, nd->inode, nd->seq));
+ nd->path.dentry, nd->inode));
}

/* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
@@ -3393,7 +3394,6 @@ static const char *open_last_lookups(struct nameidata *nd,
struct dentry *dir = nd->path.dentry;
int open_flag = op->open_flag;
bool got_write = false;
- unsigned seq;
struct inode *inode;
struct dentry *dentry;
const char *res;
@@ -3410,7 +3410,7 @@ static const char *open_last_lookups(struct nameidata *nd,
if (nd->last.name[nd->last.len])
nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
/* we _can_ be in RCU mode here */
- dentry = lookup_fast(nd, &inode, &seq);
+ dentry = lookup_fast(nd, &inode);
if (IS_ERR(dentry))
return ERR_CAST(dentry);
if (likely(dentry))
@@ -3464,7 +3464,7 @@ static const char *open_last_lookups(struct nameidata *nd,
finish_lookup:
if (nd->depth)
put_link(nd);
- res = step_into(nd, WALK_TRAILING, dentry, inode, seq);
+ res = step_into(nd, WALK_TRAILING, dentry, inode);
if (unlikely(res))
nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
return res;
--
2.30.2