Re: [PATCH 1/7] __follow_mount_rcu(): verify that mount_lock remains unchanged

Al Viro
2022-07-04T23:20:37Z

Just in case - this series is RFC only; it's very lightly
tested. Might be carved into too many steps, at that. Cumulative
delta follows; might or might not be more convenient for review.

diff --git a/fs/namei.c b/fs/namei.c
index 1f28d3f463c3..f2c99e75b578 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;
@@ -1466,8 +1470,7 @@ EXPORT_SYMBOL(follow_down);
* Try to skip to top of mountpoint pile in rcuwalk mode. Fail if
* 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)
+static bool __follow_mount_rcu(struct nameidata *nd, struct path *path)
{
struct dentry *dentry = path->dentry;
unsigned int flags = dentry->d_flags;
@@ -1496,15 +1499,12 @@ 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);
- *inode = dentry->d_inode;
- /*
- * We don't need to re-check ->d_seq after this
- * ->d_inode read - there will be an RCU delay
- * between mount hash removal and ->mnt_root
- * becoming unpinned.
- */
+ nd->next_seq = read_seqcount_begin(&dentry->d_seq);
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;
}
if (read_seqretry(&mount_lock, nd->m_seq))
@@ -1515,8 +1515,7 @@ 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)
{
bool jumped;
int ret;
@@ -1524,16 +1523,15 @@ 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)))
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) {
@@ -1546,9 +1544,6 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
dput(path->dentry);
if (path->mnt != nd->path.mnt)
mntput(path->mnt);
- } else {
- *inode = d_backing_inode(path->dentry);
- *seqp = 0; /* out of RCU mode, so the value doesn't matter */
}
return ret;
}
@@ -1607,9 +1602,7 @@ static struct dentry *__lookup_hash(const struct qstr *name,
return dentry;
}

-static struct dentry *lookup_fast(struct nameidata *nd,
- struct inode **inode,
- unsigned *seqp)
+static struct dentry *lookup_fast(struct nameidata *nd)
{
struct dentry *dentry, *parent = nd->path.dentry;
int status = 1;
@@ -1620,37 +1613,24 @@ 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);
return NULL;
}

- /*
- * This sequence count validates that the inode matches
- * the dentry name information from lookup.
- */
- *inode = d_backing_inode(dentry);
- if (unlikely(read_seqcount_retry(&dentry->d_seq, seq)))
- return ERR_PTR(-ECHILD);
-
- /*
+ /*
* This sequence count validates that the parent had no
* changes while we did the lookup of the dentry above.
- *
- * The memory barrier in read_seqcount_begin of child is
- * enough, we can use __read_seqcount_retry here.
*/
- if (unlikely(__read_seqcount_retry(&parent->d_seq, nd->seq)))
+ 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 */
@@ -1731,7 +1711,7 @@ 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;
@@ -1746,7 +1726,7 @@ 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;
@@ -1760,11 +1740,11 @@ 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))
@@ -1774,7 +1754,7 @@ 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);
@@ -1836,43 +1816,50 @@ 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 path path;
- int err = handle_mounts(nd, dentry, &path, &inode, &seq);
+ struct inode *inode;
+ int err = handle_mounts(nd, dentry, &path);

if (err < 0)
return ERR_PTR(err);
+ inode = path.dentry->d_inode;
if (likely(!d_is_symlink(path.dentry)) ||
((flags & WALK_TRAILING) && !(nd->flags & LOOKUP_FOLLOW)) ||
(flags & WALK_NOFOLLOW)) {
/* not a symlink or should not follow */
- if (!(nd->flags & LOOKUP_RCU)) {
+ if (nd->flags & LOOKUP_RCU) {
+ if (read_seqcount_retry(&path.dentry->d_seq, nd->next_seq))
+ return ERR_PTR(-ECHILD);
+ if (unlikely(!inode))
+ return ERR_PTR(-ENOENT);
+ } else {
dput(nd->path.dentry);
if (nd->path.mnt != path.mnt)
mntput(nd->path.mnt);
}
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)
+static struct dentry *follow_dotdot_rcu(struct nameidata *nd)
{
struct dentry *parent, *old;

@@ -1889,14 +1876,15 @@ 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 */
}
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)))
@@ -1907,12 +1895,11 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
return ERR_PTR(-ECHILD);
if (unlikely(nd->flags & LOOKUP_BENEATH))
return ERR_PTR(-ECHILD);
- return NULL;
+ nd->next_seq = nd->seq;
+ return nd->path.dentry;
}

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

@@ -1936,15 +1923,12 @@ 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);
- dget(nd->path.dentry);
- return NULL;
+ return dget(nd->path.dentry);
}

static const char *handle_dots(struct nameidata *nd, int type)
@@ -1952,8 +1936,6 @@ static const char *handle_dots(struct nameidata *nd, int type)
if (type == LAST_DOTDOT) {
const char *error = NULL;
struct dentry *parent;
- struct inode *inode;
- unsigned seq;

if (!nd->root.mnt) {
error = ERR_PTR(set_root(nd));
@@ -1961,17 +1943,12 @@ 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);
else
- parent = follow_dotdot(nd, &inode, &seq);
+ parent = follow_dotdot(nd);
if (IS_ERR(parent))
return ERR_CAST(parent);
- if (unlikely(!parent))
- error = step_into(nd, WALK_NOFOLLOW,
- nd->path.dentry, nd->inode, nd->seq);
- else
- error = step_into(nd, WALK_NOFOLLOW,
- parent, inode, seq);
+ error = step_into(nd, WALK_NOFOLLOW, parent);
if (unlikely(error))
return error;

@@ -1995,8 +1972,6 @@ static const char *handle_dots(struct nameidata *nd, int type)
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 +1982,7 @@ 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);
if (IS_ERR(dentry))
return ERR_CAST(dentry);
if (unlikely(!dentry)) {
@@ -2017,7 +1992,7 @@ 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);
}

/*
@@ -2372,6 +2347,8 @@ 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 +2450,8 @@ static int handle_lookup_down(struct nameidata *nd)
{
if (!(nd->flags & LOOKUP_RCU))
dget(nd->path.dentry);
- return PTR_ERR(step_into(nd, WALK_NOFOLLOW,
- nd->path.dentry, nd->inode, nd->seq));
+ nd->next_seq = nd->seq;
+ return PTR_ERR(step_into(nd, WALK_NOFOLLOW, nd->path.dentry));
}

/* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
@@ -3393,8 +3370,6 @@ 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 +3385,7 @@ 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);
if (IS_ERR(dentry))
return ERR_CAST(dentry);
if (likely(dentry))
@@ -3464,7 +3439,7 @@ 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);
if (unlikely(res))
nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
return res;