* [PATCH RFC v2 1/2] pidfs: keep pidfs dentry stashed once created
2025-06-17 15:39 [PATCH RFC v2 0/2] pidfs: keep pidfs dentry stashed once created Christian Brauner
@ 2025-06-17 15:39 ` Christian Brauner
2025-06-18 8:40 ` Christian Brauner
2025-06-17 15:39 ` [PATCH RFC v2 2/2] pidfs: remove pidfs_pid_valid() Christian Brauner
1 sibling, 1 reply; 4+ messages in thread
From: Christian Brauner @ 2025-06-17 15:39 UTC (permalink / raw)
To: Oleg Nesterov, linux-fsdevel; +Cc: Christian Brauner
Keep pidfs dentries around after a pidfd has been created for it. The
pidfs dentry will only be cleaned up once the struct pid gets reaped.
The current scheme allocated pidfs dentries on-demand repeatedly.
This scheme is reaching it's limits as it makes it impossible to pin
information that needs to be available after the task has exited or
coredumped and that should not be lost simply because the pidfd got
closed temporarily. The next opener should still see the stashed
information.
If someone opens a pidfd for a struct pid a pidfs dentry is allocated
and stashed in pid->stashed. Once the last pidfd for the struct pid is
closed the pidfs dentry is released and removed from pid->stashed.
So if 10 callers create a pidfs for the same struct pid sequentially,
i.e., each closing the pidfd before the other creates a new one then a
new pidfs dentry is allocated every time.
Because multiple tasks acquiring and releasing a pidfd for the same
struct pid can race with each another a task may still find a valid
pidfs entry from the previous task in pid->stashed and reuse it. Or it
might find a dead dentry in there and fail to reuse it and so stashes a
new pidfs dentry. Multiple tasks may race to stash a new pidfs dentry
but only one will succeed, the other ones will put their dentry.
The current scheme aims to ensure that a pidfs dentry for a struct pid
can only be created if the task is still alive or if a pidfs dentry
already existed before the task was reaped and so exit information has
been was stashed in the pidfs inode.
That's great expect that it's buggy. If a pidfs dentry is stashed in
pid->stashed after pidfs_exit() but before __unhash_process() is called
we will return a pidfd for a reaped task without exit information being
available.
The pidfds_pid_valid() check does not guard against this race as it
doens't sync at all with pidfs_exit(). The pid_has_task() check might be
successful simply because we're before __unhash_process() but after
pidfs_exit().
This switches to a scheme were pidfs entries are retained after a pidfd
was created for the struct pid. So when allocating a pidfds dentry an
extra reference is retained that is owned by the exit path and that will
be put once the task does get reaped. In the new model pidfs dentries
are still allocated on-demand but then kept until the task gets reaped.
The synchronization mechanism uses the pid->wait_pidfd.lock in struct
pid to synchronize with pidfs_exit() called when the task is reaped. If
the path_from_stashed() fastpath fails, a new pidfs dentry is allocated
and afterwards the pid->wait_pidfd.lock is taken. If no other task
managed to stash its dentry there the callers will be stashed.
When the task is reaped and calls pidfs_exit() the pid->wait_pidfd.lock
is taken. Once pidfs_exit() holds the pid->wait_pidfd.lock and sees that
no pidfs dentry is available in pid->stashed it knows that no new dentry
can be stashed while it holds the pid->wait_pidfd.lock. It thus sets a
ERR_PTR(-ESRCH) sentinel in pid->stashed. That sentinel allows
pidfs_stash_dentry() to detect that the struct pid has already been
reaped and refuse to stash a new dentry in pid->stashed. That works both
in the fast- and slowpath.
This in turn allows us to fix the bug mentioned earlier where we hand
out a pidfd for a reaped task without having exit information set as we
now sync with pidfs_exit() and thus release_task().
This also has some subtle interactions with the path_from_stashed()
fastpath that need to be considered. The path_from_stashed() fast path
will try go get a reference to an already existing pidfs dentry in
pid->stashed to avoid having to allocate and stash a pidfs dentry. If it
finds a dentry in there it will return it.
To not confuse path_from_stashed() pidfs_exit() must not replace a pidfs
dentry stashed in pid->stashed with the ERR_PTR(-ESRCH) sentinel as
path_from_stashed() could legitimately obtain another reference before
pidfs_exit() was able to call dput() to put the final pidfs dentry
reference. If it were to put the sentinel into pid->stashed it would
invalidate a struct pid even though a pidfd was just created for it.
So if a pidfs dentry is stashed in pid->stashed pidfs_exit() must leave
clearing out pid->stashed to dentry->d_prune::pidfs_dentry_prune(). When
pruning a dentry we must take care to not take the pid->wait_pidfd.lock
as this would cause a lock inversion with dentry->d_lock in
pidfs_stash_dentry(). This should fortunately not at all be necessary as
by the time we call pidfs_dentry_prune() we know that the struct pid is
dead as the task is reaped and that anyone concurrently trying to get a
reference to the stashed dentry will fail to do so.
IOW, it doesn't matter whether the path_from_stashed() fast path sees
NULL, a dead dentry, or the ERR_PTR(-ESRCH) sentinel in pid->stashed.
Any of those forces path_from_stashed() into the slowpath at which point
pid->wait_pidfd.lock must be acquired. The slowpath will then see either
a dead dentry or the ERR_PTR(-ESRCH) sentinel but never NULL and thus
fail the creation of a new pidfs dentry.
path_from_stashed() must take care to not try and take a reference on
the ERR_PTR(-ESRCH) sentinel. So stashed_dentry_get() must be prepared
to see a ERR_PTR(-ESRCH) sentinel in pid->stashed.
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/internal.h | 2 +
fs/libfs.c | 22 +++++++--
fs/pidfs.c | 143 +++++++++++++++++++++++++++++++++++++++++++++++++++-------
kernel/pid.c | 2 +-
4 files changed, 147 insertions(+), 22 deletions(-)
diff --git a/fs/internal.h b/fs/internal.h
index 393f6c5c24f6..180b367c192b 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -322,6 +322,8 @@ struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns);
struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap);
void mnt_idmap_put(struct mnt_idmap *idmap);
struct stashed_operations {
+ struct dentry *(*stash_dentry)(struct dentry **stashed,
+ struct dentry *dentry);
void (*put_data)(void *data);
int (*init_inode)(struct inode *inode, void *data);
};
diff --git a/fs/libfs.c b/fs/libfs.c
index 9ea0ecc325a8..f496373869fb 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -2128,6 +2128,8 @@ struct dentry *stashed_dentry_get(struct dentry **stashed)
dentry = rcu_dereference(*stashed);
if (!dentry)
return NULL;
+ if (IS_ERR(dentry))
+ return dentry;
if (!lockref_get_not_dead(&dentry->d_lockref))
return NULL;
return dentry;
@@ -2218,12 +2220,15 @@ static struct dentry *stash_dentry(struct dentry **stashed,
int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
struct path *path)
{
- struct dentry *dentry;
+ struct dentry *dentry, *res;
const struct stashed_operations *sops = mnt->mnt_sb->s_fs_info;
/* See if dentry can be reused. */
- path->dentry = stashed_dentry_get(stashed);
- if (path->dentry) {
+ res = stashed_dentry_get(stashed);
+ if (IS_ERR(res))
+ return PTR_ERR(res);
+ if (res) {
+ path->dentry = res;
sops->put_data(data);
goto out_path;
}
@@ -2234,8 +2239,15 @@ int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
return PTR_ERR(dentry);
/* Added a new dentry. @data is now owned by the filesystem. */
- path->dentry = stash_dentry(stashed, dentry);
- if (path->dentry != dentry)
+ if (sops->stash_dentry)
+ res = sops->stash_dentry(stashed, dentry);
+ else
+ res = stash_dentry(stashed, dentry);
+ if (IS_ERR(res))
+ return PTR_ERR(res);
+ path->dentry = res;
+ /* A dentry was reused. */
+ if (res != dentry)
dput(dentry);
out_path:
diff --git a/fs/pidfs.c b/fs/pidfs.c
index c1f0a067be40..ee5e9a18c2d3 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -25,6 +25,20 @@
#include "internal.h"
#include "mount.h"
+/*
+ * Usage:
+ * pid->wait_pidfd.lock protects:
+ * - insertion of dentry into pid->stashed
+ * - deletion of pid[PIDTYPE_TGID] task linkage
+ * - pidfs_exit() which sets pid->stashed to NULL
+ *
+ * Ordering:
+ * -> dentry->d_lock
+ * -> pid->wait_pidfd.lock
+ */
+
+#define PIDFS_PID_DEAD ERR_PTR(-ESRCH)
+
static struct kmem_cache *pidfs_cachep __ro_after_init;
/*
@@ -552,30 +566,47 @@ struct pid *pidfd_pid(const struct file *file)
* task has been reaped which cannot happen until we're out of
* release_task().
*
- * If this struct pid is referred to by a pidfd then
- * stashed_dentry_get() will return the dentry and inode for that struct
- * pid. Since we've taken a reference on it there's now an additional
- * reference from the exit path on it. Which is fine. We're going to put
- * it again in a second and we know that the pid is kept alive anyway.
+ * If this struct pid has at least once been referred to by a pidfd then
+ * pid->stashed will contain a dentry. One reference exclusively belongs
+ * to pidfs_exit(). There might of course be other references.
*
* Worst case is that we've filled in the info and immediately free the
- * dentry and inode afterwards since the pidfd has been closed. Since
- * pidfs_exit() currently is placed after exit_task_work() we know that
- * it cannot be us aka the exiting task holding a pidfd to ourselves.
+ * dentry and inode afterwards when no one holds an open pidfd anymore.
+ * Since pidfs_exit() currently is placed after exit_task_work() we know
+ * that it cannot be us aka the exiting task holding a pidfd to itself.
*/
void pidfs_exit(struct task_struct *tsk)
{
struct dentry *dentry;
+ struct pid *pid = task_pid(tsk);
might_sleep();
- dentry = stashed_dentry_get(&task_pid(tsk)->stashed);
- if (dentry) {
- struct inode *inode = d_inode(dentry);
- struct pidfs_exit_info *exit_info = &pidfs_i(inode)->__pei;
+ scoped_guard(spinlock_irq, &pid->wait_pidfd.lock) {
+ struct inode *inode;
+ struct pidfs_exit_info *exit_info;
#ifdef CONFIG_CGROUPS
struct cgroup *cgrp;
+#endif
+ dentry = rcu_dereference_raw(pid->stashed);
+ if (!dentry) {
+ /*
+ * No one held a pidfd for this struct pid.
+ * Mark it as dead so no one can add a pidfs
+ * entry anymore. We're about to be reaped and
+ * so no exit information would be available.
+ */
+ rcu_assign_pointer(pid->stashed, PIDFS_PID_DEAD);
+ return;
+ }
+
+ /* We own a reference assert that clearly. */
+ VFS_WARN_ON_ONCE(__lockref_is_dead(&dentry->d_lockref));
+ inode = d_inode(dentry);
+ exit_info = &pidfs_i(inode)->__pei;
+
+#ifdef CONFIG_CGROUPS
rcu_read_lock();
cgrp = task_dfl_cgroup(tsk);
exit_info->cgroupid = cgroup_id(cgrp);
@@ -585,8 +616,15 @@ void pidfs_exit(struct task_struct *tsk)
/* Ensure that PIDFD_GET_INFO sees either all or nothing. */
smp_store_release(&pidfs_i(inode)->exit_info, &pidfs_i(inode)->__pei);
- dput(dentry);
}
+
+ /*
+ * If there was a dentry in there we own a reference to it. So
+ * accessing pid->stashed is safe. Note, pid->stashed will be
+ * cleared by pidfs. Leave it alone as we could end up in a
+ * legitimate race with path_from_stashed()'s fast path.
+ */
+ dput(dentry);
}
#ifdef CONFIG_COREDUMP
@@ -683,9 +721,44 @@ static char *pidfs_dname(struct dentry *dentry, char *buffer, int buflen)
return dynamic_dname(buffer, buflen, "anon_inode:[pidfd]");
}
+static void pidfs_dentry_prune(struct dentry *dentry)
+{
+ struct dentry **stashed = dentry->d_fsdata;
+ struct inode *inode = d_inode(dentry);
+ struct pid *pid;
+
+ if (WARN_ON_ONCE(!stashed))
+ return;
+
+ if (!inode)
+ return;
+
+ pid = inode->i_private;
+ VFS_WARN_ON_ONCE(!pid);
+ /*
+ * We can't acquire pid->wait_pidfd.lock as we're holding
+ * dentry->d_lock and pidfs_stash_dentry() needs to be able to
+ * hold dentry->d_lock under pid->wait_pidfd.lock.
+ *
+ * Also, we don't really need it...
+ */
+ VFS_WARN_ON_ONCE(!__lockref_is_dead(&dentry->d_lockref));
+ VFS_WARN_ON_ONCE(*stashed != dentry);
+
+ /*
+ * ... Because to get here the struct pid must have been reaped
+ * already && no one holds a pidfd open anymore. The only one
+ * that can race us is pidfd_stash_dentry(). Either it sees a
+ * dead dentry in pid->stashed or it sees our sentinel marking
+ * the struct pid as dead. There's no need to synchronize this
+ * with a lock.
+ */
+ smp_store_release(stashed, PIDFS_PID_DEAD);
+}
+
const struct dentry_operations pidfs_dentry_operations = {
.d_dname = pidfs_dname,
- .d_prune = stashed_dentry_prune,
+ .d_prune = pidfs_dentry_prune,
};
static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
@@ -878,9 +951,47 @@ static void pidfs_put_data(void *data)
put_pid(pid);
}
+static struct dentry *pidfs_stash_dentry(struct dentry **stashed,
+ struct dentry *dentry)
+{
+ struct pid *pid = d_inode(dentry)->i_private;
+ struct dentry *d;
+
+ VFS_WARN_ON_ONCE(stashed != &pid->stashed);
+
+ /* We need to synchronize with pidfs_exit(). */
+ guard(spinlock_irq)(&pid->wait_pidfd.lock);
+ guard(rcu)();
+
+ d = rcu_dereference(*stashed);
+ if (!d) {
+ /*
+ * There's nothing stashed in here so no pidfs dentry
+ * exists for this task yet and we're definitely not
+ * past pidfs_exit() and everyone else will wait for us.
+ */
+ rcu_assign_pointer(*stashed, dget(dentry));
+ return dentry;
+ }
+
+ /* The struct pid is already marked dead. */
+ if (IS_ERR(d))
+ return d;
+
+ /*
+ * The struct pid isn't yet marked dead but there's a dead
+ * dentry in there which is just as well.
+ */
+ if (!lockref_get_not_dead(&d->d_lockref))
+ return ERR_PTR(-ESRCH);
+
+ return d;
+}
+
static const struct stashed_operations pidfs_stashed_ops = {
- .init_inode = pidfs_init_inode,
- .put_data = pidfs_put_data,
+ .stash_dentry = pidfs_stash_dentry,
+ .init_inode = pidfs_init_inode,
+ .put_data = pidfs_put_data,
};
static int pidfs_init_fs_context(struct fs_context *fc)
diff --git a/kernel/pid.c b/kernel/pid.c
index 8317bcbc7cf7..30518852d990 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -100,7 +100,7 @@ void put_pid(struct pid *pid)
ns = pid->numbers[pid->level].ns;
if (refcount_dec_and_test(&pid->count)) {
- WARN_ON_ONCE(pid->stashed);
+ WARN_ON_ONCE(!IS_ERR(pid->stashed));
kmem_cache_free(ns->pid_cachep, pid);
put_pid_ns(ns);
}
--
2.47.2
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH RFC v2 2/2] pidfs: remove pidfs_pid_valid()
2025-06-17 15:39 [PATCH RFC v2 0/2] pidfs: keep pidfs dentry stashed once created Christian Brauner
2025-06-17 15:39 ` [PATCH RFC v2 1/2] " Christian Brauner
@ 2025-06-17 15:39 ` Christian Brauner
1 sibling, 0 replies; 4+ messages in thread
From: Christian Brauner @ 2025-06-17 15:39 UTC (permalink / raw)
To: Oleg Nesterov, linux-fsdevel; +Cc: Christian Brauner
The validation is now completely handled in path_from_stashed().
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/pidfs.c | 48 ------------------------------------------------
1 file changed, 48 deletions(-)
diff --git a/fs/pidfs.c b/fs/pidfs.c
index ee5e9a18c2d3..9373d03fd263 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -869,53 +869,8 @@ static int pidfs_export_permission(struct handle_to_path_ctx *ctx,
return 0;
}
-static inline bool pidfs_pid_valid(struct pid *pid, const struct path *path,
- unsigned int flags)
-{
- enum pid_type type;
-
- if (flags & PIDFD_STALE)
- return true;
-
- /*
- * Make sure that if a pidfd is created PIDFD_INFO_EXIT
- * information will be available. So after an inode for the
- * pidfd has been allocated perform another check that the pid
- * is still alive. If it is exit information is available even
- * if the task gets reaped before the pidfd is returned to
- * userspace. The only exception are indicated by PIDFD_STALE:
- *
- * (1) The kernel is in the middle of task creation and thus no
- * task linkage has been established yet.
- * (2) The caller knows @pid has been registered in pidfs at a
- * time when the task was still alive.
- *
- * In both cases exit information will have been reported.
- */
- if (flags & PIDFD_THREAD)
- type = PIDTYPE_PID;
- else
- type = PIDTYPE_TGID;
-
- /*
- * Since pidfs_exit() is called before struct pid's task linkage
- * is removed the case where the task got reaped but a dentry
- * was already attached to struct pid and exit information was
- * recorded and published can be handled correctly.
- */
- if (unlikely(!pid_has_task(pid, type))) {
- struct inode *inode = d_inode(path->dentry);
- return !!READ_ONCE(pidfs_i(inode)->exit_info);
- }
-
- return true;
-}
-
static struct file *pidfs_export_open(struct path *path, unsigned int oflags)
{
- if (!pidfs_pid_valid(d_inode(path->dentry)->i_private, path, oflags))
- return ERR_PTR(-ESRCH);
-
/*
* Clear O_LARGEFILE as open_by_handle_at() forces it and raise
* O_RDWR as pidfds always are.
@@ -1032,9 +987,6 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
if (ret < 0)
return ERR_PTR(ret);
- if (!pidfs_pid_valid(pid, &path, flags))
- return ERR_PTR(-ESRCH);
-
flags &= ~PIDFD_STALE;
flags |= O_RDWR;
pidfd_file = dentry_open(&path, flags, current_cred());
--
2.47.2
^ permalink raw reply related [flat|nested] 4+ messages in thread