On Tue, Mar 04, 2025 at 08:27:29AM -0500, Jeff Layton wrote:
> On Tue, 2025-03-04 at 10:41 +0100, Christian Brauner wrote:
> > Some tools like systemd's jounral need to retrieve the exit and cgroup
> > information after a process has already been reaped. This can e.g.,
> > happen when retrieving a pidfd via SCM_PIDFD or SCM_PEERPIDFD.
> >
> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> > ---
> > fs/pidfs.c | 88 ++++++++++++++++++++++++++++++++++++----------
> > include/uapi/linux/pidfd.h | 3 +-
> > kernel/exit.c | 2 +-
> > 3 files changed, 73 insertions(+), 20 deletions(-)
> >
> > diff --git a/fs/pidfs.c b/fs/pidfs.c
> > index 258e1c13ee56..11744d7fe177 100644
> > --- a/fs/pidfs.c
> > +++ b/fs/pidfs.c
> > @@ -36,7 +36,8 @@ struct pidfs_exit_info {
> > };
> >
> > struct pidfs_inode {
> > - struct pidfs_exit_info exit_info;
> > + struct pidfs_exit_info __pei;
> > + struct pidfs_exit_info *exit_info;
> > struct inode vfs_inode;
> > };
> >
> > @@ -228,17 +229,28 @@ static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
> > return poll_flags;
> > }
> >
> > -static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long arg)
> > +static inline bool current_in_pidns(struct pid *pid)
> > +{
> > + const struct pid_namespace *ns = task_active_pid_ns(current);
> > +
> > + if (ns->level <= pid->level)
> > + return pid->numbers[ns->level].ns == ns;
> > +
> > + return false;
> > +}
> > +
> > +static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
> > {
> > struct pidfd_info __user *uinfo = (struct pidfd_info __user *)arg;
> > + struct pid *pid = pidfd_pid(file);
> > size_t usize = _IOC_SIZE(cmd);
> > struct pidfd_info kinfo = {};
> > + struct pidfs_exit_info *exit_info;
> > + struct inode *inode = file_inode(file);
> > struct user_namespace *user_ns;
> > + struct task_struct *task;
> > const struct cred *c;
> > __u64 mask;
> > -#ifdef CONFIG_CGROUPS
> > - struct cgroup *cgrp;
> > -#endif
> >
> > if (!uinfo)
> > return -EINVAL;
> > @@ -248,6 +260,37 @@ static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long
> > if (copy_from_user(&mask, &uinfo->mask, sizeof(mask)))
> > return -EFAULT;
> >
> > + task = get_pid_task(pid, PIDTYPE_PID);
> > + if (!task) {
> > + if (!(mask & PIDFD_INFO_EXIT))
> > + return -ESRCH;
> > +
> > + if (!current_in_pidns(pid))
> > + return -ESRCH;
> > + }
> > +
> > + if (mask & PIDFD_INFO_EXIT) {
> > + exit_info = READ_ONCE(pidfs_i(inode)->exit_info);
> > + if (exit_info) {
> > +#ifdef CONFIG_CGROUPS
> > + kinfo.cgroupid = exit_info->cgroupid;
> > + kinfo.mask |= PIDFD_INFO_EXIT | PIDFD_INFO_CGROUPID;
> > +#endif
> > + kinfo.exit_code = exit_info->exit_code;
> > + }
> > + }
> > +
> > + /*
> > + * If the task has already been reaped only exit information
> > + * can be provided. It's entirely possible that the task has
> > + * already been reaped but we managed to grab a reference to it
> > + * before that. So a full set of information about @task doesn't
> > + * mean it hasn't been waited upon. Similarly, a full set of
> > + * information doesn't mean that the task hasn't already exited.
> > + */
> > + if (!task)
> > + goto copy_out;
> > +
> > c = get_task_cred(task);
> > if (!c)
> > return -ESRCH;
> > @@ -267,11 +310,15 @@ static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long
> > put_cred(c);
> >
> > #ifdef CONFIG_CGROUPS
> > - rcu_read_lock();
> > - cgrp = task_dfl_cgroup(task);
> > - kinfo.cgroupid = cgroup_id(cgrp);
> > - kinfo.mask |= PIDFD_INFO_CGROUPID;
> > - rcu_read_unlock();
> > + if (!kinfo.cgroupid) {
> > + struct cgroup *cgrp;
> > +
> > + rcu_read_lock();
> > + cgrp = task_dfl_cgroup(task);
> > + kinfo.cgroupid = cgroup_id(cgrp);
> > + kinfo.mask |= PIDFD_INFO_CGROUPID;
> > + rcu_read_unlock();
> > + }
> > #endif
> >
> > /*
> > @@ -291,6 +338,7 @@ static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long
> > if (kinfo.pid == 0 || kinfo.tgid == 0 || (kinfo.ppid == 0 && kinfo.pid != 1))
> > return -ESRCH;
> >
> > +copy_out:
> > /*
> > * If userspace and the kernel have the same struct size it can just
> > * be copied. If userspace provides an older struct, only the bits that
> > @@ -325,7 +373,6 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > {
> > struct task_struct *task __free(put_task) = NULL;
> > struct nsproxy *nsp __free(put_nsproxy) = NULL;
> > - struct pid *pid = pidfd_pid(file);
> > struct ns_common *ns_common = NULL;
> > struct pid_namespace *pid_ns;
> >
> > @@ -340,13 +387,13 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > return put_user(file_inode(file)->i_generation, argp);
> > }
> >
> > - task = get_pid_task(pid, PIDTYPE_PID);
> > - if (!task)
> > - return -ESRCH;
> > -
> > /* Extensible IOCTL that does not open namespace FDs, take a shortcut */
> > if (_IOC_NR(cmd) == _IOC_NR(PIDFD_GET_INFO))
> > - return pidfd_info(task, cmd, arg);
> > + return pidfd_info(file, cmd, arg);
> > +
> > + task = get_pid_task(pidfd_pid(file), PIDTYPE_PID);
> > + if (!task)
> > + return -ESRCH;
> >
> > if (arg)
> > return -EINVAL;
> > @@ -479,10 +526,12 @@ void pidfs_exit(struct task_struct *tsk)
> > {
> > struct dentry *dentry;
> >
> > + 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)->exit_info;
> > + struct pidfs_exit_info *exit_info = &pidfs_i(inode)->__pei;
> > #ifdef CONFIG_CGROUPS
> > struct cgroup *cgrp;
> >
> > @@ -493,6 +542,8 @@ void pidfs_exit(struct task_struct *tsk)
> > #endif
> > exit_info->exit_code = tsk->exit_code;
> >
> > + /* Ensure that PIDFD_GET_INFO sees either all or nothing. */
> > + smp_store_release(&pidfs_i(inode)->exit_info, &pidfs_i(inode)->__pei);
> > dput(dentry);
> > }
> > }
> > @@ -560,7 +611,8 @@ static struct inode *pidfs_alloc_inode(struct super_block *sb)
> > if (!pi)
> > return NULL;
> >
> > - memset(&pi->exit_info, 0, sizeof(pi->exit_info));
> > + memset(&pi->__pei, 0, sizeof(pi->__pei));
> > + pi->exit_info = NULL;
> >
> > return &pi->vfs_inode;
> > }
> > diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
> > index e0abd0b18841..e5966f1a7743 100644
> > --- a/include/uapi/linux/pidfd.h
> > +++ b/include/uapi/linux/pidfd.h
> > @@ -20,6 +20,7 @@
> > #define PIDFD_INFO_PID (1UL << 0) /* Always returned, even if not requested */
> > #define PIDFD_INFO_CREDS (1UL << 1) /* Always returned, even if not requested */
> > #define PIDFD_INFO_CGROUPID (1UL << 2) /* Always returned if available, even if not requested */
> > +#define PIDFD_INFO_EXIT (1UL << 3) /* Always returned if available, even if not requested */
> >
> > #define PIDFD_INFO_SIZE_VER0 64 /* sizeof first published struct */
> >
> > @@ -86,7 +87,7 @@ struct pidfd_info {
> > __u32 sgid;
> > __u32 fsuid;
> > __u32 fsgid;
> > - __u32 spare0[1];
> > + __s32 exit_code;
> > };
> >
> > #define PIDFS_IOCTL_MAGIC 0xFF
> > diff --git a/kernel/exit.c b/kernel/exit.c
> > index 98d292120296..9916305e34d3 100644
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -250,12 +250,12 @@ void release_task(struct task_struct *p)
> > dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
> > rcu_read_unlock();
> >
> > + pidfs_exit(p);
> > cgroup_release(p);
> >
> > write_lock_irq(&tasklist_lock);
> > ptrace_release_task(p);
> > thread_pid = get_pid(p->thread_pid);
> > - pidfs_exit(p);
>
> Why move this after you just added it?
Ah sorry that should've been done in the previous patch. pidfs_exit()
needs to be called outside of tasklist_lock as it can sleep due to dput().
>
> > __exit_signal(p);
> >
> > /*
> >
>
> Everything else looks fine though. Assuming you fix the nit above, you
> can add:
>
> Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
