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?
> __exit_signal(p);
>
> /*
>
Everything else looks fine though. Assuming you fix the nit above, you
can add:
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
