On 2/28/25 13:44, Christian Brauner <brauner@xxxxxxxxxx> 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 | 70 +++++++++++++++++++++++++++++++++++++---------
> include/uapi/linux/pidfd.h | 3 +-
> 2 files changed, 59 insertions(+), 14 deletions(-)
>
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index 433f676c066c..e500bc4c5af2 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -32,11 +32,12 @@ static struct kmem_cache *pidfs_cachep __ro_after_init;
> */
> struct pidfs_exit_info {
> __u64 cgroupid;
> - __u64 exit_code;
> + __s32 exit_code;
> };
>
> struct pidfs_inode {
> - struct pidfs_exit_info exit_info;
> + struct pidfs_exit_info __pei;
> + struct pidfs_exit_info *exit_info;
> struct inode vfs_inode;
> };
>
> @@ -228,11 +229,14 @@ 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 long pidfd_info(struct file *file, struct task_struct *task,
> + unsigned int cmd, unsigned long arg)
> {
> struct pidfd_info __user *uinfo = (struct pidfd_info __user *)arg;
> 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;
> const struct cred *c;
> __u64 mask;
> @@ -248,6 +252,39 @@ static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long
> if (copy_from_user(&mask, &uinfo->mask, sizeof(mask)))
> return -EFAULT;
>
> + exit_info = READ_ONCE(pidfs_i(inode)->exit_info);
> + if (exit_info) {
> + /*
> + * TODO: Oleg, I didn't see a reason for putting
> + * retrieval of the exit status of a task behind some
> + * form of permission check. Maybe there's some
> + * potential concerns with seeing the exit status of a
> + * SIGKILLed suid binary or something but even then I'm
> + * not sure that's a problem.
> + *
> + * If we want this we could put this behind some *uid
> + * check similar to what ptrace access does by recording
> + * parts of the creds we'd need for checking this. But
> + * only if we really need it.
> + */
> + kinfo.exit_code = exit_info->exit_code;
> +#ifdef CONFIG_CGROUPS
> + kinfo.cgroupid = exit_info->cgroupid;
> + kinfo.mask |= PIDFD_INFO_EXIT | PIDFD_INFO_CGROUPID;
> +#endif
> + }
> +
> + /*
> + * 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 +304,13 @@ 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) {
> + rcu_read_lock();
> + cgrp = task_dfl_cgroup(task);
> + kinfo.cgroupid = cgroup_id(cgrp);
> + kinfo.mask |= PIDFD_INFO_CGROUPID;
> + rcu_read_unlock();
> + }
> #endif
>
> /*
> @@ -291,6 +330,7 @@ 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
> @@ -341,12 +381,13 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> }
>
> task = get_pid_task(pid, PIDTYPE_PID);
> - if (!task)
> - return -ESRCH;
Hmm, this breaks our current assumption/assertion on the API in systemd (see pidfd_get_pid_ioctl() in basic/pidfd-util.c).
Moreover, it now imposes an inconsistency: if the pidfd refers to a process from foreign pidns, the current impl treats it as if the process didn't exist, and returns -ESRCH. Now a truly exited task deviates from that...
I'd prefer to retain the current behavior of returning -ESRCH unless PIDFD_INFO_EXIT is specified in mask, in which case it's then guaranteed that -ESRCH would never be seen. IOW the caller should be explicit on what they want, which feels semantically more reasonable to me and probably even simpler?
>
> /* 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, task, cmd, arg);
> +
> + if (!task)
> + return -ESRCH;
>
> if (arg)
> return -EINVAL;
> @@ -486,7 +527,7 @@ void pidfs_exit(struct task_struct *tsk)
> struct cgroup *cgrp;
> #endif
> inode = d_inode(dentry);
> - exit_info = &pidfs_i(inode)->exit_info;
> + exit_info = &pidfs_i(inode)->__pei;
>
> /* TODO: Annoy Oleg to tell me how to do this correctly. */
> if (tsk->signal->flags & SIGNAL_GROUP_EXIT)
> @@ -501,6 +542,8 @@ void pidfs_exit(struct task_struct *tsk)
> rcu_read_unlock();
> #endif
>
> + /* Ensure that PIDFD_GET_INFO sees either all or nothing. */
> + smp_store_release(&pidfs_i(inode)->exit_info, &pidfs_i(inode)->__pei);
> dput(dentry);
> }
> }
> @@ -568,7 +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
>
> --
> 2.47.2
>
>
