[PATCH] pidfd: add ioctl to retrieve pid info

[Paul Moore]

On Wed, Oct 2, 2024 at 10:25 AM <luca.boccassi at gmail.com> wrote:
>
> From: Luca Boccassi <bluca at debian.org>
>
> A common pattern when using pid fds is having to get information
> about the process, which currently requires /proc being mounted,
> resolving the fd to a pid, and then do manual string parsing of
> /proc/N/status and friends. This needs to be reimplemented over
> and over in all userspace projects (e.g.: I have reimplemented
> resolving in systemd, dbus, dbus-daemon, polkit so far), and
> requires additional care in checking that the fd is still valid
> after having parsed the data, to avoid races.
>
> Having a programmatic API that can be used directly removes all
> these requirements, including having /proc mounted.
>
> As discussed at LPC24, add an ioctl with an extensible struct
> so that more parameters can be added later if needed. Start with
> exposing: pid, uid, gid, groupid, security label (the latter was
> requested by the LSM maintainer).
>
> Signed-off-by: Luca Boccassi <bluca at debian.org>
> ---
>  fs/pidfs.c                                    | 61 ++++++++++++++++++-
>  include/uapi/linux/pidfd.h                    | 17 ++++++
>  .../testing/selftests/pidfd/pidfd_open_test.c | 50 ++++++++++++++-
>  3 files changed, 126 insertions(+), 2 deletions(-)

...

> diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
> index 565fc0629fff..bfd0965e01f3 100644
> --- a/include/uapi/linux/pidfd.h
> +++ b/include/uapi/linux/pidfd.h
> @@ -16,6 +16,22 @@
>  #define PIDFD_SIGNAL_THREAD_GROUP      (1UL << 1)
>  #define PIDFD_SIGNAL_PROCESS_GROUP     (1UL << 2)
>
> +/* Flags for pidfd_info. */
> +#define PIDFD_INFO_PID                 (1UL << 0)
> +#define PIDFD_INFO_CREDS                   (1UL << 1)
> +#define PIDFD_INFO_CGROUPID                (1UL << 2)
> +#define PIDFD_INFO_SECURITY_CONTEXT        (1UL << 3)
> +
> +struct pidfd_info {
> +        __u64 request_mask;
> +        __u32 size;
> +        uint pid;
> +        uint uid;
> +        uint gid;
> +        __u64 cgroupid;
> +        char security_context[NAME_MAX];

[NOTE: please CC the LSM list on changes like this]

Thanks Luca :)

With the addition of the LSM syscalls we've created a lsm_ctx struct
(see include/uapi/linux/lsm.h) that properly supports multiple LSMs.
The original char ptr "secctx" approach worked back when only a single
LSM was supported at any given time, but now that multiple LSMs are
supported we need something richer, and it would be good to use this
new struct in any new userspace API.

See the lsm_get_self_attr(2) syscall for an example (defined in
security/lsm_syscalls.c but effectively implemented via
security_getselfattr() in security/security.c).

> +} __packed;
> +
>  #define PIDFS_IOCTL_MAGIC 0xFF
>
>  #define PIDFD_GET_CGROUP_NAMESPACE            _IO(PIDFS_IOCTL_MAGIC, 1)
> @@ -28,5 +44,6 @@
>  #define PIDFD_GET_TIME_FOR_CHILDREN_NAMESPACE _IO(PIDFS_IOCTL_MAGIC, 8)
>  #define PIDFD_GET_USER_NAMESPACE              _IO(PIDFS_IOCTL_MAGIC, 9)
>  #define PIDFD_GET_UTS_NAMESPACE               _IO(PIDFS_IOCTL_MAGIC, 10)
> +#define PIDFD_GET_INFO                        _IOWR(PIDFS_IOCTL_MAGIC, 11, struct pidfd_info)
>
>  #endif /* _UAPI_LINUX_PIDFD_H */

-- 
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