Re: [PATCH] liveupdate: add LIVEUPDATE_SESSION_GET_NAME ioctl

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Tue, Mar 31, 2026 at 1:56 PM <luca.boccassi@gmail.com> wrote:
>
> From: Luca Boccassi <luca.boccassi@gmail.com>
>
> Userspace when requesting a session specifies a name and gets a FD,
> but then there is no way to go back the other way and get the name
> given a LUO session FD. This is problematic especially when there
> is a userspace orchestrator that wants to check what FDs it is
> handling for clients.
>
> Resolving the /proc/self/fd/X is lossy as that has a hard limit
> of characters, so names might be truncated. Also requiring going
> through procfs and doing manual string mangling is not a nice
> pattern.

Hi Luca,

Thank you for working on this.

This needs more explanation. The way I see it, the user orchestrator
can be the only one that has access to /dev/liveupdate (and this is
actually enforced by kernel). It opens it and creates session_fds for
any client out there. So, the orchestrator already has the FDs and
sends them to clients(e.g. via SCM_RIGHTS). It then monitors the PIDs
of the clients, and if they die, it closes the session_fds for them,
as their data does not need to be preserved.

Could you please explain why the user orchestrator would need to query
the session_fds, since it is the one that creates them in the first
place and has to keep them open in order to preserve them during a
live update?

Thank you,
Pasha

>
> Add a ioctl to simply get the name from an FD.
>
> Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
> ---
> Need this for integration in systemd, so that I can check what
> FDs I get from clients and what they are, and so that I can know
> where to hand it back to
>
>  include/uapi/linux/liveupdate.h               | 19 +++++
>  kernel/liveupdate/luo_session.c               | 13 ++++
>  .../testing/selftests/liveupdate/liveupdate.c | 72 +++++++++++++++++++
>  3 files changed, 104 insertions(+)
>
> diff --git a/include/uapi/linux/liveupdate.h b/include/uapi/linux/liveupdate.h
> index 30bc66ee9436a..41dc44d239dde 100644
> --- a/include/uapi/linux/liveupdate.h
> +++ b/include/uapi/linux/liveupdate.h
> @@ -59,6 +59,7 @@ enum {
>         LIVEUPDATE_CMD_SESSION_PRESERVE_FD = LIVEUPDATE_CMD_SESSION_BASE,
>         LIVEUPDATE_CMD_SESSION_RETRIEVE_FD = 0x41,
>         LIVEUPDATE_CMD_SESSION_FINISH = 0x42,
> +       LIVEUPDATE_CMD_SESSION_GET_NAME = 0x43,
>  };
>
>  /**
> @@ -213,4 +214,22 @@ struct liveupdate_session_finish {
>  #define LIVEUPDATE_SESSION_FINISH                                      \
>         _IO(LIVEUPDATE_IOCTL_TYPE, LIVEUPDATE_CMD_SESSION_FINISH)
>
> +/**
> + * struct liveupdate_session_get_name - ioctl(LIVEUPDATE_SESSION_GET_NAME)
> + * @size:  Input; sizeof(struct liveupdate_session_get_name)
> + * @name:  Output; A null-terminated string with the full session name.
> + *
> + * Retrieves the full name of the session associated with this file descriptor.
> + * This is useful because the kernel may truncate the name shown in /proc.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +struct liveupdate_session_get_name {
> +       __u32           size;
> +       __u8            name[LIVEUPDATE_SESSION_NAME_LENGTH];
> +};
> +
> +#define LIVEUPDATE_SESSION_GET_NAME                                    \
> +       _IO(LIVEUPDATE_IOCTL_TYPE, LIVEUPDATE_CMD_SESSION_GET_NAME)
> +
>  #endif /* _UAPI_LIVEUPDATE_H */
> diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
> index 4a40c7fdfb44f..819e8864869fd 100644
> --- a/kernel/liveupdate/luo_session.c
> +++ b/kernel/liveupdate/luo_session.c
> @@ -289,10 +289,21 @@ static int luo_session_finish(struct luo_session *session,
>         return luo_ucmd_respond(ucmd, sizeof(*argp));
>  }
>
> +static int luo_session_get_name(struct luo_session *session,
> +                               struct luo_ucmd *ucmd)
> +{
> +       struct liveupdate_session_get_name *argp = ucmd->cmd;
> +
> +       strscpy((char *)argp->name, session->name, sizeof(argp->name));
> +
> +       return luo_ucmd_respond(ucmd, sizeof(*argp));
> +}
> +
>  union ucmd_buffer {
>         struct liveupdate_session_finish finish;
>         struct liveupdate_session_preserve_fd preserve;
>         struct liveupdate_session_retrieve_fd retrieve;
> +       struct liveupdate_session_get_name get_name;
>  };
>
>  struct luo_ioctl_op {
> @@ -319,6 +330,8 @@ static const struct luo_ioctl_op luo_session_ioctl_ops[] = {
>                  struct liveupdate_session_preserve_fd, token),
>         IOCTL_OP(LIVEUPDATE_SESSION_RETRIEVE_FD, luo_session_retrieve_fd,
>                  struct liveupdate_session_retrieve_fd, token),
> +       IOCTL_OP(LIVEUPDATE_SESSION_GET_NAME, luo_session_get_name,
> +                struct liveupdate_session_get_name, name),
>  };
>
>  static long luo_session_ioctl(struct file *filep, unsigned int cmd,
> diff --git a/tools/testing/selftests/liveupdate/liveupdate.c b/tools/testing/selftests/liveupdate/liveupdate.c
> index c2878e3d5ef90..510a583371fac 100644
> --- a/tools/testing/selftests/liveupdate/liveupdate.c
> +++ b/tools/testing/selftests/liveupdate/liveupdate.c
> @@ -102,6 +102,22 @@ static int create_session(int lu_fd, const char *name)
>         return args.fd;
>  }
>
> +/* Helper function to get a session name via ioctl. */
> +static int get_session_name(int session_fd, char *name, size_t name_len)
> +{
> +       struct liveupdate_session_get_name args = {};
> +
> +       args.size = sizeof(args);
> +
> +       if (ioctl(session_fd, LIVEUPDATE_SESSION_GET_NAME, &args))
> +               return -errno;
> +
> +       strncpy(name, (char *)args.name, name_len - 1);
> +       name[name_len - 1] = '\0';
> +
> +       return 0;
> +}
> +
>  /*
>   * Test Case: Create Duplicate Session
>   *
> @@ -345,4 +361,60 @@ TEST_F(liveupdate_device, preserve_unsupported_fd)
>         ASSERT_EQ(close(session_fd), 0);
>  }
>
> +/*
> + * Test Case: Get Session Name
> + *
> + * Verifies that the full session name can be retrieved from a session file
> + * descriptor via ioctl. This is important because /proc may truncate the
> + * anon_inode name.
> + */
> +TEST_F(liveupdate_device, get_session_name)
> +{
> +       char name_buf[LIVEUPDATE_SESSION_NAME_LENGTH] = {};
> +       const char *session_name = "get-name-test-session";
> +       int session_fd;
> +
> +       self->fd1 = open(LIVEUPDATE_DEV, O_RDWR);
> +       if (self->fd1 < 0 && errno == ENOENT)
> +               SKIP(return, "%s does not exist", LIVEUPDATE_DEV);
> +       ASSERT_GE(self->fd1, 0);
> +
> +       session_fd = create_session(self->fd1, session_name);
> +       ASSERT_GE(session_fd, 0);
> +
> +       ASSERT_EQ(get_session_name(session_fd, name_buf, sizeof(name_buf)), 0);
> +       ASSERT_STREQ(name_buf, session_name);
> +
> +       ASSERT_EQ(close(session_fd), 0);
> +}
> +
> +/*
> + * Test Case: Get Session Name at Maximum Length
> + *
> + * Verifies that a session name using the full LIVEUPDATE_SESSION_NAME_LENGTH
> + * (minus the null terminator) can be correctly retrieved.
> + */
> +TEST_F(liveupdate_device, get_session_name_max_length)
> +{
> +       char name_buf[LIVEUPDATE_SESSION_NAME_LENGTH] = {};
> +       char long_name[LIVEUPDATE_SESSION_NAME_LENGTH];
> +       int session_fd;
> +
> +       memset(long_name, 'A', sizeof(long_name) - 1);
> +       long_name[sizeof(long_name) - 1] = '\0';
> +
> +       self->fd1 = open(LIVEUPDATE_DEV, O_RDWR);
> +       if (self->fd1 < 0 && errno == ENOENT)
> +               SKIP(return, "%s does not exist", LIVEUPDATE_DEV);
> +       ASSERT_GE(self->fd1, 0);
> +
> +       session_fd = create_session(self->fd1, long_name);
> +       ASSERT_GE(session_fd, 0);
> +
> +       ASSERT_EQ(get_session_name(session_fd, name_buf, sizeof(name_buf)), 0);
> +       ASSERT_STREQ(name_buf, long_name);
> +
> +       ASSERT_EQ(close(session_fd), 0);
> +}
> +
>  TEST_HARNESS_MAIN
> --
> 2.47.3
>