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References: <20200725045921.2723-1-kalou@tfz.net> <20200725051547.3718-1-kalou@tfz.net>
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From:   Pascal Bouchareine <kalou@tfz.net>
Date:   Sat, 25 Jul 2020 19:11:43 -0700
Message-ID: <CAGbU3_nNeeZn8Sk28t_fqOEjBi81JXp67L7-2E+9J=8LHtcE7Q@mail.gmail.com>
Subject: Re: [PATCH v3] This command attaches a description to a file
 descriptor for troubleshooting purposes. The free string is displayed in the
 process fdinfo file for that fd /proc/pid/fdinfo/fd.
To:     linux-kernel@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jeff Layton <jlayton@poochiereds.net>,
        "J. Bruce Fields" <bfields@fieldses.org>
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Sender: linux-kernel-owner@vger.kernel.org
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Sorry about all the noise, please disregard this one.


On Sat, Jul 25, 2020 at 7:04 PM Pascal Bouchareine <kalou@tfz.net> wrote:
>
> One intended usage is to allow processes to self-document sockets
> for netstat and friends to report
>
> Signed-off-by: Pascal Bouchareine <kalou@tfz.net>
> ---
>  Documentation/filesystems/proc.rst |  3 +++
>  fs/fcntl.c                         | 19 +++++++++++++++++++
>  fs/file_table.c                    |  2 ++
>  fs/proc/fd.c                       |  5 +++++
>  include/linux/fs.h                 |  3 +++
>  include/uapi/linux/fcntl.h         |  5 +++++
>  6 files changed, 37 insertions(+)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 996f3cfe7030..ae8045650836 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -1918,6 +1918,9 @@ A typical output is::
>         flags:  0100002
>         mnt_id: 19
>
> +An optional 'desc' is set if the process documented its usage of
> +the file via the fcntl command F_SET_DESCRIPTION.
> +
>  All locks associated with a file descriptor are shown in its fdinfo too::
>
>      lock:       1: FLOCK  ADVISORY  WRITE 359 00:13:11691 0 EOF
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 2e4c0fa2074b..c1ef724a906e 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -319,6 +319,22 @@ static long fcntl_rw_hint(struct file *file, unsigned int cmd,
>         }
>  }
>
> +static long fcntl_set_description(struct file *file, char __user *desc)
> +{
> +       char *d;
> +
> +       d = strndup_user(desc, MAX_FILE_DESC_SIZE);
> +       if (IS_ERR(d))
> +               return PTR_ERR(d);
> +
> +       spin_lock(&file->f_lock);
> +       kfree(file->f_description);
> +       file->f_description = d;
> +       spin_unlock(&file->f_lock);
> +
> +       return 0;
> +}
> +
>  static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>                 struct file *filp)
>  {
> @@ -426,6 +442,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>         case F_SET_FILE_RW_HINT:
>                 err = fcntl_rw_hint(filp, cmd, arg);
>                 break;
> +       case F_SET_DESCRIPTION:
> +               err = fcntl_set_description(filp, argp);
> +               break;
>         default:
>                 break;
>         }
> diff --git a/fs/file_table.c b/fs/file_table.c
> index 656647f9575a..6673a48d2ea1 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -272,6 +272,8 @@ static void __fput(struct file *file)
>         eventpoll_release(file);
>         locks_remove_file(file);
>
> +       kfree(file->f_description);
> +
>         ima_file_free(file);
>         if (unlikely(file->f_flags & FASYNC)) {
>                 if (file->f_op->fasync)
> diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> index 81882a13212d..60b3ff971b2b 100644
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -57,6 +57,11 @@ static int seq_show(struct seq_file *m, void *v)
>                    (long long)file->f_pos, f_flags,
>                    real_mount(file->f_path.mnt)->mnt_id);
>
> +       spin_lock(&file->f_lock);
> +       if (file->f_description)
> +               seq_printf(m, "desc:\t%s\n", file->f_description);
> +       spin_unlock(&file->f_lock);
> +
>         show_fd_locks(m, file, files);
>         if (seq_has_overflowed(m))
>                 goto out;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index f5abba86107d..09717bfa4e3b 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -980,6 +980,9 @@ struct file {
>         struct address_space    *f_mapping;
>         errseq_t                f_wb_err;
>         errseq_t                f_sb_err; /* for syncfs */
> +
> +#define MAX_FILE_DESC_SIZE 256
> +       char                    *f_description;
>  } __randomize_layout
>    __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
>
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 2f86b2ad6d7e..f86ff6dc45c7 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -55,6 +55,11 @@
>  #define F_GET_FILE_RW_HINT     (F_LINUX_SPECIFIC_BASE + 13)
>  #define F_SET_FILE_RW_HINT     (F_LINUX_SPECIFIC_BASE + 14)
>
> +/*
> + * Set file description
> + */
> +#define F_SET_DESCRIPTION      (F_LINUX_SPECIFIC_BASE + 15)
> +
>  /*
>   * Valid hint values for F_{GET,SET}_RW_HINT. 0 is "not set", or can be
>   * used to clear any hints previously set.
> --
> 2.25.1
>