[PATCH v8 10/11] docs: proc: add documentation for "hidepid=4" and "subset=pidfs" options and new mount behavior

[Andy Lutomirski]

On Mon, Feb 10, 2020 at 7:06 AM Alexey Gladkov <gladkov.alexey at gmail.com> wrote:
>
> Signed-off-by: Alexey Gladkov <gladkov.alexey at gmail.com>
> ---
>  Documentation/filesystems/proc.txt | 53 ++++++++++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
>
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 99ca040e3f90..4741fd092f36 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -50,6 +50,8 @@ Table of Contents
>    4    Configuring procfs
>    4.1  Mount options
>
> +  5    Filesystem behavior
> +
>  ------------------------------------------------------------------------------
>  Preface
>  ------------------------------------------------------------------------------
> @@ -2021,6 +2023,7 @@ The following mount options are supported:
>
>         hidepid=        Set /proc/<pid>/ access mode.
>         gid=            Set the group authorized to learn processes information.
> +       subset=         Show only the specified subset of procfs.
>
>  hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories
>  (default).
> @@ -2042,6 +2045,56 @@ information about running processes, whether some daemon runs with elevated
>  privileges, whether other user runs some sensitive program, whether other users
>  run any program at all, etc.
>
> +hidepid=4 means that procfs should only contain /proc/<pid>/ directories
> +that the caller can ptrace.

I have a couple of minor nits here.

First, perhaps we could stop using magic numbers and use words.
hidepid=ptraceable is actually comprehensible, whereas hidepid=4
requires looking up what '4' means.

Second, there is PTRACE_MODE_ATTACH and PTRACE_MODE_READ.  Which is it?

> +
>  gid= defines a group authorized to learn processes information otherwise
>  prohibited by hidepid=.  If you use some daemon like identd which needs to learn
>  information about processes information, just add identd to this group.

How is this better than just creating an entirely separate mount a
different hidepid and a different gid owning it?  In any event,
usually gid= means that this gid is the group owner of inodes.  Let's
call it something different.  gid_override_hidepid might be credible.
But it's also really weird -- do different groups really see different
contents when they read a directory?