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SubjectRe: [PATCH v8 10/11] docs: proc: add documentation for "hidepid=4" and "subset=pidfs" options and new mount behavior
On Mon, Feb 10, 2020 at 7:06 AM Alexey Gladkov <gladkov.alexey@gmail.com> wrote:
>
> Signed-off-by: Alexey Gladkov <gladkov.alexey@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?

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