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

[Alexey Gladkov]

On Mon, Feb 10, 2020 at 10:29:23AM -0800, Andy Lutomirski wrote:
> 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.

Do you mean to add string aliases for the values?

hidepid=0 == hidepid=default
hidepid=1 == hidepid=restrict
hidepid=2 == hidepid=ownonly
hidepid=4 == hidepid=ptraceable

Something like that ?

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

This is PTRACE_MODE_READ.

> > +
> >  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?

I'm not sure I understand the question. Now you cannot have two proc with
different hidepid in the same pid_namespace. 

> 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?

If you use hidepid=2,gid=wheel options then the user is not in the wheel
group will see only their processes and the user in the wheel group will
see whole tree. The gid= is a kind of whitelist for hidepid=1|2.

-- 
Rgrds, legion