Re: [PATCH 1/2] Introduce new completion for Linux task capabilities

[Oliver Kiddle <opk@xxxxxxx>]

Arseny Maslennikov wrote:
> On Fri, Feb 26, 2021 at 03:50:18PM +0000, Daniel Shahaf wrote:
> > Arseny Maslennikov wrote on Fri, Feb 26, 2021 at 10:55:57 +0300:
> > > The next patch introduces a completion for setpriv(1), which actively

Thanks for the contribution. I've reviewed the second patch that Daniel
left though in general it all looks very good and there isn't much need
to comment.

> > > uses _capability_names. As for _capabilities,

Are these POSIX capabilities? The term capabilities gets wider usage so
I'd be inclined to call the function _posix_capabilities despite that
being longer. As far as I can tell, unless you count Hurd, Linux is the
only implementation of POSIX capabilities so it is fine for this
to be in the Linux directory. That said, I'm not sure I would factor it
out of _setpriv until there's at least two users of it.

> > FWIW, I think it may well be possible to write _setpriv in terms of
> > _capabilities as a black box, using some combination of _sequence,
> > matchspecs, «compadd -P», et al..
>
> Do you mean invoking «_capabilities -O array_name» to pass the -O to
> compadd, and then using the array_name in _setpriv and possible future
> users?
> As far as I understand, in that case _capabilities will have to avoid
> looping over tag labels, i. e. calling _wanted, as it does now.

No. You would call _capabilities with compadd style options to deal with
the prefixes. You have the following:

  matches=( {-,+}"${(@)^caps#cap_}" )

So _capabilities is unsuited to _setpriv because you don't want the
cap_ prefix but do want - and + prefixes. Telling compadd/completion
functions to strip a prefix is somewhat messy, you can get close with
matching control (-M 'B:=cap_') but it continues to prefer the prefix
to be present. So I'd suggest that you chop off the initial "cap_" from
everything _capabilites completes.

Adding prefixes is easier. If some other function needs capabilities
with a cap_ prefix, that can then call it with -p 'cap_',

For the - and +, we can also add these as a prefix but in their case, -P
is more appropriate because the prefix is a separate thing and it may
even be better to strip them with compset and complete them
independently so that we can indicate that they are for removing and
adding capabilities.

> > > +if [[ $OSTYPE != linux* ]]; then
> > > +    _default; return
> > > +fi
> > 
> > Why?  The code doesn't use /proc or /sbin/some-linux-only-tool, and
> > having this guard prevents people on other OSTYPE's from completing «ssh
> > $linuxbox foo --with-capability=<TAB>».
>
> I intended to play nice in case unrelated "capabilities" are present (or
> introduced later) on those OSTYPE's and pulled Oliver's trick from the
> recently posted _unshare. There might be a better way to do it; I'm open
> to discussion.

Not sure I'd bother in this case as - given the lack of setpriv commands
on other operating systems - it is unlikely to be called other than in
a case like the ssh one Daniel mentions. It mattered in the case of
unshare because unshare does exist on other OSes as an unrelated command
to remove NFS exports.

> > Suggest to list these one per line, because:
> I'd personally like them being listed one per line, too. Will do.

I'd certainly avoid exceeding 80 columns unnecessarily but if it ends up
using a lot of vertical space, grouping similar ones or by first letter
is also fine.

Arseny Maslennikov [patch 2/2] wrote:
> +__setpriv_prctl_securebits_set_element() {

I wouldn't bother with the double-underscore prefix. _git did that and
seems to have been copied elsewhere but it doesn't really serve much
purpose. I think with git it was done to make a more obvious distinction
to subcommands than just whether it was _git_… or _git-…

I'd be inclined to just name it _setpriv_prctl_securebits as that is
what it appears to complete.

> +  bits=(noroot noroot_locked
> +        no_setuid_fixup no_setuid_fixup_locked

> +  _wanted minus-plus-securebits expl 'prctl securebits' \
> +    compadd "$@" -a - matches

Minor nitpick but Etc/completion-style-guide states four spaces of
indentation for continuation lines. My own personal taste includes a
particular dislike for the variable indentation that results from lining
things up with the syntactic construct on the previous line as in the
bits=( assignment.

Also, the description should be in the singular ('prctl securebit').
Yes, there may be multiple matches, and we may be in the middle of a
comma-separated list of them but only one is completed at a time

T'd be sufficient to use _description followed by compadd rather than
_wanted. There are aspects of nested tag loops that aren't ideal so
when you can know that completion is already within one tag loop (which
_arguments does), we don't need another one.

> +__setpriv_prctl_securebits_set() {
> +  _sequence __setpriv_prctl_securebits_set_element
> +}

You can call _sequence directly from the _arguments spec. I'd probably
just do that rather than having a separate function. Convention for
separate functions is a plural name, i.e. …_sets

> +__setpriv_caps_all() {
> +  # Nonlocal expl; _description call expected.

Is that necessary? Aren't descriptions passed through with "$@"?

> +__setpriv_cap_set_element() {
> +  # We pass through arguments from _sequence.
> +  local -a Oargv=( "$@" )
> +  _alternative -O Oargv \
> +    'special-actions:drop/obtain all caps:__setpriv_caps_all' \

Wouldn't :(-all +all) do for the action here instead of needing another
separate function.
There probably ought to be a nicer way to add special all/any elements
to a list because it comes up very often.

> +    'minus-plus-caps:capabilities:__setpriv_capability_expressions' \

Again, use singular form for the description.

> +local context state state_descr line
> +typeset -A opt_args

Given that you've not used states, these are superfluous.

> +  '(- : *)*'{-d,--dump}'[display the current privilege state]' \

The exclusion list here breaks the repeatability.
I did once look into making a special case for this in _arguments but it
wasn't trivial.

> +  '--clear-groups[clear supplementary groups]' \
> +  '--groups[set supplementary groups]:groups:_groups' \

How do you specify multiple supplementary groups? If --groups is
repeated, we need * at the beginning of the spec. Otherwise, is
_sequence needed.
Are there any mutual exclusions between it and --clear-groups,
--keep-groups and --init-groups.

> +  '--inh-caps[set inheritable caps]:capability set: __setpriv_cap_set' \
> +  '--ambient-caps[set ambient caps]:capability set: __setpriv_cap_set' \

Where the _arguments descriptions are being thrown out anyway, it may be
better to leave them out entirely and just have a single space.

> +  '--securebits[set "process securebits"]:prctl securebits:__setpriv_prctl_securebits_set' \

Starting with this one is a run of plural descriptions.

Oliver