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Re: leaks fd for internal functions but not external command


From: Sam Liddicott
Subject: Re: leaks fd for internal functions but not external command
Date: Thu, 25 Jul 2019 14:03:50 +0100

Thanks for your continued engagement, it is appreciated.

I was focused on my case where I used _ as the name of the file descriptor
because I didn't want to retain it, for I was using named fd as a way to
avoid trampling on an fd already in use, rather than as an open method.

I'm writing up a shellcheck recommendation, to advise others like me on
making sure that the fd is closed. Would be OK to post it here for review,
or would it not be of interest?

Some thoughts since come to mind while writing this:

Perhaps if the named fd is also closed on the same statement that opened
it, it could be kept closed rather than saved and re-opened.
Being closed at the same statement is a strong indicator that it was only
used to aid fd transplants. Why keep open what the author closed?

{tmp_fd}>&2 2>&1 1>&${tmp_fd} {tmp_fd}<&-
                              ^^^^^^^^^^^ not even kept open the the
command it was used for

I also find that: internalize() { "$@" ; }

lets me use the word internalize as a prefix to make external or internal
commands all appear internal. it thus extends the open syntax to even
external (non-pipeline) commands, and permits a uniform clean-up without
risking closing the wrong fd. I wonder if the shell might do this
transparently in the case that a named fd was used (a mere idle wish, pay
it no heed, though it would add uniformity).

Another idea is along the lines wiping the fd variable before the statement
(as we are planning to trample on it) and closing it afterwards only if set

# close fd vars if set, preserve prior exit code
maybe-close( ) {
  set -- $? "$@"
  while test "$#" -ge 2
  do
    if test -n "${!2}"
    then eval exec "{$2}<&-"
    fi
    unset "$2"
    set -- "$1" "${@:3}"
  done
  return $1
}

tmp_fd= && echo {tmp_fd}>&2 2>&1 1>&${tmp_fd} {tmp_fd}<&- ; maybe-close
tmp_fd

they are my thoughts on avoiding the need to detect the special case of
internal commands in order to close the fd

I appreciate your time and the maintenance of bash and readline.

thanks again

Sam

On Thu, 25 Jul 2019 at 12:52, Chet Ramey <address@hidden> wrote:

> On 7/24/19 9:07 AM, Sam Liddicott wrote:
> > Thanks for that thoughtful response.
> >
> > * I understand that the design decision is to have variable file
> > descriptors to stay open after per-command redirection
>
> Yes.
>
> > * I understand that implementation constraints make it impossible to do
> > this uniformly (for external command redirection)
>
> I'm not sure "implementation constraints" is the right characterization.
> Performing redirections in child processes is how shells work.
>
> > * I understand that it is difficult for the script author to detect which
> > case his code will be
>
> I don't think this is true. The documentation is pretty clear.
>
> > The shell normally does a great job of hiding the difference between
> > internal and external commands, so even though it's very well documented,
> > most of the time the user doesn't need to be aware. This is great for the
> > user, and according to the principle of least surprise.
>
> This isn't quite the case. The shell isn't particularly obscure about
> the fact that printf has to be a builtin for `printf -v' to work, for
> instance.
>
> >
> > The syntactic sugar of having bash select a free fd (which necessary for
> > good composability of operations in complex script pipelines) is a great
> > benefit, especially when mixing with older pipelines having fixed
> numeric fd.
>
> Older shells, you mean. Yes, that was the original motivation for it.
> POSIX makes the limitation implementation-defined, and bash has never had
> it, so it's reasonably easy to choose something that won't collide. But
> it's nice not to have to.
>
> > You say that there are technical reasons why the syntactic sugar of also
> > keeping the fd open can't be implemented uniformly.
>
> Technical reasons in the sense of the relationship between parent and child
> processes, I suppose you mean.
>
> > I wonder if this puts unnecessary cognitive burden on the user, leading
> to
> > reluctance to get the benefits, or to the introduction of latent bugs.
> >
> > There is a case I explain below which can lead to a leaked fd being held
> on
> > to by subsequently invoked external processes. Of course it will
> > technically be the users fault but I'm looking at reducing the cognitive
> > burdens that make such a fault ultimately inevitable.
> >
> > The cognitive burdens of leaving the fd open are:
> >
> > 1. It breaks the normal expectation that per-command redirects are
> limited
> > to the scope of the command.
>
> Yes, it does keep the file descriptor open beyond the command. That is what
> makes the feature different.
>
>
> > A naked exec already works to hold open a variable fd in a wider scope if
> > that's what the scripter actually wants: exec {fd}>... ;
>
> Sure, you can do that too.
>
>
> > 2. As syntactic sugar it moves, not removes, the boiler-plate burden
> >
> > This naked exec (see above) saved by the syntactic sugar in the case
> where
> > the fd should remain open is offset by the naked exec now required in
> order
> > to close the fd for the traditional case that the fd should not left open
> > beyond the scope of the command.
>
> So you're saying you need to close it explicitly with `exec'. Yes, that's
> the `cost' of using the feature.
>
> >
> > 3. The unmeetable cognitive burden is that in order to safely manage the
> > previous two item, the user needs to know if the command will be external
> > or internal or a function.
>
> "Unmeetable?" Come on.
>
> >
> > This makes it hard for the user depend on this feature, because it is not
> > possible to be sure at script author time whether a command is external.
> It
> > may have become a function, (due to export -f, source, etc) which affect
> > the execution environment.
>
> I agree that export -f can change the environment from what the script
> writer expected. That one is on the user, not the script writer. But if
> it's a concern, the script writer can always use `command'.
>
> > 4. The inevitable propagation of leaked fd's
> >
> > The knowing user can remember to always use an identity wrapper function
> to
> > force treatment as external commands as internal functions in order to
> get
> > uniform behaviour, and also explicitly close the fd afterwards.
>
> Explicitly closing the fd would always work, in the case that the fd is
> opened by the parent, but will not work in the case where redirections are
> performed in the child process, since the parent will not set the correct
> shell variable.
>
>
> > But other users may not know to close the fd which was never apparent
> (due
> > invoking an external command) but which becomes an fd leak when they
> > combine with other bash features (functions wrapping of external
> commands,
> > or export -f environment that does this unawares) and those leaked fd's
> may
> > then be inherited by other invoked external processes which may hold on
> to
> > them for some time.
>
> You're mixing `users' with script writers here, unless you're talking
> about running constructs like this as one-offs in an interactive shell,
> which doesn't happen very often, if at all.
>
> >
> > This contrived example minimises the pipeline fd contortions in order to
> > show that when what was an external command then becomes an internal
> > command, it can as a consequence result in an fd leak to external
> processes
> > (bash+lsof+grep here) which may be long lived.
>
> I think there's a misunderstanding here, and it's my fault. The {var}
> syntax is identical to any other file descriptor that's the target of a
> redirection used with `exec', and the close-on-exec flag remains unset.
> There's no `leaking' that is any different than any other redirection.
> Sorry for the mistake.
>
>
> > I recognise what you say about past design decisions, but for the future,
> > as it is hard to safely get the benefit of leaving the handle open for
> > variable per-command redefines, even for users who know about it, I
> wonder
> > if the syntactic sugar might be redefined to reduce the cognitive burden
> > and widen the benefit for the most valued variable fd's feature.
>
> Yes, leaving the fd open was the design decision. The idea was to more
> closely mimic open(), giving the script writer a handle he could use to
> manage the file descriptor without using `exec'. Other than that, the
> semantics of the resulting file descriptor are the same as if it had
> been chosen by the user and used with `exec'.
>
> >
> > If the variable fd syntactic sugar were re-designed so that variable
> > handles were also limited to the scope the command, the same as for
> > external commands, the same as for numeric handles, then:
>
> If I were redesigning it today, I might choose to do that. Back in 2009,
> I made a different decision.
>
> Chet
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    address@hidden    http://tiswww.cwru.edu/~chet/
>


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