Re: RFC: fixing PR#282995

On Thu, Nov 28, 2024 at 4:36 AM Bob Bishop <rb@gid.co.uk> wrote:
>
> Hi,
>
> > On 27 Nov 2024, at 21:56, Rick Macklem <rick.macklem@gmail.com> wrote:
> >
> > Hi,
> >
> > PR#282995 reports that the "-alldirs" export option is broken,
> > since it allows an export where the directory path is not a mount point.
> >
> > I'll admit I did not recall this semantic for -alldirs and I now see it is only
> > documented in the "Examples" section of exports(5).
> >
> > Looking at the code, it appears this was broken between releng1 and
> > releng2.0 (about 30years ago) when the call to mount(2) in mountd.c
> > was changed from using the path in the exports line to using f_mntonname.
> > (The check for "it is a mount point" depended on mount(2) failing because
> > the path was not a mount point.)
> >
> > I do believe the semantic is a useful one,
>
> Why?
Suppose /cdrom is where a CD is mounted sometimes.
If this is exported when the CD is not mounted, it will export
the "/" file system.
--> This export is probably not what the sysadmin wanted.
      mountd does now generate a warning about this, which
      was how the exporter spotted the bug.
For example (the line in /etc/exports):
/cdrom -alldirs
will export "/" to "the world" if /cdrom is not mounted.

The example in the exports(5) man page claims the export
line will fail, so "/" would not be exported. This seems like
a better semantic to me.

rick

>
> > although making it that way
> > after 30years might be construed as a POLA violation?
> >
> > So, what do others think I should do with this?
> > (A) - Patch mountd to enforce the "must be a mount point when -alldirs
> >        is specified, plus update exports(5) to state this semantic clearly.
> > or
> > (B) - Patch mountd so that it enforces "must be a mount point when -alldirs
> >        is specified, but only enabled via a new mountd command line option.
> >        --> ie. Leave the default as not enforced, but allow enforcement based
> >                  on a new mountd option.
> >             - Document this in both exports(5) and mountd(8).
> > or
> > ???
>
> (C) - Patch mountd so that it enforces "must be a mount point when -alldirs
>        is specified, but provide a new mountd command line option to restore the old behaviour.
>        --> ie. Default as enforced, but allow an override based  on a new mountd option.
>             - Document this in both exports(5) and mountd(8).
>
> I think that (A) is too POLA-unfriendly.
>
> > Thanks in advance for your comments, rick
> >
>
> --
> Bob Bishop
> rb@gid.co.uk
>
>
>
>