Re: [PATCH RFC 1/3] symlink.7: document magic-links more completely

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-10-07, Jann Horn <jannh@google.com> wrote:
> On Thu, Oct 3, 2019 at 4:56 PM Aleksa Sarai <cyphar@cyphar.com> wrote:
> > Traditionally, magic-links have not been a well-understood topic in
> > Linux. Given the new changes in their semantics (related to the link
> > mode of trailing magic-links), it seems like a good opportunity to shine
> > more light on magic-links and their semantics.
> [...]
> > +++ b/man7/symlink.7
> > @@ -84,6 +84,25 @@ as they are implemented on Linux and other systems,
> >  are outlined here.
> >  It is important that site-local applications also conform to these rules,
> >  so that the user interface can be as consistent as possible.
> > +.SS Magic-links
> > +There is a special class of symlink-like objects known as "magic-links" which
> 
> I think names like that normally aren't hypenated in english, and
> instead of "magic-links", it'd be "magic links"? Just like how you
> wouldn't write "symbolic-link", but "symbolic link". But this is
> bikeshedding, and if you disagree, feel free to ignore this comment.

Looking at it now, I think you're right -- I hyphenated it here because
that's how I wrote it when documenting the feature in comments. But I
think that's because "symlink" and "magic-link" (the "abbreviated"
versions) seem to match better than "symlink" and "magic link".

I'll use "magic link" in documentation, but "magic-link" for all cases
where I would normally write "symlink".

> > +can be found in certain pseudo-filesystems such as
> > +.BR proc (5)
> > +(examples include
> > +.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .)
> > +Unlike normal symlinks, magic-links are not resolved through
> 
> nit: AFAICS symlinks are always referred to as "symbolic links"
> throughout the manpages.

:+1:

> > +pathname-expansion, but instead act as direct references to the kernel's own
> > +representation of a file handle. As such, these magic-links allow users to
> > +access files which cannot be referenced with normal paths (such as unlinked
> > +files still referenced by a running program.)
> 
> Could maybe add "and files in different mount namespaces" as another
> example here; at least for me, that's the main usecases for
> /proc/*/root.

Will do.

> [...]
> > +However, magic-links do not follow this rule. They can have a non-0777 mode,
> > +which is used for permission checks when the final
> > +component of an
> > +.BR open (2)'s
> 
> Maybe leave out the "open" part, since the same restriction has to
> also apply to other syscalls operating on files, like truncate() and
> so on?

Yes (though I've just realised I hadn't implemented that -- oops.) Given
how expansive this patchset will get -- I might end up splitting it into
the magic-link stuff (and O_EMPTYPATH) and a separate series for
openat2(2) and the path resolution restrictions.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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