Re: [PATCH man-pages v2 2/2] openat2.2: document new openat2(2) syscall

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2020-03-30, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
> Hello Aleksa,
> 
> On 2/2/20 4:19 PM, Aleksa Sarai wrote:
> > Rather than trying to merge the new syscall documentation into open.2
> > (which would probably result in the man-page being incomprehensible),
> > instead the new syscall gets its own dedicated page with links between
> > open(2) and openat2(2) to avoid duplicating information such as the list
> > of O_* flags or common errors.
> > 
> > In addition to describing all of the key flags, information about the
> > extensibility design is provided so that users can better understand why
> > they need to pass sizeof(struct open_how) and how their programs will
> > work across kernels. After some discussions with David Laight, I also
> > included explicit instructions to zero the structure to avoid issues
> > when recompiling with new headers.
> > 
> > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> 
> Thanks. I've applied this patch, but also done quite a lot of
> editing of the page. The current draft is below (and also pushed 
> to Git). Could I ask you to review the page, to see if I injected
> any error during my edits.

Looks good to me.

> In addition, I've added a number of FIXMEs in comments
> in the page source. Can you please check these, and let me
> know your thoughts.

Will do, see below.

> .\" FIXME I find the "previously-functional systems" in the previous
> .\" sentence a little odd (since openat2() ia new sysycall), so I would
> .\" like to clarify a little...
> .\" Are you referring to the scenario where someone might take an
> .\" existing application that uses openat() and replaces the uses
> .\" of openat() with openat2()? In which case, is it correct to
> .\" understand that you mean that one should not just indiscriminately
> .\" add the RESOLVE_NO_XDEV flag to all of the openat2() calls?
> .\" If I'm not on the right track, could you point me in the right
> .\" direction please.

This is mostly meant as a warning to hopefully avoid applications
because the developer didn't realise that system paths may contain
symlinks or bind-mounts. For an application which has switched to
openat2() and then uses RESOLVE_NO_SYMLINKS for a non-security reason,
it's possible that on some distributions (or future versions of a
distribution) that their application will stop working because a system
path suddenly contains a symlink or is a bind-mount.

This was a concern which was brought up on LWN some time ago. If you can
think of a phrasing that makes this more clear, I'd appreciate it.

> .\" FIXME: what specific details in symlink(7) are being referred
> .\" by the following sentence? It's not clear.

The section on magic-links, but you're right that the sentence ordering
is a bit odd. It should probably go after the first sentence.

> .\" FIXME I found the following hard to understand (in particular, the
> .\" meaning of "scoped" is unclear) , and reworded as below. Is it okay?
> .\"     Absolute symbolic links and ".." path components will be scoped to
> .\"     .IR dirfd .

Scoped does broadly mean "interpreted relative to", though the
difference is mainly that when I said scoped it's meant to be more of an
assertive claim ("the kernel promises to always treat this path inside
dirfd"). But "interpreted relative to" is a clearer way of phrasing the
semantics, so I'm okay with this change.

> .\" FIXME The next piece is unclear (to me). What kind of ".." escape
> .\" attempts does chroot() not detect that RESOLVE_IN_ROOT does?

If the root is moved, you can escape from a chroot(2). But this sentence
might not really belong in a man-page since it's describing (important)
aspects of the implementation and not the semantics.

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

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