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

Michael Kerrisk (man-pages)
2020-04-01T06:38:37Z

Hello Aleksa,

On 3/31/20 4:39 PM, Aleksa Sarai wrote:
> 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.

Thanks. I've made the text:

Applications that employ the RESOLVE_NO_XDEV flag
are encouraged to make its use configurable (unless
it is used for a specific security purpose), as bind
mounts are widely used by end-users. Setting this
flag indiscriminately—i.e., for purposes not specif‐
ically related to security—for all uses of openat2()
may result in spurious errors on previously-func‐
tional systems. This may occur if, for example, a
system pathname that is used by an application is
modified (e.g., in a new distribution release) so
that a pathname component (now) contains a bind
mount.

Okay?

>> .\" 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.

I must admit that I'm still confused. There's only the briefest of
mentions of magic links in symlink(7). Perhaps that needs to be fixed?

And, while I think of it, the text just preceding that FIXME says:

Due to the potential danger of unknowingly opening
these magic links, it may be preferable for users to
disable their resolution entirely.

This sentence reads a little strangely. Could you please give me some
concrete examples, and I will try rewording that sentence a bit.

>> .\" 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.

Okay.

>> .\" 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.

So, should I just remove the sentence?

Thanks,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/