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From: "Michael Kerrisk (man-pages)" <>
To: Aleksa Sarai <>
Cc:, Al Viro <>,
	Christian Brauner <>,
	Aleksa Sarai <>,,,,
Subject: Re: [PATCH man-pages v2 2/2] openat2.2: document new openat2(2) syscall
Date: Wed, 1 Apr 2020 08:38:29 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hello Aleksa,

On 3/31/20 4:39 PM, Aleksa Sarai wrote:
> On 2020-03-30, Michael Kerrisk (man-pages) <> 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 <>
>> 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


>> .\" 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.


>> .\" 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?



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:

  reply	other threads:[~2020-04-01  6:38 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-02 15:19 [PATCH man-pages v2 0/2] document openat2(2) Aleksa Sarai
2020-02-02 15:19 ` [PATCH man-pages v2 1/2] path_resolution.7: update to mention openat2(2) features Aleksa Sarai
2020-03-30 20:38   ` Michael Kerrisk (man-pages)
2020-02-02 15:19 ` [PATCH man-pages v2 2/2] openat2.2: document new openat2(2) syscall Aleksa Sarai
2020-03-30  9:08   ` Michael Kerrisk (man-pages)
2020-03-30  9:20     ` Aleksa Sarai
2020-03-30  9:36       ` Michael Kerrisk (man-pages)
2020-03-30  9:48         ` Aleksa Sarai
2020-03-30 20:43   ` Michael Kerrisk (man-pages)
2020-03-31 14:39     ` Aleksa Sarai
2020-04-01  6:38       ` Michael Kerrisk (man-pages) [this message]
2020-04-08 21:29         ` Michael Kerrisk (man-pages)
2020-04-12 16:49         ` Aleksa Sarai
2020-04-13  7:22           ` Michael Kerrisk (man-pages)
2020-04-14 10:35             ` Aleksa Sarai
2020-04-15 20:24               ` Michael Kerrisk (man-pages)
2020-05-04 10:17               ` Michael Kerrisk (man-pages)
2020-06-10  5:53               ` [PATCH] symlink.7: document magic-links more completely Aleksa Sarai
2020-06-19 13:00                 ` Michael Kerrisk (man-pages)

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