linux-kernel - Re: [patch] close_range.2: new page documenting close

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Date:   Thu, 10 Dec 2020 15:36:42 +0100
From:   "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Stephen Kitt <steve@....org>, linux-man@...r.kernel.org,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [patch] close_range.2: new page documenting close_range(2)

Hi Christian,

Thanks for confirming that behavior.  Seems reasonable.

I was wondering...
If this call is equivalent to unshare(2)+{close(2) in a loop},
shouldn't it fail for the same reasons those syscalls can fail?

What about the following errors?:

>From unshare(2):

       EPERM  The calling process did not have the  required  privi‐
              leges for this operation.

>From close(2):
       EBADF  fd isn't a valid open file descriptor.

OK, this one can't happen with the current code.
Let's say there are fds 1 to 10, and you call 'close_range(20,30,0)'.
It's a no-op (although it will still unshare if the flag is set).
But souldn't it fail with EBADF?

       EINTR  The close() call was interrupted by a signal; see sig‐
              nal(7).

       EIO    An I/O error occurred.

       ENOSPC, EDQUOT
              On NFS, these errors are not normally reported against
              the first write which exceeds  the  available  storage
              space,  but  instead  against  a  subsequent write(2),
              fsync(2), or close().

Thanks,

Alex


On 12/9/20 11:56 AM, Christian Brauner wrote:
> On Wed, Dec 09, 2020 at 11:44:22AM +0100, Alejandro Colomar (man-pages) wrote:
>> Hey Christian,
>>
>> I have a question for you below.
>>
>> Thanks,
> 
> Hey Alex,
> 
> Sure!

[...]

>>
>> AFAICS after reading the code, if the unsharing fails,
>> it will not close any file descriptors (please correct me if I'm wrong).
>>
>> Just wanted to be sure that it was the intended behavior with you,
>> and if so, it would be good to document it in the page.
> 
> Yes, this is intended because if the unshare fails we haven't yet
> actually started closing anything so we're before the point of no
> return where we ignore failures. So we can let userspace decide whether
> they want to retry without CLOSE_RANGE_UNSHARE.
> 
> Christian
> 

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es