GNU bug report logs - #8831
[Ubuntu10.10] Impossible to create an hardlink without write access

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Johan Oudinet <johan.oudinet <at> gmail.com>
Subject: bug#8831: closed (Re: bug#8831: [Ubuntu10.10] Impossible to
 create an hardlink without write access)
Date: Thu, 09 Jun 2011 21:24:02 +0000

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Your bug report

#8831: [Ubuntu10.10] Impossible to create an hardlink without write access

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 8831 <at> debbugs.gnu.org.

-- 
8831: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8831
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Sven Joachim <svenjoac <at> gmx.de>
To: Bob Proulx <bob <at> proulx.com>
Cc: 8831-done <at> debbugs.gnu.org, Johan Oudinet <johan.oudinet <at> gmail.com>
Subject: Re: bug#8831: [Ubuntu10.10] Impossible to create an hardlink without
	write access
Date: Thu, 09 Jun 2011 23:23:35 +0200

On 2011-06-09 22:59 +0200, Bob Proulx wrote:

> Johan Oudinet wrote:
>> GNU coreutils 8.5
>> Ubuntu 10.10 32bits Ext4
>>
>> $ sudo touch f; ln f g
>> ln: creating hard link `g' => `f': Operation not permitted
>
> Thank you for the bug report.  However I am unable to recreate this
> problem using 8.5 on my Debian system using ext3.
>
>   $ sudo touch f
>   $ ln f g
>   $ ls -ldog f g
>   -rw-r--r-- 2 0 Jun  9 14:55 f
>   -rw-r--r-- 2 0 Jun  9 14:55 g
>
> Perhaps this is a problem with ext4?  Can you run your test using
> strace so that we can see what the system is saying?  Something like
> this following command.
>
>   $ strace -e trace=file -o /tmp/ln.strace.out ln f g
>
> On my system I see this output:
>
>   ...
>   stat("g", 0x7fffa6430f10)               = -1 ENOENT (No such file or directory)
>   lstat("f", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
>   linkat(AT_FDCWD, "f", AT_FDCWD, "g", 0) = 0
>
>> However, the following commands work:
>> $ sudo touch f2; sudo chmod a+w f2; ln f2 g
>> 
>> The first call to ln should be permitted too.
>
> As far as I can tell it is permitted.  If it isn't then it would be a
> kernel limitation.

This is indeed due to an Ubuntu kernel patch that is present since
10.10.  The rationale is given in
https://wiki.ubuntu.com/SecurityTeam/Roadmap/KernelHardening#Hardlink%20Protection.
I'm taking the liberty to close the bug.

Cheers,
       Sven

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From: Johan Oudinet <johan.oudinet <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: [Ubuntu10.10] Impossible to create an hardlink without write access
Date: Thu, 9 Jun 2011 19:47:16 +0200

*** System information ***
GNU coreutils 8.5
Ubuntu 10.10 32bits Ext4

*** Error description ***
$ sudo touch f; ln f g
ln: creating hard link `g' => `f': Operation not permitted

However, the following commands work:
$ sudo touch f2; sudo chmod a+w f2; ln f2 g

The first call to ln should be permitted too.

Regards,
-- 
Johan

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