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

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

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

Message #8 received at 8831 <at> debbugs.gnu.org (full text, mbox):

From: Bob Proulx <bob <at> proulx.com>
To: Johan Oudinet <johan.oudinet <at> gmail.com>
Cc: 8831 <at> debbugs.gnu.org
Subject: Re: bug#8831: [Ubuntu10.10] Impossible to create an hardlink without
	write access
Date: Thu, 9 Jun 2011 14:59:06 -0600

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.

Bob

Message #13 received at 8831-done <at> debbugs.gnu.org (full text, mbox):

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

Message #16 received at 8831 <at> debbugs.gnu.org (full text, mbox):

From: Jim Meyering <jim <at> meyering.net>
To: 8831 <at> debbugs.gnu.org
Cc: svenjoac <at> gmx.de
Subject: Re: bug#8831: [Ubuntu10.10] Impossible to create an hardlink without
	write access
Date: Sat, 11 Jun 2011 08:37:00 +0200

Sven Joachim wrote:
> 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.

Thanks for explaining and for closing the issue.
It's too easy to forget the latter step.

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