A question about extended attributes of filesystem objects (setfattr command)

A question about extended attributes of filesystem objects (setfattr command)

[Jacky Malcles]

I have a question regarding
 Attributes of symlinks vs. the files pointed to

If I try to attach name:value pair to object symlink file
then I'll get: "Operation not permitted"

reading the man pages of setfattr (or attr) I thought that it operates
on the attributes of  the  symbolic link itself.

show:
-----
touch f
ln -s f l
setfattr -n user.filename -v ascii1 f l
setfattr -h -n user.filename  -v ascii2 f
getfattr -d f l
setfattr -h -n user.filename  -v ascii3 l
setfattr -h --no-dereference -n user.filename  -v ascii4 l
getfattr -d f l

so, my question is : what is expected ?
I've
libattr-devel-2.2.0-1
libattr-2.2.0-1
attr-2.2.0-1
and  a 2.6.7 kernel

many thanks,
regards,

-- 
 Jacky Malcles    	     B1-403   Email : Jacky.Malcles@bull.net
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 Tel : 04.76.29.73.14

Re: A question about extended attributes of filesystem objects (setfattr command)

[Michael Kerrisk]

Gidday,

> I have a question regarding
>  Attributes of symlinks vs. the files pointed to
> 
> If I try to attach name:value pair to object symlink file
> then I'll get: "Operation not permitted"

What file system are you using?  If ext2, ext3 (or patched kernel 
supporting Reiserfs EAs), did you mount with "-o user_xattr?
(The above error suggests you haven't used this option.)

> reading the man pages of setfattr (or attr) I thought that it operates
> on the attributes of  the  symbolic link itself.

No, these commands follow symbolic links.

> show:
> -----
> touch f
> ln -s f l
> setfattr -n user.filename -v ascii1 f l
> setfattr -h -n user.filename  -v ascii2 f
> getfattr -d f l
> setfattr -h -n user.filename  -v ascii3 l
> setfattr -h --no-dereference -n user.filename  -v ascii4 l
> getfattr -d f l
> 
> so, my question is : what is expected ?

attr(5) specifically notes that USER EAs are disallowed on 
symbolic links, but this is rather an issu that affects the 
use of lsetxattr(2).

Cheers,

Michael

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Re: A question about extended attributes of filesystem objects (setfattr command)

[Jacky Malcles]

Michael Kerrisk wrote:
> 
> Gidday,
> 
> > I have a question regarding
> >  Attributes of symlinks vs. the files pointed to
> >
> > If I try to attach name:value pair to object symlink file
> > then I'll get: "Operation not permitted"
> 
> What file system are you using?  If ext2, ext3 (or patched kernel
> supporting Reiserfs EAs), did you mount with "-o user_xattr?
> (The above error suggests you haven't used this option.)

# mount
/dev/sdb3 on /a type ext3 (rw,acl,user_xattr)
...etc...


> 
> > reading the man pages of setfattr (or attr) I thought that it operates
> > on the attributes of  the  symbolic link itself.
> 
> No, these commands follow symbolic links.
> 
> > show:
> > -----
> > touch f
> > ln -s f l
> > setfattr -n user.filename -v ascii1 f l
> > setfattr -h -n user.filename  -v ascii2 f
> > getfattr -d f l
> > setfattr -h -n user.filename  -v ascii3 l
> > setfattr -h --no-dereference -n user.filename  -v ascii4 l
> > getfattr -d f l
> >
> > so, my question is : what is expected ?
should have added this:

[root@t20 acl]# show
# file: f
user.filename="ascii2"

# file: l
user.filename="ascii2"

setfattr: l: Operation not permitted
setfattr: l: Operation not permitted
# file: f
user.filename="ascii2"

# file: l
user.filename="ascii2"

[root@t20 acl]#

> 
> attr(5) specifically notes that USER EAs are disallowed on
> symbolic links, but this is rather an issu that affects the
> use of lsetxattr(2).
> 
> Cheers,
> 
> Michael
> 
> --
> "Sie haben neue Mails!" - Die GMX Toolbar informiert Sie beim Surfen!
> Jetzt aktivieren unter http://www.gmx.net/info
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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-- 
 Jacky Malcles    	     B1-403   Email : Jacky.Malcles@bull.net
 Bull SA, 1 rue de Provence, B.P 208, 38432 Echirolles CEDEX, FRANCE
 Tel : 04.76.29.73.14

Re: A question about extended attributes of filesystem objects (setfattr command)

[Michael Kerrisk]

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[-- Attachment #1: Type: text/plain; charset="us-ascii", Size: 2202 bytes --]

Salut Jacky,

> > > I have a question regarding
> > >  Attributes of symlinks vs. the files pointed to
> > >
> > > If I try to attach name:value pair to object symlink file
> > > then I'll get: "Operation not permitted"
> > 
> > What file system are you using?  If ext2, ext3 (or patched kernel
> > supporting Reiserfs EAs), did you mount with "-o user_xattr?
> > (The above error suggests you haven't used this option.)
> 
> # mount
> /dev/sdb3 on /a type ext3 (rw,acl,user_xattr)
> ...etc...

Okay.

> > > reading the man pages of setfattr (or attr) I thought that it operates
> > > on the attributes of  the  symbolic link itself.
> > 
> > No, these commands follow symbolic links.
> > 
> > > show:
> > > -----
> > > touch f
> > > ln -s f l
> > > setfattr -n user.filename -v ascii1 f l
> > > setfattr -h -n user.filename  -v ascii2 f
> > > getfattr -d f l
> > > setfattr -h -n user.filename  -v ascii3 l
> > > setfattr -h --no-dereference -n user.filename  -v ascii4 l

I'm sorry -- I missed the above "--no-dereference"; that's why
my point below becomes relevant.

> > > getfattr -d f l
> > >
> > > so, my question is : what is expected ?
> should have added this:
> 
> [root@t20 acl]# show
> # file: f
> user.filename="ascii2"
> 
> # file: l
> user.filename="ascii2"
> 
> setfattr: l: Operation not permitted
> setfattr: l: Operation not permitted
> # file: f
> user.filename="ascii2"
> 
> # file: l
> user.filename="ascii2"
> 
> [root@t20 acl]#

But note the following:

> > attr(5) specifically notes that USER EAs are disallowed on
> > symbolic links, but this is rather an issu that affects the
> > use of lsetxattr(2).

or setfattr if you specify "--no-dereference"...

There is a reason for this restriction: for a symbolic link, all 
permissions are enabled for all users, and these permissions 
cannot be changed. This means that permissions cannot be used (as 
they would be with regular files) to prevent arbitrary users from 
placing user EAs on a symbolic link.  Thus all users are 
prevented from creating user EAs on the symbolic link.

Michael


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Re: A question about extended attributes of filesystem objects (setfattr command)

[Jacky Malcles]

Michael Kerrisk wrote:
> 
> Gidday,
> 
> > I have a question regarding
> >  Attributes of symlinks vs. the files pointed to
> >
> > If I try to attach name:value pair to object symlink file
> > then I'll get: "Operation not permitted"
> 
> What file system are you using?  If ext2, ext3 (or patched kernel
> supporting Reiserfs EAs), did you mount with "-o user_xattr?
> (The above error suggests you haven't used this option.)
> 
> > reading the man pages of setfattr (or attr) I thought that it operates
> > on the attributes of  the  symbolic link itself.
> 
> No, these commands follow symbolic links.
> 
> > show:
> > -----
> > touch f
> > ln -s f l
> > setfattr -n user.filename -v ascii1 f l
> > setfattr -h -n user.filename  -v ascii2 f
> > getfattr -d f l
> > setfattr -h -n user.filename  -v ascii3 l
> > setfattr -h --no-dereference -n user.filename  -v ascii4 l
> > getfattr -d f l
> >
> > so, my question is : what is expected ?
>

thanks Michael, 
below is what I was trying to find out !!
> attr(5) specifically notes that USER EAs are disallowed on
> symbolic links, but this is rather an issu that affects the
> use of lsetxattr(2).
> 
> Cheers,
> 
> Michael
> 
> --
> "Sie haben neue Mails!" - Die GMX Toolbar informiert Sie beim Surfen!
> Jetzt aktivieren unter http://www.gmx.net/info
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

-- 
 Jacky Malcles    	     B1-403   Email : Jacky.Malcles@bull.net
 Bull SA, 1 rue de Provence, B.P 208, 38432 Echirolles CEDEX, FRANCE
 Tel : 04.76.29.73.14