RE: [RFC][PATCH] extended attributes

[Luka Renko]

In http://acl.bestbits.at/man/extattr.5.html there is a statement:

   Device special files cannot be associated with extended user attributes 

What is the reason for this limitation? Why should there be a difference
between regular files/directories and special files (device files)?

I am also thinking in terms of HSM application (or DMAPI if you want). Where
do you want HSM attributes to be placed? I thought it should be in trusted,
because we might need access to them from user space. Other option is system
(that would require accessing them from kernel code) or user (might be
problematic, since regular user with write permission might remove them...
Actually, where are XFS guys storing DMAPI attributes today?

Regards,
Luka

> -----Original Message-----
> From: Nathan Scott [mailto:[e-mail]] 
> Sent: Wednesday, November 07, 2001 01:12
> To: Linus Torvalds; Andreas Gruenbacher
> Cc: [e-mail]; 
> [e-mail]; [e-mail]; 
> [e-mail]
> Subject: [RFC][PATCH] extended attributes
> 
> 
> hi folks,
> 
> I've been discussing a filesystem extended attributes API 
> with Andreas Gruenbacher (maintainer of the ext2/ext3 
> extended attributes patch[1]) which is suitable for other 
> Linux filesystems as well, in an effort to remove the 
> differences between our current implementations and to help 
> out the people building services layered above this 
> (especially Samba). In doing so we have reviewed the earlier 
> discussion[2,3] on this topic, and have attempted to produce 
> a new interface which I believe satisfies many of the issues 
> and ideas put forward there, while at the same time ensuring 
> that the interface is simple, and remains true to the design 
> of extended attributes being name:value pairs.
> 
> A manual page describing the system call interface can be 
> found here[4]. We're very interested in feedback on this.  In 
> partiular, Linus - would you consider the patch below, which 
> reserves system call numbers for this interface?  That would 
> be a big help to our collaborative effort.
> 
> We have written most of the code for XFS, and Andreas is 
> working away on the ext2/ext3 version.  Switching to a new 
> syscall interface is going to cause several compatibility 
> issues for our existing users, of course, so is not something 
> we want to rush into before soliciting feedback and
> (hopefully) getting some system call numbers reserved - 
> otherwise we may find ourselves needing to do a similar 
> transition again later.
> 
> As a test case for the interface, we will now be able to use 
> the same POSIX ACL userspace[1,5] between XFS and ext2 
> without any on-disk format changes in XFS - this was an 
> important interface design goal for us XFS folk, where our 
> format is fixed in stone as it is also used by IRIX.
> 
> We have also begun discussions with some of the LSM 
> developers, with the goal of implementing POSIX capabilities 
> and POSIX MAC (mandatory access
> control) security extensions in Linux also,  Here we again 
> expect to be able to provide a filesystem independent view of 
> these attributes, while still preserving the on-disk XFS 
> format for these attributes using the simple namespace 
> abstraction mechanism this new interface provides.
> 
> I've included some pointers[6,7,8,9,10] to other projects, 
> developers, discussions, etc. which I've come across who are 
> in some way or another interested in an extended attributes 
> implementation in the base kernel
> - just as examples of how various people are using (or 
> planning to use) the current ext2/ext3 and XFS interfaces on Linux.
> 
> cheers.
> 
> -- 
> Nathan
> 
> 
> [1] Extended attributes for ext2/ext3 and POSIX ACLs
>     http://acl.bestbits.at/
> [2] fs-devel extended attributes discussion
>     http://marc.theaimsgroup.com/?l=linux-fsdevel&m=97222475218787&w=2
> [3] Andrew Gildfind's interface comparison whitepaper
>     http://acl.bestbits.at/pre/gildfind-acls.pdf
> [4] New extattr(2) system call man pages
>     http://acl.bestbits.at/man/extattr.2.html
>     http://acl.bestbits.at/man/extattr.5.html
>     
> http://oss.sgi.com/cgi-bin/cvsweb.cgi/linux-2.4-xfs/cmd/attr2/
man/man2/extattr.2
> [5] Common POSIX ACL implementation for Linux
>     
> http://acl.bestbits.at/pipermail/acl-devel/2001-February/000495.html
>     http://www.samba.org/samba/whatsnew/samba-2.2.0.html
> [6] Andrew Morgan's Filesystem Capability patches
>     http://www.kernel.org/pub/linux/libs/security/linux-privs/README
> [7] LSM - Linux Security Module project
>     http://www.linuxsecurity.com/articles/forums_article-2854.html
>     
> http://mail.wirex.com/pipermail/linux-security-module/2001-Oct
ober/002310.html
[8] DMAPI/XDSM specification - implemented in XFS via extended attributes
    http://www.opengroup.org/onlinepubs/9657099/
    http://oss.sgi.com/projects/xfs/dmapi.html
[9] SnapFS snapshot filesystem
    http://lwn.net/2001/0308/a/snapfs.php3
[10] Will Dyson's resurrection of BeFS for Linux 2.4
    http://cs.earlham.edu/~will/software/linux/kernel/BeFS.html
    http://marc.theaimsgroup.com/?l=linux-fsdevel&m=100431033704112&w=2

diff -Naur 2.4.14-pristine/arch/i386/kernel/entry.S
2.4.14-reserved/arch/i386/kernel/entry.S
--- 2.4.14-pristine/arch/i386/kernel/entry.S	Sat Nov  3 12:18:49 2001
+++ 2.4.14-reserved/arch/i386/kernel/entry.S	Wed Nov  7 10:02:59 2001
@@ -622,6 +622,9 @@
 	.long SYMBOL_NAME(sys_ni_syscall)	/* Reserved for Security */
 	.long SYMBOL_NAME(sys_gettid)
 	.long SYMBOL_NAME(sys_readahead)	/* 225 */
+	.long SYMBOL_NAME(sys_ni_syscall)	/* reserved for extattr  */
+	.long SYMBOL_NAME(sys_ni_syscall)	/* reserved for lextattr */
+	.long SYMBOL_NAME(sys_ni_syscall)	/* reserved for fextattr */
 
 	.rept NR_syscalls-(.-sys_call_table)/4
 		.long SYMBOL_NAME(sys_ni_syscall)
diff -Naur 2.4.14-pristine/include/asm-i386/unistd.h
2.4.14-reserved/include/asm-i386/unistd.h
--- 2.4.14-pristine/include/asm-i386/unistd.h	Thu Oct 18 03:03:03 2001
+++ 2.4.14-reserved/include/asm-i386/unistd.h	Wed Nov  7 10:02:59 2001
@@ -230,6 +230,9 @@
 #define __NR_security		223	/* syscall for security modules */
 #define __NR_gettid		224
 #define __NR_readahead		225
+#define __NR_extattr		226	/* syscall for extended attributes
*/
+#define __NR_lextattr		227	/* syscall for extended attributes
*/
+#define __NR_fextattr		228	/* syscall for extended attributes
*/
 
 /* user-visible error numbers are in the range -1 - -124: see
<asm-i386/errno.h> */