Indication of extended attributes availability.

[Robert Watson]

On Fri, 21 Mar 2008, Václav Haisman wrote:

> I would like to have some sort of indication of extended attributes 
> availability for given FS. It seems that things like this (MAC, ACLs etc.) 
> are indicated using mount flags and those are available through statfs() 
> call. The following is tentative patch that would expose extended attributes 
> availability as mount flag. It is completely untested. I would just like to 
> know if it is a viable approach to the problem or should I scratch it and 
> try something else?

I think the preferred programmatic approach is actually via fpathconf(2).  I 
don't know if any other OS's have assigned a _PC constant for extended 
attributes, but if they have we should probably use the same one.  However, I 
guess there's a meta-question: is your goal to allow programs to be able to 
tell if extended attributes are available, or for administrators to be able to 
tell?

Robert N M Watson
Computer Laboratory
University of Cambridge

>
>
> --- ./sys/mount.h~	2007-04-10 12:12:44.000000000 +0200
> +++ ./sys/mount.h	2008-03-21 22:48:04.436630277 +0100
> @@ -218,7 +218,6 @@ void          __mnt_vnode_markerfree(str
> #define	MNT_SYNCHRONOUS	0x00000002	/* filesystem written 
> synchronously */
> #define	MNT_NOEXEC	0x00000004	/* can't exec from filesystem 
> */
> #define	MNT_NOSUID	0x00000008	/* don't honor setuid bits on 
> fs */
> -#define	MNT_NODEV	0		/* Deprecated option */
> #define	MNT_UNION	0x00000020	/* union with underlying 
> filesystem */
> #define	MNT_ASYNC	0x00000040	/* filesystem written 
> asynchronously */
> #define	MNT_SUIDDIR	0x00100000	/* special handling of SUID 
> on dirs */
> @@ -246,6 +245,7 @@ void          __mnt_vnode_markerfree(str
>  * but visible to the user.
>  * XXX some of these are not quite right.. (I've never seen the root flag 
> set)
>  */
> +#define MNT_EXTATTR	0x00000010	/* filesystem with extended 
> attributes */
> #define	MNT_LOCAL	0x00001000	/* filesystem is stored 
> locally */
> #define	MNT_QUOTA	0x00002000	/* quotas are enabled on 
> filesystem */
> #define	MNT_ROOTFS	0x00004000	/* identifies the root 
> filesystem */
> @@ -265,7 +265,8 @@ void          __mnt_vnode_markerfree(str
> 			MNT_ROOTFS	| MNT_NOATIME	| MNT_NOCLUSTERR| \
> 			MNT_NOCLUSTERW	| MNT_SUIDDIR	| MNT_SOFTDEP	| \
> 			MNT_IGNORE	| MNT_EXPUBLIC	| MNT_NOSYMFOLLOW | \
> -			MNT_JAILDEVFS	| MNT_MULTILABEL | MNT_ACLS)
> +			MNT_JAILDEVFS	| MNT_MULTILABEL | MNT_ACLS     | \
> +			MNT_EXTATTR)
>
> /* Mask of flags that can be updated. */
> #define	MNT_UPDATEMASK (MNT_NOSUID	| MNT_NOEXEC	| \
> --- ./ufs/ufs/ufs_extattr.c~	2006-03-13 04:08:08.000000000 +0100
> +++ ./ufs/ufs/ufs_extattr.c	2008-03-21 22:59:50.766939565 +0100
> @@ -220,6 +220,10 @@ ufs_extattr_start(struct mount *mp, stru
>
> 	ump->um_extattr.uepm_ucred = crhold(td->td_ucred);
>
> +	MNT_ILOCK(mp);
> +	mp->f_flags |= MNT_EXTATTR;
> +	MNT_IUNLOCK(mp);
> +
> unlock:
> 	ufs_extattr_uepm_unlock(ump, td);
>
> @@ -549,6 +553,10 @@ ufs_extattr_stop(struct mount *mp, struc
> 		    uele->uele_attrname, td);
> 	}
>
> +	MNT_ILOCK(mp);
> +	mp->f_flags &= ~MNT_EXTATTR;
> +	MNT_IUNLOCK(mp);
> +
> 	ump->um_extattr.uepm_flags &= ~UFS_EXTATTR_UEPM_STARTED;
>
> 	crfree(ump->um_extattr.uepm_ucred);
>
> --
> VH
>
>