dtrace discuss - Jan 2008 - Obtaining file attributes from open()

Hello all,

I''m trying to determine the file attributes (mode, owner, group) from
an
open() via the syscall probe. I have found fds[] and fileinfo_t and also

    curthread->t_procp->p_user.u_finfo.fi_list[arg0].uf_file->f_vnode

which gets me as far as the vnode.  But I don''t see how to get to
struct
vattr from there, or if there is a function that will enable me to access
these in the predicate.

Many thanks for any illumination,

Martin.

On Fri, Jan 04, 2008 at 07:08:55PM +0100, Martin Carpenter
wrote:
> 
> Hello all,
> 
> I''m trying to determine the file attributes (mode, owner, group)
from an
> open() via the syscall probe. I have found fds[] and fileinfo_t and also
> 
>    
curthread->t_procp->p_user.u_finfo.fi_list[arg0].uf_file->f_vnode
> 
> which gets me as far as the vnode.  But I don''t see how to get to
struct
> vattr from there, or if there is a function that will enable me to access
> these in the predicate.
> 
> Many thanks for any illumination,
> 
> Martin.
There is no way to statically examine that data because it''s up to
each filesystem to implement a function, VOP_GETATTR(), that fills
that structure in, and there is no requirement everything in sitting
in an in-core vattr_t already.  And if the object is not in core,
that function will need to read it in from disk etc.  So you can''t
generically use DTrace to examine in-memory data and be assured of
getting this data.  You''ll need to look at the code for the underlying
filesystem of your open() and, assuming all the data is in-memory,
write a function for that filesystem to get what you need.

As one example if you look at ufs_getattr(), you will see that you
can do a relatively straightforward conversion from f_vnode to the
ufs inode structure, and from there get at i_uid, i_gid, i_mode, etc.
ZFS is more complicated due to the use of FUIDs, but in the common
case you should be able to get what you need from the znode.

-Mike

-- 
Mike Shapiro, Solaris Kernel Development. blogs.sun.com/mws/

Oops, I forgot to copy the list.

Martin.

On Sun, 2008-01-06 at 18:22 +0100, Martin Carpenter
wrote:
> On Fri, 2008-01-04 at 10:15 -0800, Mike Shapiro wrote:
> > As one example if you look at ufs_getattr(), you will see that you
> > can do a relatively straightforward conversion from f_vnode to the
> > ufs inode structure, and from there get at i_uid, i_gid, i_mode, etc.
> 
> Thanks for that rapid and pedagogic response - that just enough to get
> me out of my rut. Fortunately I''m on UFS, so this:
> 
>   inode >     (struct inode
>
*)curthread->t_procp->p_user.u_finfo.fi_list[arg1].uf_file->f_vnode->v_data;
>   printf("%-5d", inode->i_ic.ic_uid);
> 
> was all I needed.
> 
> Thanks again.
> 
> Martin.
> 
>