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SubjectRe: [PATCH] Initial support for struct vfs_cred [0/1]
On Sun, 2002-09-01 at 21:25, Trond Myklebust wrote:
> >>>>> " " == Luca Barbieri <> writes:
> > But you'll need to modify the declaration of the various
> > function pointers whose implementations might need credentials
> > and modify all functions that call them and deal with
> > permissions. Instead with my proposal the credentials are
> Yes... And this is a useful activity in itself, as the existence of
> all these hacks that temporarily change uid/gid/whatever... show.
I disagree.
It reduces performance, and results in huge patches.
Furthermore you can't predict whether the target of a function pointer
will need credentials or not.

> > automatically immutable across the syscall without needing to
> > worry at all about locks, counts and sharing.
> I still have no opinion about your proposal for implementing CLONE_CRED.
> What I fail to see is why you appear to insist it would be
> incompatible with the idea of copy-on-write VFS credentials (which I
> explained are interesting for other purposes).
I don't insist on that (see below).
> I also fail to understand why you insist that we need to drop the idea
> of copy-on-write credentials in order to optimize for this fringe case
> in which somebody calls sys_access() or exec with euid != fsuid.
I do not propose to remove the idea of copy-on-write credentials, just
to use copy-on-write only for groups and "copy-always" for uid and gid
so that access to uid and gid doesn't need to go through the credentials

You have code like this:
+ cred = get_current_vfscred();
fdata->fd_do_lml = 0;
- fdata->fd_fsuid = current->fsuid;
- fdata->fd_fsgid = current->fsgid;
+ fdata->fd_fsuid = cred->uid;
+ fdata->fd_fsgid = cred->gid;
fdata->fd_mode = file->f_dentry->d_inode->i_mode;
- fdata->fd_ngroups = current->ngroups;
- for (i=0 ; i<current->ngroups ; i++)
- fdata->fd_groups[i] = current->groups[i];
+ fdata->fd_ngroups = cred->ngroups;
+ for (i=0 ; i<cred->ngroups ; i++)
+ fdata->fd_groups[i] = cred->groups[i];
fdata->fd_bytes_written = 0; /*when open,written data is zero*/
file->private_data = fdata;
+ put_vfscred(cred);

The get/put here only make sense if something else can modify the
credentials without it.
Otherwise, you can skip them since the reference count will always be >
0 because nothing else can change the current->vfscred pointer, and that
will contribute to the reference count.

What I'm trying to do is to design credentials in such a way that, even
with shared credentials, the get/put won't be necessary.
I'm also trying to put such credentials directly in task_struct so that
the code doesn't need to get a pointer from task_struct and then fetch
the credentials from the structure it points to.

The order of likelyhood of credential operations is assumed to be the
1. Use/copy/local temporary modification of uid and gid
2. Use/copy/local temporary modification of groups
3. Permanent modification of something
* Local temporary modification means that the modification only has
effect on the current task and is revert before the end of the syscall

To optimize for this I propose to:
1. Put uid and gid directly in task_struct (optimize for 1)
2. Use copy-on-write on groups so that we avoid copying an big array
(optimize for 2.copy)
3. Change credentials only when not performing a syscall (optimize for 1
and 2)

> "changing fsuid/fsgid is *not* the common case that needs optimization."
I completely agree with this, that's the whole point!

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