[Devel] Re: [PATCH 4/6] cr: checkpoint and restore task credentials

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
> 
> 
> Serge E. Hallyn wrote:
> > Quoting Oren Laadan (orenl at cs.columbia.edu):
> >>>  /* dump the task_struct of a given task */
> >>>  static int checkpoint_task_struct(struct ckpt_ctx *ctx, struct task_struct *t)
> >>>  {
> >>>  	struct ckpt_hdr_task *h;
> >>>  	int ret;
> >>> +	int realcred_ref, ecred_ref;
> >>> +
> >>> +	realcred_ref = checkpoint_obj(ctx, t->real_cred, CKPT_OBJ_CRED);
> >>> +	if (realcred_ref < 0)
> >>> +		return realcred_ref;
> >>> +
> >>> +	ecred_ref = checkpoint_obj(ctx, t->cred, CKPT_OBJ_CRED);
> >>> +	if (ecred_ref < 0)
> >>> +		return ecred_ref;
> >> Is this safe even if the checkpointed task's state changes ?
> >> (e.g. unfrozen - yes, I know there is a patch in the works to
> >> prevent this; but if we ever want to checkpoint STOPPED tasks...
> >> for instance).
> >>
> >> Would incrementing the refcount on t->{cred,real_cred} help ?
> > 
> > Doesn't checkpoint_obj already do that through obj_new?
> > 
> 
> No, it does not. There is a (potentially long) window of opportunity
> between the callback invoked from checkpoint_obj() - where the pointer
> is used, and when checkpoint_obj() later takes the extra reference.
> 
> See for comparison checkpoint_mm_obj(), it safely grabs the task->mm
> (with a reference) around the invocation of checkpoint_obj().

Hmm, ok.  Will do.

-serge
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