Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

Joe Perches
2011-05-11T01:20:52Z

On Tue, 2011-05-10 at 18:10 -0700, John Stultz wrote:
> On Tue, 2011-05-10 at 17:51 -0700, Joe Perches wrote:
> > On Tue, 2011-05-10 at 17:23 -0700, John Stultz wrote:
> > > Acessing task->comm requires proper locking. However in the past
> > > access to current->comm could be done without locking. This
> > > is no longer the case, so all comm access needs to be done
> > > while holding the comm_lock.
> > Could misuse of %ptc (not using current) cause system lockup?
> It very well could. Although I don't see other %p options tring to
> handle invalid pointers. Any suggestions on how to best handle this?

The only one I know of is ipv6 which copies a 16 byte buffer
in case the pointed to value is unaligned. I suppose %pI6c
could be a problem or maybe %pS too, but it hasn't been in
practice. The use of %ptc somehow seemed more error prone.

> Most users are current, so forcing the more rare
> non-current users to copy it to a buffer first and use the normal %s
> would not be of much impact.
>
> Although I'm not sure if there's precedent for a %p value that didn't
> take a argument. Thoughts on that? Anyone else have an opinion here?

The uses of %ptc must add an argument or else gcc will complain.
I suggest you just ignore the argument value and use current.