lib/vsprintf: add %pT format specifier

lib/vsprintf: add %pT format specifier

Since task_struct->comm can be modified by other threads while the current
thread is reading it, it is recommended to use get_task_comm() for reading
it.

However, since get_task_comm() holds task_struct->alloc_lock spinlock,
some users cannot use get_task_comm().  Also, a lot of users are directly
reading from task_struct->comm even if they can use get_task_comm().  Such
users might obtain inconsistent result.

This patch introduces %pT format specifier for printing task_struct->comm.
Currently %pT does not provide consistency.  I'm planning to change to
use RCU in the future.  By using RCU, the comm name read from
task_struct->comm will be guaranteed to be consistent.  But before
modifying set_task_comm() to use RCU, we need to kill direct ->comm users
who do not use get_task_comm().

An example for converting direct ->comm users is shown below.  Since many
debug printings use p == current, you can pass NULL instead of p if p ==
current.

  pr_info("comm=%s\n", p->comm);       => pr_info("comm=%pT\n", p);
  pr_info("comm=%s\n", current->comm); => pr_info("comm=%pT\n", NULL);

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>