fs/proc: Stop trying to report thread stacks

fs/proc: Stop trying to report thread stacks

This reverts more of:

  b76437579d13 ("procfs: mark thread stack correctly in proc/<pid>/maps")

... which was partially reverted by:

  65376df58217 ("proc: revert /proc/<pid>/maps [stack:TID] annotation")

Originally, /proc/PID/task/TID/maps was the same as /proc/TID/maps.

In current kernels, /proc/PID/maps (or /proc/TID/maps even for
threads) shows "[stack]" for VMAs in the mm's stack address range.

In contrast, /proc/PID/task/TID/maps uses KSTK_ESP to guess the
target thread's stack's VMA.  This is racy, probably returns garbage
and, on arches with CONFIG_TASK_INFO_IN_THREAD=y, is also crash-prone:
KSTK_ESP is not safe to use on tasks that aren't known to be running
ordinary process-context kernel code.

This patch removes the difference and just shows "[stack]" for VMAs
in the mm's stack range.  This is IMO much more sensible -- the
actual "stack" address really is treated specially by the VM code,
and the current thread stack isn't even well-defined for programs
that frequently switch stacks on their own.

Reported-by: Jann Horn <[email protected]>
Signed-off-by: Andy Lutomirski <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Linux API <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Tycho Andersen <[email protected]>
Link: http://lkml.kernel.org/r/3e678474ec14e0a0ec34c611016753eea2e1b8ba.1475257877.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>