coredump: elf_core_dump: skip kernel threads

linux_binfmt->core_dump() runs before the process does exit_aio(), this
means that we can hit the kernel thread which shares the same ->mm.
Afaics, nothing really bad can happen, but perhaps it makes sense to fix
this minor bug.

It is sad we have to iterate over all threads in system and use
GFP_ATOMIC.  Hopefully we can kill theses ugly do_each_thread()s, but this
needs some nontrivial changes in mm_struct and do_coredump.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_elf.c

@@ -1520,6 +1520,9 @@ static int fill_note_info(struct elfhdr *elf, int phdrs,
 	rcu_read_lock();
 	do_each_thread(g, p)
 		if (p->mm == dump_task->mm) {
+			if (p->flags & PF_KTHREAD)
+				continue;
+
 			t = kzalloc(offsetof(struct elf_thread_core_info,
 					     notes[info->thread_notes]),
 				    GFP_ATOMIC);
@@ -1724,6 +1727,9 @@ static int fill_note_info(struct elfhdr *elf, int phdrs,
 		rcu_read_lock();
 		do_each_thread(g, p)
 			if (current->mm == p->mm && current != p) {
+				if (p->flags & PF_KTHREAD)
+					continue;
+
 				ets = kzalloc(sizeof(*ets), GFP_ATOMIC);
 				if (!ets) {
 					rcu_read_unlock();

fs/binfmt_elf_fdpic.c

@@ -1626,6 +1626,9 @@ static int elf_fdpic_core_dump(long signr, struct pt_regs *regs,
 		rcu_read_lock();
 		do_each_thread(g,p)
 			if (current->mm == p->mm && current != p) {
+				if (p->flags & PF_KTHREAD)
+					continue;
+
 				tmp = kzalloc(sizeof(*tmp), GFP_ATOMIC);
 				if (!tmp) {
 					rcu_read_unlock();