fs/proc/task_mmu.c: do not show VmExe bigger than total executable virtual memory

If start_code / end_code pointers are screwed then "VmExe" could be
bigger than total executable virtual memory and "VmLib" becomes
negative:

  VmExe:	  294320 kB
  VmLib:	18446744073709327564 kB

VmExe and VmLib documented as text segment and shared library code size.

Now their sum will be always equal to mm->exec_vm which sums size of
executable and not writable and not stack areas.

I've seen this for huge (>2Gb) statically linked binary which has whole
world inside.  For it start_code ..  end_code range also covers one of
rodata sections.  Probably this is bug in customized linker, elf loader
or both.

Anyway CONFIG_CHECKPOINT_RESTORE allows to change these pointers, thus
we cannot trust them without validation.

Link: http://lkml.kernel.org/r/150728955451.743749.11276392315459539583.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -47,8 +47,11 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
 	if (hiwater_rss < mm->hiwater_rss)
 		hiwater_rss = mm->hiwater_rss;
 
-	text = (PAGE_ALIGN(mm->end_code) - (mm->start_code & PAGE_MASK)) >> 10;
-	lib = (mm->exec_vm << (PAGE_SHIFT-10)) - text;
+	/* split executable areas between text and lib */
+	text = PAGE_ALIGN(mm->end_code) - (mm->start_code & PAGE_MASK);
+	text = min(text, mm->exec_vm << PAGE_SHIFT);
+	lib = (mm->exec_vm << PAGE_SHIFT) - text;
+
 	swap = get_mm_counter(mm, MM_SWAPENTS);
 	seq_printf(m,
 		"VmPeak:\t%8lu kB\n"
@@ -76,7 +79,9 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
 		file << (PAGE_SHIFT-10),
 		shmem << (PAGE_SHIFT-10),
 		mm->data_vm << (PAGE_SHIFT-10),
-		mm->stack_vm << (PAGE_SHIFT-10), text, lib,
+		mm->stack_vm << (PAGE_SHIFT-10),
+		text >> 10,
+		lib >> 10,
 		mm_pgtables_bytes(mm) >> 10,
 		swap << (PAGE_SHIFT-10));
 	hugetlb_report_usage(m, mm);