mm: Make copy_from_user() in migrate.c statically predictable

x86-32 has had a static test for copy_on_user() overflow for a while.
This test currently fails in mm/migrate.c resulting in an
allyesconfig/allmodconfig build failure on x86-32:

In function ‘copy_from_user’,
    inlined from ‘do_pages_stat’ at
    /home/hpa/kernel/git/mm/migrate.c:1012:
/home/hpa/kernel/git/arch/x86/include/asm/uaccess_32.h:212: error:
    call to ‘copy_from_user_overflow’ declared

Make the logic more explicit and therefore easier for gcc to
understand.

v2: rewrite the loop entirely using a more normal structure for a
    chunked-data loop (Linus Torvalds)

Reported-by: Len Brown <lenb@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Reviewed-and-Tested-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Arjan van de Ven <arjan@linux.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 21 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 9a0db5bbabe4..880bd592d38e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1002,33 +1002,27 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
 #define DO_PAGES_STAT_CHUNK_NR 16
         const void __user *chunk_pages[DO_PAGES_STAT_CHUNK_NR];
         int chunk_status[DO_PAGES_STAT_CHUNK_NR];
-        unsigned long i, chunk_nr = DO_PAGES_STAT_CHUNK_NR;
-        int err;
 
-        for (i = 0; i < nr_pages; i += chunk_nr) {
-                if (chunk_nr > nr_pages - i)
-                        chunk_nr = nr_pages - i;
+        while (nr_pages) {
+                unsigned long chunk_nr;
 
-                err = copy_from_user(chunk_pages, &pages[i],
-                                     chunk_nr * sizeof(*chunk_pages));
-                if (err) {
-                        err = -EFAULT;
-                        goto out;
-                }
+                chunk_nr = nr_pages;
+                if (chunk_nr > DO_PAGES_STAT_CHUNK_NR)
+                        chunk_nr = DO_PAGES_STAT_CHUNK_NR;
+
+                if (copy_from_user(chunk_pages, pages, chunk_nr * sizeof(*chunk_pages)))
+                        break;
 
                 do_pages_stat_array(mm, chunk_nr, chunk_pages, chunk_status);
 
-                err = copy_to_user(&status[i], chunk_status,
-                                   chunk_nr * sizeof(*chunk_status));
-                if (err) {
-                        err = -EFAULT;
-                        goto out;
-                }
-        }
-        err = 0;
+                if (copy_to_user(status, chunk_status, chunk_nr * sizeof(*status)))
+                        break;
 
-out:
-        return err;
+                pages += chunk_nr;
+                status += chunk_nr;
+                nr_pages -= chunk_nr;
+        }
+        return nr_pages ? -EFAULT : 0;
 }
 
 /*