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

[H . Peter Anvin]

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 âcopy_from_userâ,
    inlined from âdo_pages_statâ at
    /home/hpa/kernel/git/mm/migrate.c:1012:
/home/hpa/kernel/git/arch/x86/include/asm/uaccess_32.h:212: error:
    call to âcopy_from_user_overflowâ 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@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
---
 mm/migrate.c |   36 +++++++++++++++---------------------
 1 files changed, 15 insertions(+), 21 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 9a0db5b..880bd59 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;
 }
 
 /*
-- 
1.6.5.2


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