linux-kernel - [PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 21 Jun 2017 20:55:28 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Xishi Qiu <qiuxishi@...wei.com>
Cc:     Hao Lee <haolee.swjtu@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback

Since current_order starts as MAX_ORDER-1 and is then only
decremented, the second half of the loop condition seems
superfluous. However, if order is 0, we may decrement current_order
past 0, making it UINT_MAX. This is obviously too subtle ([1], [2]).

Since we need to add some comment anyway, change the two variables to
signed, making the counting-down for loop look more familiar, and
apparently also making gcc generate slightly smaller code.

[1] https://lkml.org/lkml/2016/6/20/493
[2] https://lkml.org/lkml/2017/6/19/345

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
Michal, something like this, perhaps?

mm/page_alloc.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2302f250d6b1..e656f4da9772 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2204,19 +2204,23 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
  * list of requested migratetype, possibly along with other pages from the same
  * block, depending on fragmentation avoidance heuristics. Returns true if
  * fallback was found so that __rmqueue_smallest() can grab it.
+ *
+ * The use of signed ints for order and current_order is a deliberate
+ * deviation from the rest of this file, to make the for loop
+ * condition simpler.
  */
 static inline bool
-__rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
+__rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
 {
 	struct free_area *area;
-	unsigned int current_order;
+	int current_order;
 	struct page *page;
 	int fallback_mt;
 	bool can_steal;
 
 	/* Find the largest possible block of pages in the other list */
 	for (current_order = MAX_ORDER-1;
-				current_order >= order && current_order <= MAX_ORDER-1;
+				current_order >= order;
 				--current_order) {
 		area = &(zone->free_area[current_order]);
 		fallback_mt = find_suitable_fallback(area, current_order,
-- 
2.11.0