[PATCH] mm: slub: Really fix slab walking for init_on_free

[Laura Abbott <labbott@redhat.com>]

Commit 1b7e816fc80e ("mm: slub: Fix slab walking for init_on_free")
fixed one problem with the slab walking but missed a key detail:
When walking the list, the head and tail pointers need to be updated
since we end up reversing the list as a result. Without doing this,
bulk free is broken. One way this is exposed is a NULL pointer with
slub_debug=F:

=============================================================================
BUG skbuff_head_cache (Tainted: G                T): Object already free
-----------------------------------------------------------------------------

INFO: Slab 0x000000000d2d2f8f objects=16 used=3 fp=0x0000000064309071 flags=0x3fff00000000201
BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 0 PID: 0 Comm: swapper/0 Tainted: G    B           T 5.3.8 #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:print_trailer+0x70/0x1d5
Code: 28 4d 8b 4d 00 4d 8b 45 20 81 e2 ff 7f 00 00 e8 86 ce ef ff 8b 4b 20 48 89 ea 48 89 ee 4c 29 e2 48 c7 c7 90 6f d4 89 48 01 e9 <48> 33 09 48 33 8b 70 01 00 00 e8 61 ce ef ff f6 43 09 04 74 35 8b
RSP: 0018:ffffbf7680003d58 EFLAGS: 00010046
RAX: 000000000000005d RBX: ffffa3d2bb08e540 RCX: 0000000000000000
RDX: 00005c2d8fdc2000 RSI: 0000000000000000 RDI: ffffffff89d46f90
RBP: 0000000000000000 R08: 0000000000000242 R09: 000000000000006c
R10: 0000000000000000 R11: 0000000000000030 R12: ffffa3d27023e000
R13: fffff11080c08f80 R14: ffffa3d2bb047a80 R15: 0000000000000002
FS:  0000000000000000(0000) GS:ffffa3d2be400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000007a6c4000 CR4: 00000000000006f0
Call Trace:
 <IRQ>
 free_debug_processing.cold.37+0xc9/0x149
 ? __kfree_skb_flush+0x30/0x40
 ? __kfree_skb_flush+0x30/0x40
 __slab_free+0x22a/0x3d0
 ? tcp_wfree+0x2a/0x140
 ? __sock_wfree+0x1b/0x30
 kmem_cache_free_bulk+0x415/0x420
 ? __kfree_skb_flush+0x30/0x40
 __kfree_skb_flush+0x30/0x40
 net_rx_action+0x2dd/0x480
 __do_softirq+0xf0/0x246
 irq_exit+0x93/0xb0
 do_IRQ+0xa0/0x110
 common_interrupt+0xf/0xf
 </IRQ>

Given we're now almost identical to the existing debugging
code which correctly walks the list, combine with that.

Link: https://lkml.kernel.org/r/20191104170303.GA50361@gandi.net
Reported-by: Thibaut Sautereau <thibaut.sautereau@clip-os.org>
Fixes: 1b7e816fc80e ("mm: slub: Fix slab walking for init_on_free")
Signed-off-by: Laura Abbott <labbott@redhat.com>
---
 mm/slub.c | 39 +++++++++------------------------------
 1 file changed, 9 insertions(+), 30 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index dac41cf0b94a..d2445dd1c7ed 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1432,12 +1432,15 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 	void *old_tail = *tail ? *tail : *head;
 	int rsize;
 
-	if (slab_want_init_on_free(s)) {
-		void *p = NULL;
+	/* Head and tail of the reconstructed freelist */
+	*head = NULL;
+	*tail = NULL;
 
-		do {
-			object = next;
-			next = get_freepointer(s, object);
+	do {
+		object = next;
+		next = get_freepointer(s, object);
+
+		if (slab_want_init_on_free(s)) {
 			/*
 			 * Clear the object and the metadata, but don't touch
 			 * the redzone.
@@ -1447,29 +1450,8 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 							   : 0;
 			memset((char *)object + s->inuse, 0,
 			       s->size - s->inuse - rsize);
-			set_freepointer(s, object, p);
-			p = object;
-		} while (object != old_tail);
-	}
-
-/*
- * Compiler cannot detect this function can be removed if slab_free_hook()
- * evaluates to nothing.  Thus, catch all relevant config debug options here.
- */
-#if defined(CONFIG_LOCKDEP)	||		\
-	defined(CONFIG_DEBUG_KMEMLEAK) ||	\
-	defined(CONFIG_DEBUG_OBJECTS_FREE) ||	\
-	defined(CONFIG_KASAN)
 
-	next = *head;
-
-	/* Head and tail of the reconstructed freelist */
-	*head = NULL;
-	*tail = NULL;
-
-	do {
-		object = next;
-		next = get_freepointer(s, object);
+		}
 		/* If object's reuse doesn't have to be delayed */
 		if (!slab_free_hook(s, object)) {
 			/* Move object to the new freelist */
@@ -1484,9 +1466,6 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 		*tail = NULL;
 
 	return *head != NULL;
-#else
-	return true;
-#endif
 }
 
 static void *setup_object(struct kmem_cache *s, struct page *page,
-- 
2.21.0