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Subject[PATCH] slab: fix oops when reading /proc/slab_allocators
Date
commit 'b1cb098: change the management method of free objects of the slab'
introduces bug on slab leak detector('/proc/slab_allocators'). This
detector works like as following decription.

1. traverse all objects on all the slabs.
2. determine whether it is active or not.
3. if active, print who allocate this object.

commit 'b1cb098' changes the way how to manage free objects, so logic
determining whether it is active or not is also changed. In before, we
regard object in cpu caches as inactive one, but, with this commit, we
mistakenly regard object in cpu caches as active one.

This intoduces kernel oops if DEBUG_PAGEALLOC is enabled.
If DEBUG_PAGEALLOC is enabled, kernel_map_pages() is used to detect
who corrupt free memory in the slab. It unmaps page table mapping if
object is free and map it if object is active. When slab leak detector
check object in cpu caches, it mistakenly think this object active so
try to access object memory to retrieve caller of allocation. At this
point, page table mapping to this object doesn't exist, so oops occurs.

Following is oops message reported from Dave.
https://lkml.org/lkml/2014/3/6/626

It blew up when something tried to read /proc/slab_allocators
(Just cat it, and you should see the oops below)

Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
Modules linked in:
[snip...]
CPU: 1 PID: 9386 Comm: trinity-c33 Not tainted 3.14.0-rc5+ #131
task: ffff8801aa46e890 ti: ffff880076924000 task.ti: ffff880076924000
RIP: 0010:[<ffffffffaa1a8f4a>] [<ffffffffaa1a8f4a>] handle_slab+0x8a/0x180
RSP: 0018:ffff880076925de0 EFLAGS: 00010002
RAX: 0000000000001000 RBX: 0000000000000000 RCX: 000000005ce85ce7
RDX: ffffea00079be100 RSI: 0000000000001000 RDI: ffff880107458000
RBP: ffff880076925e18 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 000000000000000f R12: ffff8801e6f84000
R13: ffffea00079be100 R14: ffff880107458000 R15: ffff88022bb8d2c0
FS: 00007fb769e45740(0000) GS:ffff88024d040000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff8801e6f84ff8 CR3: 00000000a22db000 CR4: 00000000001407e0
DR0: 0000000002695000 DR1: 0000000002695000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000070602
Stack:
ffff8802339dcfc0 ffff88022bb8d2c0 ffff880107458000 ffff88022bb8d2c0
ffff8802339dd008 ffff8802339dcfc0 ffffea00079be100 ffff880076925e68
ffffffffaa1ad9be ffff880203fe4f00 ffff88022bb8d318 0000000076925e98
Call Trace:
[<ffffffffaa1ad9be>] leaks_show+0xce/0x240
[<ffffffffaa1e6c0e>] seq_read+0x28e/0x490
[<ffffffffaa23008d>] proc_reg_read+0x3d/0x80
[<ffffffffaa1c026b>] vfs_read+0x9b/0x160
[<ffffffffaa1c0d88>] SyS_read+0x58/0xb0
[<ffffffffaa7420aa>] tracesys+0xd4/0xd9
Code: f5 00 00 00 0f 1f 44 00 00 48 63 c8 44 3b 0c 8a 0f 84 e3 00 00 00 83 c0 01 44 39 c0 72 eb 41 f6 47 1a 01 0f 84 e9 00 00 00 89 f0 <4d> 8b 4c 04 f8 4d 85 c9 0f 84 88 00 00 00 49 8b 7e 08 4d 8d 46
RIP [<ffffffffaa1a8f4a>] handle_slab+0x8a/0x180
RSP <ffff880076925de0>
CR2: ffff8801e6f84ff8

To fix the problem, I introduces object status buffer on each slab.
With this, we can track object status precisely, so slab leak detector
would not access active object and no kernel oops would occur.
Memory overhead caused by this fix is only imposed to
CONFIG_DEBUG_SLAB_DELAK which is mainly used for debugging, so memory
overhead isn't big problem.

Reported-by: Dave Jones <davej@redhat.com>
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
---
This patch is based on v3.15-rc1.

diff --git a/mm/slab.c b/mm/slab.c
index 388cb1a..101eae4 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -386,6 +386,41 @@ static void **dbg_userword(struct kmem_cache *cachep, void *objp)

#endif

+#define OBJECT_FREE (0)
+#define OBJECT_ACTIVE (1)
+
+#ifdef CONFIG_DEBUG_SLAB_LEAK
+
+static void set_obj_status(struct page *page, int idx, int val)
+{
+ int freelist_size;
+ int *status;
+ struct kmem_cache *cachep = page->slab_cache;
+
+ freelist_size = ALIGN(cachep->num * sizeof(freelist_idx_t),
+ sizeof(int));
+ status = (int *)page->freelist + (freelist_size / sizeof(int));
+ status[idx] = val;
+}
+
+static inline unsigned int get_obj_status(struct page *page, int idx)
+{
+ int freelist_size;
+ int *status;
+ struct kmem_cache *cachep = page->slab_cache;
+
+ freelist_size = ALIGN(cachep->num * sizeof(freelist_idx_t),
+ sizeof(int));
+ status = (int *)page->freelist + (freelist_size / sizeof(int));
+
+ return status[idx];
+}
+
+#else
+static inline void set_obj_status(struct page *page, int idx, int val) {}
+
+#endif
+
/*
* Do not go above this order unless 0 objects fit into the slab or
* overridden on the command line.
@@ -581,7 +616,10 @@ static int calculate_nr_objs(size_t slab_size, size_t buffer_size,
{
int nr_objs;
size_t freelist_size;
+ int extra_space = 0;

+ if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK))
+ extra_space = sizeof(int);
/*
* Ignore padding for the initial guess. The padding
* is at most @align-1 bytes, and @buffer_size is at
@@ -590,15 +628,23 @@ static int calculate_nr_objs(size_t slab_size, size_t buffer_size,
* into the memory allocation when taking the padding
* into account.
*/
- nr_objs = slab_size / (buffer_size + idx_size);
+ nr_objs = slab_size / (buffer_size + idx_size + extra_space);

/*
* This calculated number will be either the right
* amount, or one greater than what we want.
*/
- freelist_size = slab_size - nr_objs * buffer_size;
- if (freelist_size < ALIGN(nr_objs * idx_size, align))
+ while (nr_objs) {
+ int aligned_size;
+
+ freelist_size = slab_size - nr_objs * buffer_size;
+ aligned_size = nr_objs * (idx_size + extra_space);
+ aligned_size = ALIGN(ALIGN(aligned_size, sizeof(int)), align);
+ if (freelist_size >= aligned_size)
+ break;
+
nr_objs--;
+ }

return nr_objs;
}
@@ -611,6 +657,8 @@ static void cache_estimate(unsigned long gfporder, size_t buffer_size,
unsigned int *num)
{
int nr_objs;
+ int idx_size = sizeof(freelist_idx_t);
+ int extra_space = 0;
size_t mgmt_size;
size_t slab_size = PAGE_SIZE << gfporder;

@@ -634,8 +682,13 @@ static void cache_estimate(unsigned long gfporder, size_t buffer_size,

} else {
nr_objs = calculate_nr_objs(slab_size, buffer_size,
- sizeof(freelist_idx_t), align);
- mgmt_size = ALIGN(nr_objs * sizeof(freelist_idx_t), align);
+ idx_size, align);
+
+ if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK))
+ extra_space = sizeof(int);
+ mgmt_size =
+ ALIGN(nr_objs * (idx_size + extra_space), sizeof(int));
+ mgmt_size = ALIGN(mgmt_size, align);
}
*num = nr_objs;
*left_over = slab_size - nr_objs*buffer_size - mgmt_size;
@@ -2285,8 +2338,13 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
if (!cachep->num)
return -E2BIG;

- freelist_size =
- ALIGN(cachep->num * sizeof(freelist_idx_t), cachep->align);
+ freelist_size = cachep->num * sizeof(freelist_idx_t);
+ /* extra space to store alloc/free status of object */
+ if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK)) {
+ freelist_size += cachep->num * sizeof(int);
+ freelist_size = ALIGN(freelist_size, sizeof(int));
+ }
+ freelist_size = ALIGN(freelist_size, cachep->align);

/*
* If the slab has been placed off-slab, and we have enough space then
@@ -2300,6 +2358,10 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
if (flags & CFLGS_OFF_SLAB) {
/* really off slab. No need for manual alignment */
freelist_size = cachep->num * sizeof(freelist_idx_t);
+ if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK)) {
+ freelist_size += cachep->num * sizeof(int);
+ freelist_size = ALIGN(freelist_size, sizeof(int));
+ }

#ifdef CONFIG_PAGE_POISONING
/* If we're going to use the generic kernel_map_pages()
@@ -2625,6 +2687,7 @@ static void cache_init_objs(struct kmem_cache *cachep,
if (cachep->ctor)
cachep->ctor(objp);
#endif
+ set_obj_status(page, i, OBJECT_FREE);
set_free_obj(page, i, i);
}
}
@@ -2833,6 +2896,7 @@ static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
BUG_ON(objnr >= cachep->num);
BUG_ON(objp != index_to_obj(cachep, page, objnr));

+ set_obj_status(page, objnr, OBJECT_FREE);
if (cachep->flags & SLAB_POISON) {
#ifdef CONFIG_DEBUG_PAGEALLOC
if ((cachep->size % PAGE_SIZE)==0 && OFF_SLAB(cachep)) {
@@ -2966,6 +3030,8 @@ static inline void cache_alloc_debugcheck_before(struct kmem_cache *cachep,
static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
gfp_t flags, void *objp, unsigned long caller)
{
+ struct page *page;
+
if (!objp)
return objp;
if (cachep->flags & SLAB_POISON) {
@@ -2996,6 +3062,9 @@ static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
*dbg_redzone1(cachep, objp) = RED_ACTIVE;
*dbg_redzone2(cachep, objp) = RED_ACTIVE;
}
+
+ page = virt_to_head_page(objp);
+ set_obj_status(page, obj_to_index(cachep, page, objp), OBJECT_ACTIVE);
objp += obj_offset(cachep);
if (cachep->ctor && cachep->flags & SLAB_POISON)
cachep->ctor(objp);
@@ -4232,21 +4301,12 @@ static void handle_slab(unsigned long *n, struct kmem_cache *c,
struct page *page)
{
void *p;
- int i, j;
+ int i;

if (n[0] == n[1])
return;
for (i = 0, p = page->s_mem; i < c->num; i++, p += c->size) {
- bool active = true;
-
- for (j = page->active; j < c->num; j++) {
- /* Skip freed item */
- if (get_free_obj(page, j) == i) {
- active = false;
- break;
- }
- }
- if (!active)
+ if (get_obj_status(page, i) != OBJECT_ACTIVE)
continue;

if (!add_caller(n, (unsigned long)*dbg_userword(c, p)))
--
1.7.9.5


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