mm/slub: Fix incorrect checkings of s->offset

PATCH Superseded Waiman Long 2 participants 6 messages Apr 27, 2020

Subsystems (2)

Memory management Slab allocator

lore

Share & tools

lore

Waiman Long

author

2020-04-27 02:03 UTC #0 lore raw

PATCH 1/1 mm/slub: Fix incorrect checkings of s->offset

In a couple of places in the slub memory allocator, the code uses "s->offset" as a check to see if the free pointer is put right after the object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").

As a result, echoing "1" into the validate sysfs file, e.g. of dentry, may cause a bunch of "Freepointer corrupt" error reports to appear with the system in panic afterwards.

To fix it, use the check "s->offset == s->inuse" instead.

Fixes 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")

Signed-off-by Waiman Long <[email protected]>

---
 mm/slub.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

mm/slub.c +7 ~~-11~~

index 0e736d66bb42..99952d01e7e0 100644
@@ -556,10 +556,8 @@ static struct track *get_track(struct kmem_cache *s, void *object,
 {
 	struct track *p;
 
-	if (s->offset)
-		p = object + s->offset + sizeof(void *);
-	else
-		p = object + s->inuse;
+	p = object + s->inuse +
+	    ((s->offset == s->inuse) ? sizeof(void *) : 0);
 
 	return p + alloc;
 }

@@ -693,10 +691,8 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
 		print_section(KERN_ERR, "Redzone ", p + s->object_size,
 			s->inuse - s->object_size);
 
-	if (s->offset)
-		off = s->offset + sizeof(void *);
-	else
-		off = s->inuse;
+	off = s->inuse +
+	      ((s->offset == s->inuse) ? sizeof(void *) : 0);
 
 	if (s->flags & SLAB_STORE_USER)
 		off += 2 * sizeof(struct track);

@@ -790,7 +786,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
  * object address
  * 	Bytes of the object to be managed.
  * 	If the freepointer may overlay the object then the free
- * 	pointer is the first word of the object.
+ *	pointer is at the middle of the object.
  *
  * 	Poisoning uses 0x6b (POISON_FREE) and the last byte is
  * 	0xa5 (POISON_END)

@@ -826,7 +822,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct page *page, u8 *p)
 {
 	unsigned long off = s->inuse;	/* The end of info */
 
-	if (s->offset)
+	if (s->offset == s->inuse)
 		/* Freepointer is placed after the object. */
 		off += sizeof(void *);

@@ -915,7 +911,7 @@ static int check_object(struct kmem_cache *s, struct page *page,
 		check_pad_bytes(s, page, p);
 	}
 
-	if (!s->offset && val == SLUB_RED_ACTIVE)
+	if ((s->offset != s->inuse) && val == SLUB_RED_ACTIVE)
 		/*
 		 * Object and freepointer overlap. Cannot check
 		 * freepointer while object is allocated.

Signature · 1 line

2.18.1

Matthew Wilcox

2020-04-27 12:38 UTC #1 lore raw

Quoted 10 lines · Waiman Long

In a couple of places in the slub memory allocator, the code uses "s->offset" as a check to see if the free pointer is put right after the object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").
As a result, echoing "1" into the validate sysfs file, e.g. of dentry, may cause a bunch of "Freepointer corrupt" error reports to appear with the system in panic afterwards.
To fix it, use the check "s->offset == s->inuse" instead.

I think a little refactoring would make this more clear.

unsigned int track_offset(const struct kmem_cache *s)

{
	return s->inuse + (s->offset == s->inuse) ? sizeof(void *) : 0;
}

mm/slub.c static struct track *get_track(struct kmem_cache *s, void *object, · line 560

Quoted 10 lines

mm/slub.c +2 -4

@@ -556,10 +556,8 @@ static struct track *get_track(struct kmem_cache *s, void *object,
 {
 	struct track *p;
 
-	if (s->offset)
-		p = object + s->offset + sizeof(void *);
-	else
-		p = object + s->inuse;
+	p = object + s->inuse +
+	    ((s->offset == s->inuse) ? sizeof(void *) : 0);

	p = object + track_offset(s);

mm/slub.c static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p) · line 695

Quoted 12 lines

mm/slub.c +2 -4

 	return p + alloc;
 }

@@ -693,10 +691,8 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
 		print_section(KERN_ERR, "Redzone ", p + s->object_size,
 			s->inuse - s->object_size);
 
-	if (s->offset)
-		off = s->offset + sizeof(void *);
-	else
-		off = s->inuse;
+	off = s->inuse +
+	      ((s->offset == s->inuse) ? sizeof(void *) : 0);

	off = track_offset(s);

mm/slub.c static int check_pad_bytes(struct kmem_cache *s, struct page *page, u8 *p) · line 827

Quoted 8 lines

mm/slub.c +1 -1

@@ -826,7 +822,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct page *page, u8 *p)
 {
 	unsigned long off = s->inuse;	/* The end of info */
 
-	if (s->offset)
+	if (s->offset == s->inuse)
 		/* Freepointer is placed after the object. */
 		off += sizeof(void *);

	unsigned long off = track_offset(s);

Waiman Long

author

2020-04-27 13:18 UTC #2 lore raw

Quoted 10 lines · Matthew Wilcox

Quoted 9 lines · Waiman Long
In a couple of places in the slub memory allocator, the code uses "s->offset" as a check to see if the free pointer is put right after the object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").
As a result, echoing "1" into the validate sysfs file, e.g. of dentry, may cause a bunch of "Freepointer corrupt" error reports to appear with the system in panic afterwards.

Quoted 7 lines

Quoted 1 line
To fix it, use the check "s->offset == s->inuse" instead.
I think a little refactoring would make this more clear.
unsigned int track_offset(const struct kmem_cache *s)
{
	return s->inuse + (s->offset == s->inuse) ? sizeof(void *) : 0;
}

Yes, that was what I am thinking of doing in v2.

Thanks,
Longman

Waiman Long

author

2020-04-27 13:29 UTC #3 lore raw

Quoted 12 lines · Waiman Long

Quoted 11 lines · Matthew Wilcox
Quoted 10 lines · Waiman Long
In a couple of places in the slub memory allocator, the code uses "s->offset" as a check to see if the free pointer is put right after the object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").
As a result, echoing "1" into the validate sysfs file, e.g. of dentry, may cause a bunch of "Freepointer corrupt" error reports to appear with the system in panic afterwards.
To fix it, use the check "s->offset == s->inuse" instead.

Quoted 8 lines

Quoted 6 lines
I think a little refactoring would make this more clear.
unsigned int track_offset(const struct kmem_cache *s)
{
return s->inuse + (s->offset == s->inuse) ? sizeof(void *) : 0; }
Yes, that was what I am thinking of doing in v2.

BTW, "+" has a higher priority than "?:". So we need a parenthesis around "?:".

Cheers,
Longman

Matthew Wilcox

2020-04-27 13:38 UTC #4 lore raw

Quoted 21 lines · Waiman Long

Quoted 20 lines · Waiman Long
Quoted 18 lines · Matthew Wilcox
Quoted 11 lines · Waiman Long
In a couple of places in the slub memory allocator, the code uses "s->offset" as a check to see if the free pointer is put right after the
object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").
As a result, echoing "1" into the validate sysfs file, e.g. of dentry, may cause a bunch of "Freepointer corrupt" error reports to appear with the system in panic afterwards.
To fix it, use the check "s->offset == s->inuse" instead.
I think a little refactoring would make this more clear.
unsigned int track_offset(const struct kmem_cache *s)
{
return s->inuse + (s->offset == s->inuse) ? sizeof(void *) : 0; }

Quoted 4 lines

Quoted 1 line
Yes, that was what I am thinking of doing in v2.
BTW, "+" has a higher priority than "?:". So we need a parenthesis around "?:".

That seems like a good reason to not use ?:

unsigned int track_offset(const struct kmem_cache *s)

{
	if (s->offset != s->inuse)
		return s->inuse;
	return s->inuse + sizeof(void *);
}

Also this needs a comment about why we're doing this ... something about the freelist pointer, I think?

Waiman Long

author

2020-04-27 13:56 UTC #5 lore raw

Quoted 27 lines · Matthew Wilcox

Quoted 16 lines · Waiman Long
Quoted 15 lines · Waiman Long
Quoted 14 lines · Matthew Wilcox
Quoted 11 lines · Waiman Long
In a couple of places in the slub memory allocator, the code uses "s->offset" as a check to see if the free pointer is put right after the
object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").
As a result, echoing "1" into the validate sysfs file, e.g. of dentry, may cause a bunch of "Freepointer corrupt" error reports to appear with the system in panic afterwards.
To fix it, use the check "s->offset == s->inuse" instead.
I think a little refactoring would make this more clear.
Quoted 7 lines
Quoted 5 lines
Quoted 4 lines
unsigned int track_offset(const struct kmem_cache *s)
{
return s->inuse + (s->offset == s->inuse) ? sizeof(void *) : 0;
}
Yes, that was what I am thinking of doing in v2.
BTW, "+" has a higher priority than "?:". So we need a parenthesis around "?:".
That seems like a good reason to not use ?:
unsigned int track_offset(const struct kmem_cache *s)

Quoted 9 lines

{
	if (s->offset != s->inuse)
		return s->inuse;
	return s->inuse + sizeof(void *);
}
Also this needs a comment about why we're doing this ... something about the freelist pointer, I think?

I can see a simple if-else to make it easier to read.

Thanks,
Longman