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SubjectRe: [PATCH] mm/slub: Fix incorrect checkings of s->offset
On Sun, Apr 26, 2020 at 10:02:12PM -0400, Waiman Long wrote:
> 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;
}

> @@ -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);

> 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);

> @@ -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);

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