Re: [PATCH] mm/slub: Fix incorrect checkings of s->offset

[Matthew Wilcox <willy@infradead.org>]

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);