Re: [PATCH v2 0/5] mm/slub: Improve data handling of krealloc() when orig_size is enabled

[Vlastimil Babka <vbabka@suse.cz>]

On 10/14/24 14:52, Feng Tang wrote:
> On Mon, Oct 14, 2024 at 10:53:32AM +0200, Vlastimil Babka wrote:
>> On 10/14/24 09:52, Feng Tang wrote:
>> > On Fri, Oct 04, 2024 at 05:52:10PM +0800, Vlastimil Babka wrote:
>> > Thanks for the suggestion!
>> > 
>> > As there were error report about the NULL slab for big kmalloc object, how
>> > about the following code for 
>> > 
>> > __do_krealloc(const void *p, size_t new_size, gfp_t flags)
>> > {
>> > 	void *ret;
>> > 	size_t ks = 0;
>> > 	int orig_size = 0;
>> > 	struct kmem_cache *s = NULL;
>> > 
>> > 	/* Check for double-free. */
>> > 	if (likely(!ZERO_OR_NULL_PTR(p))) {
>> > 		if (!kasan_check_byte(p))
>> > 			return NULL;
>> > 
>> > 		ks = ksize(p);
>> 
>> I think this will result in __ksize() doing
>>   skip_orig_size_check(folio_slab(folio)->slab_cache, object);
>> and we don't want that?
> 
> I think that's fine. As later code will re-set the orig_size anyway.

But you also read it first.

>> > 		/* Some objects have no orig_size, like big kmalloc case */
>> > 		if (is_kfence_address(p)) {
>> > 			orig_size = kfence_ksize(p);
>> > 		} else if (virt_to_slab(p)) {
>> > 			s = virt_to_cache(p);
>> > 			orig_size = get_orig_size(s, (void *)p);

here.

>> > 		}

>> Also the checks below repeat some of the checks of ksize().
> 
> Yes, there is some redundancy, mostly the virt_to_slab() 
> 
>> So I think in __do_krealloc() we should do things manually to determine ks
>> and not call ksize(). Just not break any of the cases ksize() handles
>> (kfence, large kmalloc).
> 
> OK, originally I tried not to expose internals of __ksize(). Let me
> try this way.

ksize() makes assumptions that a user outside of slab itself is calling it.

But we (well mostly Kees) also introduced kmalloc_size_roundup() to avoid
querying ksize() for the purposes of writing beyond the original
kmalloc(size) up to the bucket size. So maybe we can also investigate if the
skip_orig_size_check() mechanism can be removed now?

Still I think __do_krealloc() should rather do its own thing and not call
ksize().

> Thanks,
> Feng
> 
>> 
>> > 
>> > 	} else {
>> > 		goto alloc_new;
>> > 	}
>> > 
>> > 	/* If the object doesn't fit, allocate a bigger one */
>> > 	if (new_size > ks)
>> > 		goto alloc_new;
>> > 
>> > 	/* Zero out spare memory. */
>> > 	if (want_init_on_alloc(flags)) {
>> > 		kasan_disable_current();
>> > 		if (orig_size && orig_size < new_size)
>> > 			memset((void *)p + orig_size, 0, new_size - orig_size);
>> > 		else
>> > 			memset((void *)p + new_size, 0, ks - new_size);
>> > 		kasan_enable_current();
>> > 	}
>> > 
>> > 	/* Setup kmalloc redzone when needed */
>> > 	if (s && slub_debug_orig_size(s) && !is_kfence_address(p)) {
>> > 		set_orig_size(s, (void *)p, new_size);
>> > 		if (s->flags & SLAB_RED_ZONE && new_size < ks)
>> > 			memset_no_sanitize_memory((void *)p + new_size,
>> > 						SLUB_RED_ACTIVE, ks - new_size);
>> > 	}
>> > 
>> > 	p = kasan_krealloc((void *)p, new_size, flags);
>> > 	return (void *)p;
>> > 
>> > alloc_new:
>> > 	ret = kmalloc_node_track_caller_noprof(new_size, flags, NUMA_NO_NODE, _RET_IP_);
>> > 	if (ret && p) {
>> > 		/* Disable KASAN checks as the object's redzone is accessed. */
>> > 		kasan_disable_current();
>> > 		memcpy(ret, kasan_reset_tag(p), orig_size ?: ks);
>> > 		kasan_enable_current();
>> > 	}
>> > 
>> > 	return ret;
>> > }