Re: [PATCH] mm: kmemleak: check boundary of objects allocated with physical address when scan

[Patrick Wang]

On 2022/6/1 00:29, Catalin Marinas wrote:
> On Tue, May 31, 2022 at 11:08:23PM +0800, Patrick Wang wrote:
> > @@ -1132,8 +1135,13 @@ EXPORT_SYMBOL(kmemleak_no_scan);
> >   void __ref kmemleak_alloc_phys(phys_addr_t phys, size_t size, int min_count,
> >   			       gfp_t gfp)
> >   {
> > -	if (PHYS_PFN(phys) >= min_low_pfn && PHYS_PFN(phys) < max_low_pfn)
> > -		kmemleak_alloc(__va(phys), size, min_count, gfp);
> > +	pr_debug("%s(0x%p, %zu, %d)\n", __func__, __va(phys), size, min_count);
>
> I'd print just phys here since that's the function argument.

Will do.

> > +	if (kmemleak_enabled && (unsigned long)__va(phys) >= PAGE_OFFSET &&
> > +	    !IS_ERR(__va(phys)))
> > +		/* create object with OBJECT_PHYS flag */
> > +		create_object((unsigned long)__va(phys), size, min_count,
> > +			      gfp, true);
>
> Do we still need to check for __va(phys) >= PAGE_OFFSET? Also I don't
> think IS_ERR(__va(phys)) makes sense, we can't store an error in a
> physical address. The kmemleak_alloc_phys() function is only called on
> successful allocation, so shouldn't bother with error codes.

In this commit:
972fa3a7c17c(mm: kmemleak: alloc gray object for reserved
region with direct map)

The kmemleak_alloc_phys() function is called directly by passing
physical address from devicetree. So I'm concerned that could
__va() => __pa() convert always get the phys back? I thought
check for __va(phys) might help, but it probably dosen't work
and using IS_ERR is indeed inappropriate.

We might have to store phys in object and convert it via __va()
for normal use like:

#define object_pointer(obj)	\
	(obj->flags & OBJECT_PHYS ? (unsigned long)__va((void *)obj->pointer)	\
				: obj->pointer)

> > @@ -1436,6 +1441,13 @@ static void kmemleak_scan(void)
> >   			dump_object_info(object);
> >   		}
> >   #endif
> > +
> > +		/* outside lowmem, make it black */
>
> Maybe a bit more verbose:
>
> 		/* ignore objects outside lowmem (paint them black) */

Will do.

> > +		if (object->flags & OBJECT_PHYS)
> > +			if (PHYS_PFN(__pa((void *)object->pointer)) < min_low_pfn ||
> > +			    PHYS_PFN(__pa((void *)object->pointer)) >= max_low_pfn)
> > +				__paint_it(object, KMEMLEAK_BLACK);
>
> I'd skip the checks if the object is OBJECT_NO_SCAN (side-effect of
> __paint_it()) so that the next scan won't have to go through the __pa()
> checks again. It's also probably more correct to check the upper object
> boundary). Something like:
>
> 		if ((object->flags & OBJECT_PHYS) &&
> 		    !(object->flags & OBJECT_NO_SCAN)) {
> 			unsigned long phys = __pa((void *)object->pointer);
> 			if (PHYS_PFN(phys) < min_low_pfn ||
> 			    PHYS_PFN(phys + object->size) >= max_low_pfn)
> 				__paint_it(object, KMEMLEAK_BLACK);
> 		}

Right, much more thorough. Will do.

Thanks,
Patrick