Re: [PATCH v2 1/2] mm: vmalloc: implement vrealloc()

[Feng Tang]

On Mon, Sep 02, 2024 at 10:56:57AM +0200, Vlastimil Babka wrote:
> On 9/2/24 09:04, Feng Tang wrote:
> > On Mon, Sep 02, 2024 at 09:36:26AM +0800, Tang, Feng wrote:
> >> On Tue, Jul 30, 2024 at 08:15:34PM +0800, Vlastimil Babka wrote:
> >> > On 7/30/24 3:35 AM, Danilo Krummrich wrote:
> > [...]
> >> > 
> >> > Let's say we kmalloc(56, __GFP_ZERO), we get an object from kmalloc-64
> >> > cache. Since commit 946fa0dbf2d89 ("mm/slub: extend redzone check to
> >> > extra allocated kmalloc space than requested") and preceding commits, if
> >> > slub_debug is enabled (red zoning or user tracking), only the 56 bytes
> >> > will be zeroed. The rest will be either unknown garbage, or redzone.
> >> 
> >> Yes.
> >> 
> >> > 
> >> > Then we might e.g. krealloc(120) and get a kmalloc-128 object and 64
> >> > bytes (result of ksize()) will be copied, including the garbage/redzone.
> >> > I think it's fixable because when we do this in slub_debug, we also
> >> > store the original size in the metadata, so we could read it back and
> >> > adjust how many bytes are copied.
> >> 
> >> krealloc() --> __do_krealloc() --> ksize()
> >> When ksize() is called, as we don't know what user will do with the
> >> extra space ([57, 64] here), the orig_size check will be unset by
> >> __ksize() calling skip_orig_size_check(). 
> >> 
> >> And if the newsize is bigger than the old 'ksize', the 'orig_size'
> >> will be correctly set for the newly allocated kmalloc object.
> 
> Yes, but the memcpy() to the new object will be done using ksize() thus
> include the redzone, e.g. [57, 64]

Right.

> 
> >> For the 'unstable' branch of -mm tree, which has all latest patches
> >> from Danilo, I run some basic test and it seems to be fine. 
> 
> To test it would not always be enough to expect some slub_debug to fail,
> you'd e.g. have to kmalloc(48, GFP_KERNEL | GFP_ZERO), krealloc(128,
> GFP_KERNEL | GFP_ZERO) and then verify there are zeroes from 48 to 128. I
> suspect there won't be zeroes from 48 to 64 due to redzone.

Yes, you are right.
 
> (this would have made a great lib/slub_kunit.c test :))

Agree.

> > when doing more test, I found one case matching Vlastimil's previous
> > concern, that if we kzalloc a small object, and then krealloc with
> > a slightly bigger size which can still reuse the kmalloc object,
> > some redzone will be preserved.
> > 
> > With test code like: 
> > 
> > 	buf = kzalloc(36, GFP_KERNEL);
> > 	memset(buf, 0xff, 36);
> > 
> > 	buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);
> > 
> > Data after kzalloc+memset :
> > 
> > 	ffff88802189b040: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  
> > 	ffff88802189b050: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  
> > 	ffff88802189b060: ff ff ff ff cc cc cc cc cc cc cc cc cc cc cc cc  
> > 	ffff88802189b070: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc  
> > 
> > Data after krealloc:
> > 
> > 	ffff88802189b040: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > 	ffff88802189b050: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > 	ffff88802189b060: ff ff ff ff cc cc cc cc cc cc cc cc cc cc cc cc
> > 	ffff88802189b070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > 
> > If we really want to make [37, 48] to be zeroed too, we can lift the
> > get_orig_size() from slub.c to slab_common.c and use it as the start
> > of zeroing in krealloc().
> 
> Or maybe just move krealloc() to mm/slub.c so there are no unnecessary calls
> between the files.
> 
> We should also set a new orig_size in cases we are shrinking or enlarging
> within same object (i.e. 48->40 or 48->64). In case of shrinking, we also
> might need to redzone the shrinked area (i.e. [40, 48]) or later checks will
> fail.  But if the current object is from kfence, then probably not do any of
> this... sigh this gets complicated. And really we need kunit tests for all
> the scenarios :/

Good point! will think about and try to implement it to ensure the
orig_size and kmalloc-redzone check setting is kept. 

Thanks,
Feng