Re: Can kfree() sleep at runtime?

From: Jia-Ju Bai
Date: Thu May 31 2018 - 21:12:49 EST



On 2018/5/31 22:08, Matthew Wilcox wrote:
On Thu, May 31, 2018 at 09:10:07PM +0800, Jia-Ju Bai wrote:
I write a static analysis tool (DSAC), and it finds that kfree() can sleep.

Here is the call path for kfree().
Please look at it *from the bottom up*.

[FUNC] alloc_pages(GFP_KERNEL)
arch/x86/mm/pageattr.c, 756: alloc_pages in split_large_page
arch/x86/mm/pageattr.c, 1283: split_large_page in __change_page_attr
Here's your bug. Coming from kfree(), we can't end up in the
split_large_page() path. __change_page_attr may be called in several
different circumstances in which it would have to split a large page,
but the path from kfree() is not one of them.

I think the path from kfree() will lead to the 'level == PG_LEVEL_4K'
path, but I'm not really familiar with this x86 code.

Thanks for reply :)
But from the code in my call path, I cannot find why kfree() will only lead to the 'level == PG_LEVEL_4K' path.
Could you please explain it in more detail?


Best wishes,
Jia-Ju Bai