linux-kernel - Re: [PATCH] damon: vaddr-test: fix a missing check on list iterator

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Date:   Mon, 28 Mar 2022 07:51:04 +0000
From:   sj@...nel.org
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     sj@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] damon: vaddr-test: fix a missing check on list iterator

Hi Xiaomeng,

On Sun, 27 Mar 2022 16:03:45 +0800 Xiaomeng Tong <xiam0nd.tong@...il.com> wrote:

> The bug is here:
> 	KUNIT_EXPECT_EQ(test, r->ar.start, start + i * expected_width);
> 	KUNIT_EXPECT_EQ(test, r->ar.end, end);
> 
> For the damon_for_each_region(), just like list_for_each_entry(),
> the list iterator 'drm_crtc' will point to a bogus position
> containing HEAD if the list is empty or no element is found.
> This case must be checked before any use of the iterator,
> otherwise it will lead to a invalid memory access.

We ensure 'damon_va_evenly_split_region()' successes before executing the loop,
so the issue cannot occur.  That said, I think this patch makes code better to
read.  Could you please resend this patch after fixing the commit message?

> 
> To fix this bug, just mov two KUNIT_EXPECT_EQ() into the loop

s/mov/move

> when found.
> 
> Cc: stable@...r.kernel.org
> Fixes: 044cd9750fe01 ("mm/damon/vaddr-test: split a test function having >1024 bytes frame size")
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>
> ---
>  mm/damon/vaddr-test.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/damon/vaddr-test.h b/mm/damon/vaddr-test.h
> index 6a1b9272ea12..98b7a9f54b35 100644
> --- a/mm/damon/vaddr-test.h
> +++ b/mm/damon/vaddr-test.h
> @@ -281,14 +281,16 @@ static void damon_test_split_evenly_succ(struct kunit *test,
>  	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), nr_pieces);

As mentioned above, this will ensure the loop will not result in the bogus
pointer problem.

>  
>  	damon_for_each_region(r, t) {
> -		if (i == nr_pieces - 1)
> +		if (i == nr_pieces - 1) {
> +			KUNIT_EXPECT_EQ(test,
> +				r->ar.start, start + i * expected_width);
> +			KUNIT_EXPECT_EQ(test, r->ar.end, end);
>  			break;
> +		}
>  		KUNIT_EXPECT_EQ(test,
>  				r->ar.start, start + i++ * expected_width);
>  		KUNIT_EXPECT_EQ(test, r->ar.end, start + i * expected_width);
>  	}
> -	KUNIT_EXPECT_EQ(test, r->ar.start, start + i * expected_width);
> -	KUNIT_EXPECT_EQ(test, r->ar.end, end);
>  	damon_free_target(t);
>  }
>  
> -- 
> 2.17.1
> 
> 


Thanks,
SJ