linux-kernel - Re: [PATCH] mm: list_lru: Fix NULL pointer dereference in list_lru

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Message-ID: <8ce42a2e-783f-4244-8e75-21dcd578adf5@linux.dev>
Date: Fri, 12 Jul 2024 12:28:57 +0800
From: Youling Tang <youling.tang@...ux.dev>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Youling Tang <tangyouling@...inos.cn>
Subject: Re: [PATCH] mm: list_lru: Fix NULL pointer dereference in
 list_lru_add()

Hi, Kent

On 12/07/2024 12:07, Kent Overstreet wrote:
> On Fri, Jul 12, 2024 at 11:25:54AM GMT, Youling Tang wrote:
>> From: Youling Tang <tangyouling@...inos.cn>
>>
>> Note that list_lru_from_memcg_idx() may return NULL, so it is necessary
>> to error handle the return value to avoid triggering NULL pointer
>> dereference BUG.
>>
>> The issue was triggered for discussion [1],
>> Link [1]: https://lore.kernel.org/linux-bcachefs/84de6cb1-57bd-42f7-8029-4203820ef0b4@linux.dev/T/#m901bb26cdb1d9d4bacebf0d034f0a5a712cc93a6
> I see no explanation for why this is the correct fix, and I doubt it is.
> What's the real reason for the NULL lru_list_one, and why doesn't this
> come up on other filesystems?
We can break it down into two questions (independent of each other):
1) Error handling is necessary when l (lru_list_one) is NULL here.
2) Why does marking SLAB_ACCOUNT in bcachefs cause lru_list_one
    to be NULL?

This patch only fixes the first issue, and the real cause of the
second issue still needs to be analyzed.

Thanks,
Youling.