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

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Message-ID: <oxzh24oit3ulkl5at66c4g3bsyo7z6jd54757yrkgqbeftfgjz@qlzktn3nrkpl>
Date: Fri, 12 Jul 2024 11:49:05 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Youling Tang <youling.tang@...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()

On Fri, Jul 12, 2024 at 12:28:57PM GMT, Youling Tang wrote:
> 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.

No, you're just hiding the actual bug - since I wasn't clear, I'm naking
this patch.