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Subject: Re: [PATCH v2] md/md-multipath: guard against a possible NULL
 dereference
To:     Daniil Tatianin <d-tatianin@yandex-team.ru>,
        Song Liu <song@kernel.org>
Cc:     linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230421062110.368253-1-d-tatianin@yandex-team.ru>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <eba0985d-27fc-ce0f-537a-b8b713d75262@huaweicloud.com>
Date:   Sun, 23 Apr 2023 11:08:38 +0800
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Precedence: bulk

Hi,

?? 2023/04/21 14:21, Daniil Tatianin д??:
> mempool_alloc might fail to allocate a slot, in which case we will end
> up dereferencing a NULL mp_bh pointer.
> 
> Found by Linux Verification Center (linuxtesting.org) with the SVACE
> static analysis tool.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---
> Changes since v1: fixed a typo
> ---
>   drivers/md/md-multipath.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/md/md-multipath.c b/drivers/md/md-multipath.c
> index 66edf5e72bd6..8dfa353440e5 100644
> --- a/drivers/md/md-multipath.c
> +++ b/drivers/md/md-multipath.c
> @@ -108,6 +108,8 @@ static bool multipath_make_request(struct mddev *mddev, struct bio * bio)
>   		return true;
>   
>   	mp_bh = mempool_alloc(&conf->pool, GFP_NOIO);

The comment in mempool_alloc says:

Note that due to preallocation, this function *never* fails when called
from process contexts.

And there are many places to use this way, so I think this patch is not
needed.

Thanks,
Kuai
> +	if (!mp_bh)
> +		return false;
>   
>   	mp_bh->master_bio = bio;
>   	mp_bh->mddev = mddev;
>