linux-kernel - Re: [PATCH 1/4] scsi: core: constify pointer to scsi_host

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Message-ID: <Ynt0aFMX+z/UhGJ2@infradead.org>
Date:   Wed, 11 May 2022 01:31:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     John Garry <john.garry@...wei.com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Ewan D. Milne" <emilne@...hat.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        Doug Gilbert <dgilbert@...erlog.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        james.smart@...adcom.com
Subject: Re: [PATCH 1/4] scsi: core: constify pointer to scsi_host_template

On Mon, May 09, 2022 at 03:50:33PM +0100, John Garry wrote:
> If you check scsi_device_dev_release(), we try to do a 'get' - if it fails,
> then we nullify hostt->module. I think that is important as then we call
> execute_in_process_context(), whose worker does the 'put'. However, the
> 'put' will get upset if the refcnt was 0, which it would be if the earlier
> 'get' fails - hence the nullify is to avoid that possibility. So whatever
> you do needs to handle that. Details are in f2b85040

Yikes, that code is completely and utterly buggy and does not account
for all the cases why try_module_get can fail.  I think we always have
a reference here and could use __module_get, but what we have is
certainly unsafe and a good reason why the host template should be
constifyed.